Noel Q. Hoffer, Krishna Neupane, Andrew G. T. Pyo, and Michael T. Woodside
Transition paths represent the parts of a reaction where the energy barrier separating products and reactants is crossed. They are essential to understanding reaction mechanisms, yet many of their properties remain unstudied. Here, we report measurements of the average shape of transition paths, studying the folding of DNA hairpins as a model system for folding reactions. Individual transition paths were detected in the folding trajectories of hairpins with different sequences held under tension in optical tweezers, and path shapes were computed by averaging all transitions in the time domain, 〈t(x)〉, or by averaging transitions of a given duration in the extension domain, 〈x(t|τ)〉τ. Whereas 〈t(x)〉 was close to straight, with only a subtle curvature, 〈x(t|τ)〉τ had more pronounced curvature that fit well to theoretical expectations for the dominant transition path, returning diffusion coefficients similar to values obtained previously from independent methods. Simulations suggested that 〈t(x)〉 provided a less reliable representation of the path shape than 〈x(t|τ)〉τ, because it was far more sensitive to the effects of coupling the molecule to the experimental force probe. Intriguingly, the path shape variance was larger for some hairpins than others, indicating sequence-dependent changes in the diversity of transition paths reflective of differences in the character of the energy barriers, such as the width of the barrier saddle-point or the presence of parallel paths through multiple barriers between the folded and unfolded states. These studies of average path shapes point the way forward for probing the rich information contained in path shape fluctuations.
DOI
Thursday, June 27, 2019
Optical-force-directed single-particle-based track etching in polystyrene films
Shuangshuang Wang and Tao Ding
The establishment of optical trapping theory by A Ashkin almost half a century ago led to the trapping and manipulation of micro-/nano-objects and atoms by laser beams, which is now applied in many fields. However, in a complex system where multi-physical effects interact synergistically, light–matter interaction becomes dynamic and complicated and is still poorly understood. Here, by utilising plasmonic heating, nanolithography of polystyrene (PS) films and nanomanipulation of gold nanoparticles are realised. We find that laser power and PS film thickness as well as particle shape strongly affect the etching behaviour of the PS films, which is a synergistic effect of photothermal ablation and optical forces. Theoretical calculations and simulations rationalise the proposed mechanism, which is also verified by experimental observation. This understanding not only sheds light on how the synergistic effect of photo-ablation and optical forces acts on the particles and polymer films, but also provides a guideline for light-directed single-particle-based nanolithography and nanomanipulation.
DOI
The establishment of optical trapping theory by A Ashkin almost half a century ago led to the trapping and manipulation of micro-/nano-objects and atoms by laser beams, which is now applied in many fields. However, in a complex system where multi-physical effects interact synergistically, light–matter interaction becomes dynamic and complicated and is still poorly understood. Here, by utilising plasmonic heating, nanolithography of polystyrene (PS) films and nanomanipulation of gold nanoparticles are realised. We find that laser power and PS film thickness as well as particle shape strongly affect the etching behaviour of the PS films, which is a synergistic effect of photothermal ablation and optical forces. Theoretical calculations and simulations rationalise the proposed mechanism, which is also verified by experimental observation. This understanding not only sheds light on how the synergistic effect of photo-ablation and optical forces acts on the particles and polymer films, but also provides a guideline for light-directed single-particle-based nanolithography and nanomanipulation.
DOI
Microscope images of strongly scattering objects via vectorial transfer matrices: Modeling and an experimental verification
Alexander B. Stilgoe, Vincent L. Y. Loke, Anatolii V. Kashchuk, Timo A. Nieminen, and Halina Rubinsztein-Dunlop
We present an accurate and efficient method for calculating the image of a mesoscopic particle that is captured using a high-numerical-aperture objective lens. We test various scattering models of silica, plastic, and birefringent vaterite spheres in water. We show that the calculated images accurately replicate experimental observations. This method uses the idea that the optical system can be represented as a product of matrices acting on the electromagnetic field in a truncated Hilbert space representation. A general reusable matrix encapsulating the polarization, limited capture angle, or beam shaping in the microscope can be applied to find the image and not be limited to a particular particle shape or medium. We show that the image obtained from this method can be used to determine and match particle properties. We also use incoherent averaging on multiple T-matrices to produce polychromatic images. Data obtained using this method could be used as an input to track the general behavior of particles in suspension or within an optical trap.
DOI
We present an accurate and efficient method for calculating the image of a mesoscopic particle that is captured using a high-numerical-aperture objective lens. We test various scattering models of silica, plastic, and birefringent vaterite spheres in water. We show that the calculated images accurately replicate experimental observations. This method uses the idea that the optical system can be represented as a product of matrices acting on the electromagnetic field in a truncated Hilbert space representation. A general reusable matrix encapsulating the polarization, limited capture angle, or beam shaping in the microscope can be applied to find the image and not be limited to a particular particle shape or medium. We show that the image obtained from this method can be used to determine and match particle properties. We also use incoherent averaging on multiple T-matrices to produce polychromatic images. Data obtained using this method could be used as an input to track the general behavior of particles in suspension or within an optical trap.
DOI
Spin–Orbit Interaction of Light in Plasmonic Lattices
Shai Tsesses, Kobi Cohen, Evgeny Ostrovsky, Bergin Gjonaj, Guy Bartal
In the past decade, the spin–orbit interaction (SOI) of light has been a driving force in the design of metamaterials, metasurfaces, and schemes for light-matter interaction. A hallmark of the spin–orbit interaction of light is the spin-based plasmonic effect, converting spin angular momentum of propagating light to near-field orbital angular momentum. Although this effect has been thoroughly investigated in circular symmetry, it has yet to be characterized in a noncircular geometry, where whirling, periodic plasmonic fields are expected. Using phase-resolved near-field microscopy, we experimentally demonstrate the SOI of circularly polarized light in nanostructures possessing dihedral symmetry. We show how interaction with hexagonal slits results in four topologically different plasmonic lattices, controlled by engineered boundary conditions, and reveal a cyclic nature of the spin-based plasmonic effect which does not exist for circular symmetry. Finally, we calculate the optical forces generated by the plasmonic lattices, predicting that light with mere spin angular momentum can exert torque on a multitude of particles in an ordered fashion to form an optical nanomotor array. Our findings may be of use in both biology and chemistry, as a means for simultaneous trapping, manipulation, and excitation of multiple objects, controlled by the polarization of light.
In the past decade, the spin–orbit interaction (SOI) of light has been a driving force in the design of metamaterials, metasurfaces, and schemes for light-matter interaction. A hallmark of the spin–orbit interaction of light is the spin-based plasmonic effect, converting spin angular momentum of propagating light to near-field orbital angular momentum. Although this effect has been thoroughly investigated in circular symmetry, it has yet to be characterized in a noncircular geometry, where whirling, periodic plasmonic fields are expected. Using phase-resolved near-field microscopy, we experimentally demonstrate the SOI of circularly polarized light in nanostructures possessing dihedral symmetry. We show how interaction with hexagonal slits results in four topologically different plasmonic lattices, controlled by engineered boundary conditions, and reveal a cyclic nature of the spin-based plasmonic effect which does not exist for circular symmetry. Finally, we calculate the optical forces generated by the plasmonic lattices, predicting that light with mere spin angular momentum can exert torque on a multitude of particles in an ordered fashion to form an optical nanomotor array. Our findings may be of use in both biology and chemistry, as a means for simultaneous trapping, manipulation, and excitation of multiple objects, controlled by the polarization of light.
Recent progress in near-field nanolithography using light interactions with colloidal particles: from nanospheres to three-dimensional nanostructures
Xu A Zhang, I-Te Chen and Chih-Hao Chang
The advance of nanotechnology is firmly rooted in the development of cost-effective, versatile, and easily accessible nanofabrication techniques. The ability to pattern complex two-dimensional and three-dimensional nanostructured materials are particularly desirable, since they can have novel physical properties that are not found in bulk materials. This review article will report recent progress in utilizing self-assembly of colloidal particles for nanolithography. In these techniques, the near-field interactions of light and colloids are the sole mechanisms employed to generate the intensity distributions for patterning. Based on both 'bottom-up' self-assembly and 'top-down' lithography approaches, these processes are highly versatile and can take advantage of a number of optical effects, allowing the complex 3D nanostructures to be patterned using single exposures. There are several key advantages including low equipment cost, facile structure design, and patterning scalability, which will be discussed in detail. We will outline the underlying optical effects, review the geometries that can be fabricated, discuss key limitations, and highlight potential applications in nanophotonics, optoelectronic devices, and nanoarchitectured materials.
DOI
The advance of nanotechnology is firmly rooted in the development of cost-effective, versatile, and easily accessible nanofabrication techniques. The ability to pattern complex two-dimensional and three-dimensional nanostructured materials are particularly desirable, since they can have novel physical properties that are not found in bulk materials. This review article will report recent progress in utilizing self-assembly of colloidal particles for nanolithography. In these techniques, the near-field interactions of light and colloids are the sole mechanisms employed to generate the intensity distributions for patterning. Based on both 'bottom-up' self-assembly and 'top-down' lithography approaches, these processes are highly versatile and can take advantage of a number of optical effects, allowing the complex 3D nanostructures to be patterned using single exposures. There are several key advantages including low equipment cost, facile structure design, and patterning scalability, which will be discussed in detail. We will outline the underlying optical effects, review the geometries that can be fabricated, discuss key limitations, and highlight potential applications in nanophotonics, optoelectronic devices, and nanoarchitectured materials.
DOI
Wednesday, June 26, 2019
Dystrophy-associated caveolin-3 mutations reveal that caveolae couple IL6/STAT3 signaling with mechanosensing in human muscle cells
Melissa Dewulf, Darius Vasco Köster, Bidisha Sinha, Christine Viaris de Lesegno, Valérie Chambon, Anne Bigot, Mona Bensalah, Elisa Negroni, Nicolas Tardif, Joanna Podkalicka, Ludger Johannes, Pierre Nassoy, Gillian Butler-Browne, Christophe Lamaze & Cedric M. Blouin
Caveolin-3 is the major structural protein of caveolae in muscle. Mutations in the CAV3 gene cause different types of myopathies with altered membrane integrity and repair, expression of muscle proteins, and regulation of signaling pathways. We show here that myotubes from patients bearing the CAV3 P28L and R26Q mutations present a dramatic decrease of caveolae at the plasma membrane, resulting in abnormal response to mechanical stress. Mutant myotubes are unable to buffer the increase in membrane tension induced by mechanical stress. This results in impaired regulation of the IL6/STAT3 signaling pathway leading to its constitutive hyperactivation and increased expression of muscle genes. These defects are fully reversed by reassembling functional caveolae through expression of caveolin-3. Our study reveals that under mechanical stress the regulation of mechanoprotection by caveolae is directly coupled with the regulation of IL6/STAT3 signaling in muscle cells and that this regulation is absent in Cav3-associated dystrophic patients.
DOI
Caveolin-3 is the major structural protein of caveolae in muscle. Mutations in the CAV3 gene cause different types of myopathies with altered membrane integrity and repair, expression of muscle proteins, and regulation of signaling pathways. We show here that myotubes from patients bearing the CAV3 P28L and R26Q mutations present a dramatic decrease of caveolae at the plasma membrane, resulting in abnormal response to mechanical stress. Mutant myotubes are unable to buffer the increase in membrane tension induced by mechanical stress. This results in impaired regulation of the IL6/STAT3 signaling pathway leading to its constitutive hyperactivation and increased expression of muscle genes. These defects are fully reversed by reassembling functional caveolae through expression of caveolin-3. Our study reveals that under mechanical stress the regulation of mechanoprotection by caveolae is directly coupled with the regulation of IL6/STAT3 signaling in muscle cells and that this regulation is absent in Cav3-associated dystrophic patients.
DOI
Time-resolved nano-Newton force spectroscopy in air and vacuum using a load cell of ultra micro-balance
Biswajit Panda, Mehra S. Sidhu, Pooja Munjal, Shivali Sokhi, and Kamal P. Singh
We demonstrate a simple and versatile nanomechanical force measuring setup with 1 nN precision in air and vacuum using a load cell of an ultra-microbalance. We validate stability, precision, and linearity of the load cell with simple tests. The setup is customized to measure stress-strain response of biomaterials (silk, leaf, and flower) and capillary force in fluids. We isolated an optical pull force induced by a Watt-level laser reflected from a mirror/solid surface in air, in addition to optical push force. Furthermore, we add an interferometric probe to directly measure nanoscale deflection of cantilever of the load cell in real-time, thus bypassing its conventional electromagnetic readout, to improve speed and precision of the instrument. We demonstrate nanomechanical force measurement in high vacuum with the same precision and employ radiation pressure to calibrate the load cell for various precision measurements.
DOI
We demonstrate a simple and versatile nanomechanical force measuring setup with 1 nN precision in air and vacuum using a load cell of an ultra-microbalance. We validate stability, precision, and linearity of the load cell with simple tests. The setup is customized to measure stress-strain response of biomaterials (silk, leaf, and flower) and capillary force in fluids. We isolated an optical pull force induced by a Watt-level laser reflected from a mirror/solid surface in air, in addition to optical push force. Furthermore, we add an interferometric probe to directly measure nanoscale deflection of cantilever of the load cell in real-time, thus bypassing its conventional electromagnetic readout, to improve speed and precision of the instrument. We demonstrate nanomechanical force measurement in high vacuum with the same precision and employ radiation pressure to calibrate the load cell for various precision measurements.
DOI
Structural basis for reversible amyloids of hnRNPA1 elucidates their role in stress granule assembly
Xinrui Gui, Feng Luo, Yichen Li, Heng Zhou, Zhenheng Qin, Zhenying Liu, Jinge Gu, Muyun Xie, Kun Zhao, Bin Dai, Woo Shik Shin, Jianhua He, Lin He, Lin Jiang, Minglei Zhao, Bo Sun, Xueming Li, Cong Liu & Dan Li
Subcellular membrane-less organelles consist of proteins with low complexity domains. Many of them, such as hnRNPA1, can assemble into both a polydisperse liquid phase and an ordered solid phase of amyloid fibril. The former mirrors biological granule assembly, while the latter is usually associated with neurodegenerative disease. Here, we observe a reversible amyloid formation of hnRNPA1 that synchronizes with liquid–liquid phase separation, regulates the fluidity and mobility of the liquid-like droplets, and facilitates the recruitment of hnRNPA1 into stress granules. We identify the reversible amyloid-forming cores of hnRNPA1 (named hnRACs). The atomic structures of hnRACs reveal a distinct feature of stacking Asp residues, which contributes to fibril reversibility and explains the irreversible pathological fibril formation caused by the Asp mutations identified in familial ALS. Our work characterizes the structural diversity and heterogeneity of reversible amyloid fibrils and illuminates the biological function of reversible amyloid formation in protein phase separation.
DOI
Subcellular membrane-less organelles consist of proteins with low complexity domains. Many of them, such as hnRNPA1, can assemble into both a polydisperse liquid phase and an ordered solid phase of amyloid fibril. The former mirrors biological granule assembly, while the latter is usually associated with neurodegenerative disease. Here, we observe a reversible amyloid formation of hnRNPA1 that synchronizes with liquid–liquid phase separation, regulates the fluidity and mobility of the liquid-like droplets, and facilitates the recruitment of hnRNPA1 into stress granules. We identify the reversible amyloid-forming cores of hnRNPA1 (named hnRACs). The atomic structures of hnRACs reveal a distinct feature of stacking Asp residues, which contributes to fibril reversibility and explains the irreversible pathological fibril formation caused by the Asp mutations identified in familial ALS. Our work characterizes the structural diversity and heterogeneity of reversible amyloid fibrils and illuminates the biological function of reversible amyloid formation in protein phase separation.
DOI
Microfabrication of Concave Micromirror for Microbial Cell Trapping Using Köhler Illumination by XeF2 Vapor Etching
Akihiro Matsutani, Mina Sato, Koichi Hasebe, and Ayako Takada
We demonstrated a Si-based concave micromirror array for cell trapping that was fabricated by XeF2 vapor etching. We also examined the optical properties of the focal image of each concave micromirror. In addition, yeast cell trapping was realized at the focal point of a concave micromirror of approximately 35 μm diameter by Köhler illumination using a halogen lamp. The proposed process is useful for the microfabrication of various Si-based microstructure devices, such as microchannels and micro-electromechanical systems (MEMS).
DOI
We demonstrated a Si-based concave micromirror array for cell trapping that was fabricated by XeF2 vapor etching. We also examined the optical properties of the focal image of each concave micromirror. In addition, yeast cell trapping was realized at the focal point of a concave micromirror of approximately 35 μm diameter by Köhler illumination using a halogen lamp. The proposed process is useful for the microfabrication of various Si-based microstructure devices, such as microchannels and micro-electromechanical systems (MEMS).
DOI
Power-exponent helico-conical optical beams
Shubo Cheng, Tian Xia, Mengsi Liu, Yuanyuan Jin, Geng Zhang, Yan Xiong, Shaohua Tao
A new kind of beam, i.e., power-exponent helico-conical (PEHC) optical beams, has been proposed in this paper. The proposed beams possess a variety of interesting properties different from optical vortex beams. The intensities of the proposed optical beams at the focal plane are analyzed theoretically and experimentally. The far field mappings are also theoretically analyzed. The results demonstrate that the proposed beams have the ring-broken openings which can be adjusted by modifying the exponent n. The proposed beams can be useful for the extension applications of helico-conical optical beams, especially for optical trapping, guiding, and sorting.
DOI
A new kind of beam, i.e., power-exponent helico-conical (PEHC) optical beams, has been proposed in this paper. The proposed beams possess a variety of interesting properties different from optical vortex beams. The intensities of the proposed optical beams at the focal plane are analyzed theoretically and experimentally. The far field mappings are also theoretically analyzed. The results demonstrate that the proposed beams have the ring-broken openings which can be adjusted by modifying the exponent n. The proposed beams can be useful for the extension applications of helico-conical optical beams, especially for optical trapping, guiding, and sorting.
DOI
Data on the target search by a single protein on DNA measured with ultrafast force-clamp spectroscopy
Carina Monico, Alessia Tempestini, Lucia Gardini, Francesco Saverio Pavone, Marco Capitanio
The mechanism by which proteins are able to find small cognate sequences in the range from few to few tens of base pairs amongst the millions of non-specific chromosomal DNA has been puzzling researchers for decades. Single molecule techniques based on fluorescence have been successfully applied to investigate this process but are inherently limited in terms of spatial and temporal resolution. We previously showed that ultrafast force-clamp spectroscopy, a single molecule technique based on laser tweezers, can be applied to the study of protein-DNA interaction attaining sub-millisecond and few base-pair resolution. Here, we share experimental records of interactions between a single lactose repressor protein and DNA collected under different forces using our technique [1]. The data can be valuable for researchers interested in the study of protein-DNA interaction and the mechanism of DNA target search, both from an experimental and modeling point of view. The data is related to the research article “Sliding of a single lac repressor protein along DNA is tuned by DNA sequence and molecular switching” [2].
DOI
The mechanism by which proteins are able to find small cognate sequences in the range from few to few tens of base pairs amongst the millions of non-specific chromosomal DNA has been puzzling researchers for decades. Single molecule techniques based on fluorescence have been successfully applied to investigate this process but are inherently limited in terms of spatial and temporal resolution. We previously showed that ultrafast force-clamp spectroscopy, a single molecule technique based on laser tweezers, can be applied to the study of protein-DNA interaction attaining sub-millisecond and few base-pair resolution. Here, we share experimental records of interactions between a single lactose repressor protein and DNA collected under different forces using our technique [1]. The data can be valuable for researchers interested in the study of protein-DNA interaction and the mechanism of DNA target search, both from an experimental and modeling point of view. The data is related to the research article “Sliding of a single lac repressor protein along DNA is tuned by DNA sequence and molecular switching” [2].
DOI
Design of a Single Nanoparticle Trapping Device Based on Bow-Tie-Shaped Photonic Crystal Nanobeam Cavities
Yan Gao; Yaocheng Shi
Photonic crystal (PhC) cavities have been widely utilized for the optical trapping. However, it is still challenging to achieve high-efficiency optical trapping of ultrasmall nanoparticles. In this paper, we show optical trapping of a 3 nm size single nanoparticle by using an ultrahigh Q/V bow-tie-shaped PhC nanobeam cavity. For the trapping of a single polystyrene nanoparticle with the radius as small as 3 nm, a maximum trapping force of 1.2 × 10 5 pN/mW is theoretically obtained, which is at least one order of magnitude higher than the previous results. Furthermore, the calculated sensitivity for the cavity is around 350 nm/RIU, which provides a valid solution for monitoring the trapping process of the nanoparticles with ultrasmall size. We believe that such structure with characteristics of extreme light confinement and high trapping efficiency will be conducive to the development of multifunctional on-chip trapping devices.
Photonic crystal (PhC) cavities have been widely utilized for the optical trapping. However, it is still challenging to achieve high-efficiency optical trapping of ultrasmall nanoparticles. In this paper, we show optical trapping of a 3 nm size single nanoparticle by using an ultrahigh Q/V bow-tie-shaped PhC nanobeam cavity. For the trapping of a single polystyrene nanoparticle with the radius as small as 3 nm, a maximum trapping force of 1.2 × 10 5 pN/mW is theoretically obtained, which is at least one order of magnitude higher than the previous results. Furthermore, the calculated sensitivity for the cavity is around 350 nm/RIU, which provides a valid solution for monitoring the trapping process of the nanoparticles with ultrasmall size. We believe that such structure with characteristics of extreme light confinement and high trapping efficiency will be conducive to the development of multifunctional on-chip trapping devices.
Efficient Optical Trapping of Nano-Particle via Waveguide-Coupled Hybrid Plasmonic Nano-Taper
Yi-Chang Lin ; Po-Tsung Lee
For manipulating nano-bio-specimens, we propose a tweezing device by integrating a triangular-shaped photonic-plasmonic nano-taper on top of a coupling waveguide. The device can exert strong trapping force on nano-particle owing to efficient optical energy usage and accessible field distribution. Working principle and optical characteristics of the device are fully investigated and discussed. The optimized device shows an excellent trapping capability with low threshold power of 3.57 mW for stable trapping n 100 nm polystyrene particle. Simple geometry and high tolerance to pattern misalignment between trap unit and coupling waveguide are beneficial for realistic fabrication. Furthermore, the footprint of trap unit is only 400 nm × 625 nm. We believe this design will be very useful to the development of lab-on-a-chip system.
For manipulating nano-bio-specimens, we propose a tweezing device by integrating a triangular-shaped photonic-plasmonic nano-taper on top of a coupling waveguide. The device can exert strong trapping force on nano-particle owing to efficient optical energy usage and accessible field distribution. Working principle and optical characteristics of the device are fully investigated and discussed. The optimized device shows an excellent trapping capability with low threshold power of 3.57 mW for stable trapping n 100 nm polystyrene particle. Simple geometry and high tolerance to pattern misalignment between trap unit and coupling waveguide are beneficial for realistic fabrication. Furthermore, the footprint of trap unit is only 400 nm × 625 nm. We believe this design will be very useful to the development of lab-on-a-chip system.
Tuesday, June 25, 2019
Narrow-Line Cooling and Imaging of Ytterbium Atoms in an Optical Tweezer Array
S. Saskin, J. T. Wilson, B. Grinkemeyer, and J. D. Thompson
Engineering controllable, strongly interacting many-body quantum systems is at the frontier of quantum simulation and quantum information processing. Arrays of laser-cooled neutral atoms in optical tweezers have emerged as a promising platform because of their flexibility and the potential for strong interactions via Rydberg states. Existing neutral atom array experiments utilize alkali atoms, but alkaline-earth atoms offer many advantages in terms of coherence and control, and also open the door to new applications in precision measurement and time keeping. In this Letter, we present a technique to trap individual alkaline-earth-like ytterbium (Yb) atoms in optical tweezer arrays. The narrow 1S0−3P1 intercombination line is used for both cooling and imaging in a magic-wavelength optical tweezer at 532 nm. The low Doppler temperature allows for imaging near the saturation intensity, resulting in a very high atom detection fidelity. We demonstrate the imaging fidelity concretely by observing rare (<1 in 104 images) spontaneous quantum jumps into and out of a metastable state. We also demonstrate stochastic loading of atoms into a two-dimensional, 144-site tweezer array. This platform will enable advances in quantum information processing, quantum simulation, and precision measurement. The demonstrated narrow-line Doppler imaging may also be applied in tweezer arrays or quantum gas microscopes using other atoms with similar transitions, such as erbium and dysprosium.
DOI
Engineering controllable, strongly interacting many-body quantum systems is at the frontier of quantum simulation and quantum information processing. Arrays of laser-cooled neutral atoms in optical tweezers have emerged as a promising platform because of their flexibility and the potential for strong interactions via Rydberg states. Existing neutral atom array experiments utilize alkali atoms, but alkaline-earth atoms offer many advantages in terms of coherence and control, and also open the door to new applications in precision measurement and time keeping. In this Letter, we present a technique to trap individual alkaline-earth-like ytterbium (Yb) atoms in optical tweezer arrays. The narrow 1S0−3P1 intercombination line is used for both cooling and imaging in a magic-wavelength optical tweezer at 532 nm. The low Doppler temperature allows for imaging near the saturation intensity, resulting in a very high atom detection fidelity. We demonstrate the imaging fidelity concretely by observing rare (<1 in 104 images) spontaneous quantum jumps into and out of a metastable state. We also demonstrate stochastic loading of atoms into a two-dimensional, 144-site tweezer array. This platform will enable advances in quantum information processing, quantum simulation, and precision measurement. The demonstrated narrow-line Doppler imaging may also be applied in tweezer arrays or quantum gas microscopes using other atoms with similar transitions, such as erbium and dysprosium.
DOI
The Ribosome Cooperates with a Chaperone to Guide Multi-domain Protein Folding
Kaixian Liu, Kevin Maciuba, Christian M.Kaiser
Multi-domain proteins, containing several structural units within a single polypeptide, constitute a large fraction of all proteomes. Co-translational folding is assumed to simplify the conformational search problem for large proteins, but the events leading to correctly folded, functional structures remain poorly characterized. Similarly, how the ribosome and molecular chaperones promote efficient folding remains obscure. Using optical tweezers, we have dissected early folding events of nascent elongation factor G, a multi-domain protein that requires chaperones for folding. The ribosome and the chaperone trigger factor reduce inter-domain misfolding, permitting folding of the N-terminal G-domain. Successful completion of this step is a crucial prerequisite for folding of the next domain. Unexpectedly, co-translational folding does not proceed unidirectionally; emerging unfolded polypeptide can denature an already-folded domain. Trigger factor, but not the ribosome, protects against denaturation. The chaperone thus serves a previously unappreciated function, helping multi-domain proteins overcome inherent challenges during co-translational folding.
DOI
Multi-domain proteins, containing several structural units within a single polypeptide, constitute a large fraction of all proteomes. Co-translational folding is assumed to simplify the conformational search problem for large proteins, but the events leading to correctly folded, functional structures remain poorly characterized. Similarly, how the ribosome and molecular chaperones promote efficient folding remains obscure. Using optical tweezers, we have dissected early folding events of nascent elongation factor G, a multi-domain protein that requires chaperones for folding. The ribosome and the chaperone trigger factor reduce inter-domain misfolding, permitting folding of the N-terminal G-domain. Successful completion of this step is a crucial prerequisite for folding of the next domain. Unexpectedly, co-translational folding does not proceed unidirectionally; emerging unfolded polypeptide can denature an already-folded domain. Trigger factor, but not the ribosome, protects against denaturation. The chaperone thus serves a previously unappreciated function, helping multi-domain proteins overcome inherent challenges during co-translational folding.
DOI
Surface Tensions of Picoliter Droplets with Sub-Millisecond Surface Age
Rachael E. H. Miles, Michael W. J. Glerum, Hallie C. Boyer, Jim S. Walker, Cari S. Dutcher, Bryan R. Bzdek
Aerosols are key components of the atmosphere and play important roles in many industrial processes. Because aerosol particles have high surface-to-volume ratios, their surface properties are especially important. However, direct measurement of the surface properties of aerosol particles is challenging. In this work, we describe an approach to measure the surface tension of picoliter volume droplets with surface age <1 ms by resolving their dynamic oscillations in shape immediately after ejection from a microdroplet dispenser. Droplet shape oscillations are monitored by highly time-resolved (500 ns) stroboscopic imaging, and droplet surface tension is accurately retrieved across a wide range of droplet sizes (10–25 μm radius) and surface ages (down to ∼100 μs). The approach is validated for droplets containing sodium chloride, glutaric acid, and water, which all show no variation in surface tension with surface age. Experimental results from the microdroplet dispenser approach are compared to complementary surface tension measurements of 5–10 μm radius droplets with aged surfaces using a holographic optical tweezers approach and predictions of surface tension using a statistical thermodynamic model. These approaches combined will allow investigation of droplet surface tension across a wide range of droplet sizes, compositions, and surface ages.
DOI
Aerosols are key components of the atmosphere and play important roles in many industrial processes. Because aerosol particles have high surface-to-volume ratios, their surface properties are especially important. However, direct measurement of the surface properties of aerosol particles is challenging. In this work, we describe an approach to measure the surface tension of picoliter volume droplets with surface age <1 ms by resolving their dynamic oscillations in shape immediately after ejection from a microdroplet dispenser. Droplet shape oscillations are monitored by highly time-resolved (500 ns) stroboscopic imaging, and droplet surface tension is accurately retrieved across a wide range of droplet sizes (10–25 μm radius) and surface ages (down to ∼100 μs). The approach is validated for droplets containing sodium chloride, glutaric acid, and water, which all show no variation in surface tension with surface age. Experimental results from the microdroplet dispenser approach are compared to complementary surface tension measurements of 5–10 μm radius droplets with aged surfaces using a holographic optical tweezers approach and predictions of surface tension using a statistical thermodynamic model. These approaches combined will allow investigation of droplet surface tension across a wide range of droplet sizes, compositions, and surface ages.
DOI
Light‐Driven Self‐Healing of Nanoparticle‐Based Metamolecules
Dr. Fan Nan, Prof. Zijie Yan
Metamolecules and crystals consisting of nanoscale building blocks offer rich models to study colloidal chemistry, materials science, and photonics. Herein we demonstrate the self‐assembly of colloidal Ag nanoparticles into quasi‐one‐dimensional metamolecules with an intriguing self‐healing ability in a linearly polarized optical field. By investigating the spatial stability of the metamolecules, we found that the origin of self‐healing is the inhomogeneous interparticle electrodynamic interactions enhanced by the formation of unusual nanoparticle dimers, which minimize the free energy of the whole structure. The equilibrium configuration and self‐healing behavior can be further tuned by modifying the electrical double layers surrounding the nanoparticles. Our results reveal a unique route to build self‐healing colloidal structures assembled from simple metal nanoparticles. This approach could potentially lead to reconfigurable plasmonic devices for photonic and sensing applications.
DOI
Metamolecules and crystals consisting of nanoscale building blocks offer rich models to study colloidal chemistry, materials science, and photonics. Herein we demonstrate the self‐assembly of colloidal Ag nanoparticles into quasi‐one‐dimensional metamolecules with an intriguing self‐healing ability in a linearly polarized optical field. By investigating the spatial stability of the metamolecules, we found that the origin of self‐healing is the inhomogeneous interparticle electrodynamic interactions enhanced by the formation of unusual nanoparticle dimers, which minimize the free energy of the whole structure. The equilibrium configuration and self‐healing behavior can be further tuned by modifying the electrical double layers surrounding the nanoparticles. Our results reveal a unique route to build self‐healing colloidal structures assembled from simple metal nanoparticles. This approach could potentially lead to reconfigurable plasmonic devices for photonic and sensing applications.
DOI
Slow and steady wins the race: physical limits on the rate of viral DNA packaging
Paul J Jardine
During the assembly of dsDNA viruses such as the tailed bacteriophages and herpesviruses, the viral chromosome is compacted to near crystalline density inside a preformed head shell. DNA translocation is driven by powerful ring ATPase motors that couple ATP binding, hydrolysis, and release to force generation and movement. Studies of the motor of the bacteriophage phi29 have revealed a complex mechanochemistry behind this process that slows as the head fills. Recent studies of the physical behavior of packaging DNA suggest that surprisingly long-time scales of relaxation of DNA inside the head and jamming phenomena during packaging create the physical need for regulation of the rate of packaging. Studies of DNA packaging in viral systems have, therefore, revealed fundamental insight into the complex behavior of DNA and the need for biological systems to accommodate these physical constraints.
DOI
During the assembly of dsDNA viruses such as the tailed bacteriophages and herpesviruses, the viral chromosome is compacted to near crystalline density inside a preformed head shell. DNA translocation is driven by powerful ring ATPase motors that couple ATP binding, hydrolysis, and release to force generation and movement. Studies of the motor of the bacteriophage phi29 have revealed a complex mechanochemistry behind this process that slows as the head fills. Recent studies of the physical behavior of packaging DNA suggest that surprisingly long-time scales of relaxation of DNA inside the head and jamming phenomena during packaging create the physical need for regulation of the rate of packaging. Studies of DNA packaging in viral systems have, therefore, revealed fundamental insight into the complex behavior of DNA and the need for biological systems to accommodate these physical constraints.
DOI
Studying the rigidity of red blood cells induced by Plasmodium falciparum infection
Apurba Paul, Ghania Ramdani, Utpal Tatu, Gordon Langsley & Vasant Natarajan
We study the effect of different chemical moieties on the rigidity of red blood cells (RBCs) induced by Plasmodium falciparum infection, and the bystander effect previously found. The infected cells are obtained from a culture of parasite-infected RBCs grown in the laboratory. The rigidity of RBCs is measured by looking at the Brownian fluctuations of individual cells in an optical-tweezers trap. The results point towards increased intracellular cyclic adenosine monophosphate (cAMP) levels as being responsible for the increase in rigidity.
DOI
We study the effect of different chemical moieties on the rigidity of red blood cells (RBCs) induced by Plasmodium falciparum infection, and the bystander effect previously found. The infected cells are obtained from a culture of parasite-infected RBCs grown in the laboratory. The rigidity of RBCs is measured by looking at the Brownian fluctuations of individual cells in an optical-tweezers trap. The results point towards increased intracellular cyclic adenosine monophosphate (cAMP) levels as being responsible for the increase in rigidity.
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Digital video microscopy enhanced by deep learning
Saga Helgadottir, Aykut Argun, and Giovanni Volpe
Single particle tracking is essential in many branches of science and technology, from the measurement of biomolecular forces to the study of colloidal crystals. Standard methods rely on algorithmic approaches; by fine-tuning several user-defined parameters, these methods can be highly successful at tracking a well-defined kind of particle under low-noise conditions with constant and homogenous illumination. Here, we introduce an alternative data-driven approach based on a convolutional neural network, which we name DeepTrack. We show that DeepTrack outperforms algorithmic approaches, especially in the presence of noise and under poor illumination conditions. We use DeepTrack to track an optically trapped particle under very noisy and unsteady illumination conditions, where standard algorithmic approaches fail. We then demonstrate how DeepTrack can also be used to track multiple particles and non-spherical objects such as bacteria, also at very low signal-to-noise ratios. In order to make DeepTrack readily available for other users, we provide a Python software package, which can be easily personalized and optimized for specific applications.
DOI
Single particle tracking is essential in many branches of science and technology, from the measurement of biomolecular forces to the study of colloidal crystals. Standard methods rely on algorithmic approaches; by fine-tuning several user-defined parameters, these methods can be highly successful at tracking a well-defined kind of particle under low-noise conditions with constant and homogenous illumination. Here, we introduce an alternative data-driven approach based on a convolutional neural network, which we name DeepTrack. We show that DeepTrack outperforms algorithmic approaches, especially in the presence of noise and under poor illumination conditions. We use DeepTrack to track an optically trapped particle under very noisy and unsteady illumination conditions, where standard algorithmic approaches fail. We then demonstrate how DeepTrack can also be used to track multiple particles and non-spherical objects such as bacteria, also at very low signal-to-noise ratios. In order to make DeepTrack readily available for other users, we provide a Python software package, which can be easily personalized and optimized for specific applications.
DOI
Monday, June 24, 2019
Optical radiation force expression for a cylinder exhibiting rotary polarization in plane quasi-standing, standing, or progressive waves
F. G. Mitri
A generalized analytical expression for the radiation force of plane quasi-standing, standing, or progressive electromagnetic (EM) waves is derived for a circular cylinder exhibiting rotary polarization in a TM-polarized incident field. Such a material, allowing rotary polarization, produces cross-polarized waves in the scattered field, which contribute to the radiation force experienced by the cylindrical object as shown here. As an example of a material exhibiting rotary polarization, a perfect electromagnetic conductor (PEMC) nonabsorptive cylinder is chosen to illustrate the analysis. In contrast with perfect electrical conductors (PECs), perfect magnetic conductors (PMCs), or conventional dielectric materials, the radiation force on a PEMC cylinder shows a direct dependency on the expansion coefficients of the cross-polarized waves, which do not exist for PECs, PMCs, or standard dielectrics. Extra new terms contribute to the generalized radiation force series expansions for plane quasi-standing, standing, or progressive waves. Numerical predictions demonstrate the possibility of trapping a circular-shaped cylinder material with rotary polarization in-plane quasi-standing or standing waves. Furthermore, the scattering, extinction, and absorption energy efficiencies for the nonabsorptive PEMC cylinder are computed, which validate the radiation force results from the standpoint of the law of energy conservation applied to EM scattering. The exact analytical radiation force expression for a PEMC cylinder of any arbitrary radius α (i.e., much smaller, comparable, or much larger than the wavelength of the illuminating incident field) in quasi-standing, standing, or progressive waves is also applicable to chiral, plasma, topological insulator, liquid crystal tubular phantom, or any other material exhibiting rotary polarization.
DOI
A generalized analytical expression for the radiation force of plane quasi-standing, standing, or progressive electromagnetic (EM) waves is derived for a circular cylinder exhibiting rotary polarization in a TM-polarized incident field. Such a material, allowing rotary polarization, produces cross-polarized waves in the scattered field, which contribute to the radiation force experienced by the cylindrical object as shown here. As an example of a material exhibiting rotary polarization, a perfect electromagnetic conductor (PEMC) nonabsorptive cylinder is chosen to illustrate the analysis. In contrast with perfect electrical conductors (PECs), perfect magnetic conductors (PMCs), or conventional dielectric materials, the radiation force on a PEMC cylinder shows a direct dependency on the expansion coefficients of the cross-polarized waves, which do not exist for PECs, PMCs, or standard dielectrics. Extra new terms contribute to the generalized radiation force series expansions for plane quasi-standing, standing, or progressive waves. Numerical predictions demonstrate the possibility of trapping a circular-shaped cylinder material with rotary polarization in-plane quasi-standing or standing waves. Furthermore, the scattering, extinction, and absorption energy efficiencies for the nonabsorptive PEMC cylinder are computed, which validate the radiation force results from the standpoint of the law of energy conservation applied to EM scattering. The exact analytical radiation force expression for a PEMC cylinder of any arbitrary radius α (i.e., much smaller, comparable, or much larger than the wavelength of the illuminating incident field) in quasi-standing, standing, or progressive waves is also applicable to chiral, plasma, topological insulator, liquid crystal tubular phantom, or any other material exhibiting rotary polarization.
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The unorthodox chromosomal organisation of the dinoflagellates
I Ian Hu, Ross Waller
Dinoflagellates nuclei are unlike any other. They have: 1) highly inflated genome sizes, 2) practically lack histones and nucleosomal organization; 3) have permanently condensed liquid crystalline chromosomes throughout the life cycle, and; 4) contain a novel a major new nuclear DNA-binding protein. This protein, called Dinoflagellate/Viral NucleoProtein (DVNP) is small in size (10–20 kDa), highly positively charged (30–40 % R+K), and its gene is one of the most highly transcribed in dinoflagellate cells. It has no homology to histone proteins, has no homologues in either eukaryotes or prokaryotes, but is found in a number of marine large DNA viruses. To understand the role of DVNP in the dinoflagellate nuclei we have expressed and purified DVNP and are studying the properties of this novel protein and its interaction with DNA. We show that DVNP is a monomer in solution, but upon exposure to DNA it rapidly binds to and compacts DNA into complexes micrometers in size. Using single-molecule imaging and optical tweezers, DVNP is seen to compact DNA a rates of over 50 µm/sec and change the mechanical properties of DNA. Most interestingly, the DVNP/DNA aggregates show a propensity to travel along the DNA strand en masse. Together, these observations suggest that DVNP plays a central role in the novel model for chromatin management found in dinoflagellates.
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Dinoflagellates nuclei are unlike any other. They have: 1) highly inflated genome sizes, 2) practically lack histones and nucleosomal organization; 3) have permanently condensed liquid crystalline chromosomes throughout the life cycle, and; 4) contain a novel a major new nuclear DNA-binding protein. This protein, called Dinoflagellate/Viral NucleoProtein (DVNP) is small in size (10–20 kDa), highly positively charged (30–40 % R+K), and its gene is one of the most highly transcribed in dinoflagellate cells. It has no homology to histone proteins, has no homologues in either eukaryotes or prokaryotes, but is found in a number of marine large DNA viruses. To understand the role of DVNP in the dinoflagellate nuclei we have expressed and purified DVNP and are studying the properties of this novel protein and its interaction with DNA. We show that DVNP is a monomer in solution, but upon exposure to DNA it rapidly binds to and compacts DNA into complexes micrometers in size. Using single-molecule imaging and optical tweezers, DVNP is seen to compact DNA a rates of over 50 µm/sec and change the mechanical properties of DNA. Most interestingly, the DVNP/DNA aggregates show a propensity to travel along the DNA strand en masse. Together, these observations suggest that DVNP plays a central role in the novel model for chromatin management found in dinoflagellates.
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Modification of microparticles due to intense laser manipulation
Frank Wieben, Jan Schablinski, and Dietmar Block
Single micron-sized melamine-formaldehyde particles are levitated in the sheath of an rf-plasma and exposed to an intense laser beam, while being trapped in optical tweezers. A reversible change in the particles' properties is observed and quantitatively analyzed using reference particles. The investigations indicate a gain in particle charge where the initial charge restores within minutes. Possible reasons for these findings are discussed.
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Single micron-sized melamine-formaldehyde particles are levitated in the sheath of an rf-plasma and exposed to an intense laser beam, while being trapped in optical tweezers. A reversible change in the particles' properties is observed and quantitatively analyzed using reference particles. The investigations indicate a gain in particle charge where the initial charge restores within minutes. Possible reasons for these findings are discussed.
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Propagation characteristics and optical forces exerted upon a Rayleigh dielectric sphere for a controllable dark-hollow beam
Zhirong Liu, Xun Wang, Kelin Huang
Propagation characteristics and optical forces exerted upon a Rayleigh dielectric sphere for a novel controllable dark-hollow beam (CDHB) are analyzed theoretically and illustrated numerically. In view of the unique focusing characteristics that a sharp, peak-centered, and adjustable configuration would be produced in the focal region, a tightly focused CDHB could be exploited to trap and manipulate particles with high-refractive index in the focal vicinity. Furthermore, it is significant that the CDHB tweezers’ trapping efficiency and optical trap stiffness could be finely controlled by adjusting either the targeted beam’s order or the central dark size controlling parameter. Finally, the trapping stability conditions are analyzed. The results obtained here are of interest and importance in optical manipulations and investigations for a CDHB.
DOI
Propagation characteristics and optical forces exerted upon a Rayleigh dielectric sphere for a novel controllable dark-hollow beam (CDHB) are analyzed theoretically and illustrated numerically. In view of the unique focusing characteristics that a sharp, peak-centered, and adjustable configuration would be produced in the focal region, a tightly focused CDHB could be exploited to trap and manipulate particles with high-refractive index in the focal vicinity. Furthermore, it is significant that the CDHB tweezers’ trapping efficiency and optical trap stiffness could be finely controlled by adjusting either the targeted beam’s order or the central dark size controlling parameter. Finally, the trapping stability conditions are analyzed. The results obtained here are of interest and importance in optical manipulations and investigations for a CDHB.
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Graphene-tuned optical manipulation on microparticle by Bessel beam
Xiaoran Hou, Dongliang Gao, and Lei Gao
We study the optical force on the graphene-coated low-index microparticle by the first-order Bessel beam lighting. We theoretically demonstrate that the optical scattering pulling force is realized near the Fano resonance due to the interference between electric dipole mode and quadrupole one. Moreover, the optical force can be further enhanced and flexibly tuned by controlling the conductivity of the graphene. In order to transport the particle over a long distance, the stability for optical trapping at transverse plane is also analyzed. Our study might offer a new thought to trap and transport dielectric or plasmonic particles, as well as provide potential applications in optical manipulation and optoelectronic devices.
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We study the optical force on the graphene-coated low-index microparticle by the first-order Bessel beam lighting. We theoretically demonstrate that the optical scattering pulling force is realized near the Fano resonance due to the interference between electric dipole mode and quadrupole one. Moreover, the optical force can be further enhanced and flexibly tuned by controlling the conductivity of the graphene. In order to transport the particle over a long distance, the stability for optical trapping at transverse plane is also analyzed. Our study might offer a new thought to trap and transport dielectric or plasmonic particles, as well as provide potential applications in optical manipulation and optoelectronic devices.
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A Disease-Causing Intronic Point Mutation C19G Alters Tau Exon 10 Splicing via RNA Secondary Structure Rearrangement
Jiazi Tan, Lixia Yang, Alan Ann Lerk Ong, Jiahao Shi, Zhensheng Zhong, Mun Leng Lye, Shiyi Liu, Jolanta Lisowiec-Wachnicka, Ryszard Kierzek, Xavier Roca, Gang Chen
Alternative splicing of MAPT cassette exon 10 produces tau isoforms with four microtubule-binding repeat domains (4R) upon exon inclusion or three repeats (3R) upon exon skipping. In human neurons, deviations from the ∼1:1 physiological 4R:3R ratio lead to frontotemporal dementia with Parkinsonism linked to chromosome 17 (FTDP-17). Certain FTDP-17-associated mutations affect a regulatory hairpin that sequesters the exon 10 5′ splice site (5′ss, located at the exon 10–intron 10 junction). These mutations tend to increase the 4R:3R ratio by destabilizing the hairpin, thereby improving 5′ss recognition by U1 snRNP. Interestingly, a single C-to-G mutation at the 19th nucleotide in intron 10 (C19G or +19G) decreases the level of exon 10 inclusion significantly from 56% to 1%, despite the disruption of a G-C base pair in the bottom stem of the hairpin. Here, we show by biophysical characterization, including thermal melting, fluorescence, and single-molecule mechanical unfolding using optical tweezers, that the +19G mutation alters the structure of the bottom stem, resulting in the formation of a new bottom stem with enhanced stability. The cell culture alternative splicing patterns of a series of minigenes reveal that the splicing activities of the mutants with destabilizing mutations on the top stem can be compensated in a position-dependent manner by the +19G mutation in the bottom stem. We observed an excellent correlation between the level of exon 10 inclusion and the rate of mechanical unfolding at 10 pN, indicating that the unfolding of the splice site hairpins (to facilitate subsequent binding of U1 snRNA) may be aided by helicases or other proteins.
DOI
Alternative splicing of MAPT cassette exon 10 produces tau isoforms with four microtubule-binding repeat domains (4R) upon exon inclusion or three repeats (3R) upon exon skipping. In human neurons, deviations from the ∼1:1 physiological 4R:3R ratio lead to frontotemporal dementia with Parkinsonism linked to chromosome 17 (FTDP-17). Certain FTDP-17-associated mutations affect a regulatory hairpin that sequesters the exon 10 5′ splice site (5′ss, located at the exon 10–intron 10 junction). These mutations tend to increase the 4R:3R ratio by destabilizing the hairpin, thereby improving 5′ss recognition by U1 snRNP. Interestingly, a single C-to-G mutation at the 19th nucleotide in intron 10 (C19G or +19G) decreases the level of exon 10 inclusion significantly from 56% to 1%, despite the disruption of a G-C base pair in the bottom stem of the hairpin. Here, we show by biophysical characterization, including thermal melting, fluorescence, and single-molecule mechanical unfolding using optical tweezers, that the +19G mutation alters the structure of the bottom stem, resulting in the formation of a new bottom stem with enhanced stability. The cell culture alternative splicing patterns of a series of minigenes reveal that the splicing activities of the mutants with destabilizing mutations on the top stem can be compensated in a position-dependent manner by the +19G mutation in the bottom stem. We observed an excellent correlation between the level of exon 10 inclusion and the rate of mechanical unfolding at 10 pN, indicating that the unfolding of the splice site hairpins (to facilitate subsequent binding of U1 snRNA) may be aided by helicases or other proteins.
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Optical trapping of sub-wavelength objects with point-like slot waveguides
Mufei Xiao
An optical tweezer capable of picking up dielectric as well as metallic objects of sizes much smaller than the light wavelength is proposed based on point-like two-dimensionally slotted waveguides. Slot waveguides transport light based on evanescent modes, therefore, with sub-wavelength resolution. A point-like slot waveguide confines light within a sub-wavelength cross-section, and a strong gradient electric field is formed. A radiation force is induced by the strong gradient field, and the force is strong enough for optically trapping and manipulating small objects of different sizes and refractive indice with sub-wavelength resolution. As an example, the proposed device is numerically simulated with a rectangular slot waveguide so as to confirm its feasibility. Since the assumed device profile and materials are common for chip formation, the proposed device has the potential to become a lab-on-a-chip opto-electric tool for biologic analysis.
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An optical tweezer capable of picking up dielectric as well as metallic objects of sizes much smaller than the light wavelength is proposed based on point-like two-dimensionally slotted waveguides. Slot waveguides transport light based on evanescent modes, therefore, with sub-wavelength resolution. A point-like slot waveguide confines light within a sub-wavelength cross-section, and a strong gradient electric field is formed. A radiation force is induced by the strong gradient field, and the force is strong enough for optically trapping and manipulating small objects of different sizes and refractive indice with sub-wavelength resolution. As an example, the proposed device is numerically simulated with a rectangular slot waveguide so as to confirm its feasibility. Since the assumed device profile and materials are common for chip formation, the proposed device has the potential to become a lab-on-a-chip opto-electric tool for biologic analysis.
DOI
Friday, June 21, 2019
Replicative DNA polymerases promote active displacement of SSB proteins during lagging strand synthesis
Fernando Cerrón Sara de Lorenzo Kateryna M Lemishko Grzegorz L Ciesielski Laurie S Kaguni Francisco J Cao Borja Ibarra
Genome replication induces the generation of large stretches of single-stranded DNA (ssDNA) intermediates that are rapidly protected by single-stranded DNA-binding (SSB) proteins. To date, the mechanism by which tightly bound SSBs are removed from ssDNA by the lagging strand DNA polymerase without compromising the advance of the replication fork remains unresolved. Here, we aimed to address this question by measuring, with optical tweezers, the real-time replication kinetics of the human mitochondrial and bacteriophage T7 DNA polymerases on free-ssDNA, in comparison with ssDNA covered with homologous and non-homologous SSBs under mechanical tension. We find important differences between the force dependencies of the instantaneous replication rates of each polymerase on different substrates. Modeling of the data supports a mechanism in which strong, specific polymerase-SSB interactions, up to ∼12 kBT, are required for the polymerase to dislodge SSB from the template without compromising its instantaneous replication rate, even under stress conditions that may affect SSB–DNA organization and/or polymerase–SSB communication. Upon interaction, the elimination of template secondary structure by SSB binding facilitates the maximum replication rate of the lagging strand polymerase. In contrast, in the absence of polymerase–SSB interactions, SSB poses an effective barrier for the advance of the polymerase, slowing down DNA synthesis.
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Genome replication induces the generation of large stretches of single-stranded DNA (ssDNA) intermediates that are rapidly protected by single-stranded DNA-binding (SSB) proteins. To date, the mechanism by which tightly bound SSBs are removed from ssDNA by the lagging strand DNA polymerase without compromising the advance of the replication fork remains unresolved. Here, we aimed to address this question by measuring, with optical tweezers, the real-time replication kinetics of the human mitochondrial and bacteriophage T7 DNA polymerases on free-ssDNA, in comparison with ssDNA covered with homologous and non-homologous SSBs under mechanical tension. We find important differences between the force dependencies of the instantaneous replication rates of each polymerase on different substrates. Modeling of the data supports a mechanism in which strong, specific polymerase-SSB interactions, up to ∼12 kBT, are required for the polymerase to dislodge SSB from the template without compromising its instantaneous replication rate, even under stress conditions that may affect SSB–DNA organization and/or polymerase–SSB communication. Upon interaction, the elimination of template secondary structure by SSB binding facilitates the maximum replication rate of the lagging strand polymerase. In contrast, in the absence of polymerase–SSB interactions, SSB poses an effective barrier for the advance of the polymerase, slowing down DNA synthesis.
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Microtubule end conversion mediated by motors and diffusing proteins with no intrinsic microtubule end-binding activity
Manas Chakraborty, Ekaterina V. Tarasovetc, Anatoly V. Zaytsev, Maxim Godzi, Ana C. Figueiredo, Fazly I. Ataullakhanov & Ekaterina L. Grishchuk
Accurate chromosome segregation relies on microtubule end conversion, the ill-understood ability of kinetochores to transit from lateral microtubule attachment to durable association with dynamic microtubule plus-ends. The molecular requirements for this conversion and the underlying biophysical mechanisms are elusive. We reconstituted end conversion in vitro using two kinetochore components: the plus end–directed kinesin CENP-E and microtubule-binding Ndc80 complex, combined on the surface of a microbead. The primary role of CENP-E is to ensure close proximity between Ndc80 complexes and the microtubule plus-end, whereas Ndc80 complexes provide lasting microtubule association by diffusing on the microtubule wall near its tip. Together, these proteins mediate robust plus-end coupling during several rounds of microtubule dynamics, in the absence of any specialized tip-binding or regulatory proteins. Using a Brownian dynamics model, we show that end conversion is an emergent property of multimolecular ensembles of microtubule wall-binding proteins with finely tuned force-dependent motility characteristics.
DOI
Accurate chromosome segregation relies on microtubule end conversion, the ill-understood ability of kinetochores to transit from lateral microtubule attachment to durable association with dynamic microtubule plus-ends. The molecular requirements for this conversion and the underlying biophysical mechanisms are elusive. We reconstituted end conversion in vitro using two kinetochore components: the plus end–directed kinesin CENP-E and microtubule-binding Ndc80 complex, combined on the surface of a microbead. The primary role of CENP-E is to ensure close proximity between Ndc80 complexes and the microtubule plus-end, whereas Ndc80 complexes provide lasting microtubule association by diffusing on the microtubule wall near its tip. Together, these proteins mediate robust plus-end coupling during several rounds of microtubule dynamics, in the absence of any specialized tip-binding or regulatory proteins. Using a Brownian dynamics model, we show that end conversion is an emergent property of multimolecular ensembles of microtubule wall-binding proteins with finely tuned force-dependent motility characteristics.
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Microsphere enhanced optical imaging and patterning: From physics to applications featured
Lianwei Chen, Yan Zhou, Yang Li, and Minghui Hong
The diffraction limit is a fundamental barrier in optical science and engineering. It limits the minimum feature size in surface patterning technologies, such as lithography and laser direct writing. It also restricts the resolution for optical imaging, which includes different kinds of microscopes. Microspheres have been demonstrated as a powerful platform to challenge the diffraction limit. Microspheres can manipulate the light in a novel way that conventional optical components cannot achieve. In this review, we summarize the fundamental physical mechanisms and the related applications of microspheres in two primary research directions: first, to focus light energy on the sample surface, which leads to nano-patterning and achieves a sub-100 nm feature size and second, to manipulate light reflected back from the sample surface, which forms the foundation of super-resolution optical imaging to observe nano-structures. We also analyze key features, development, limitation, and opportunities of the nano-patterning and nano-imaging systems based on the microsphere.
DOI
The diffraction limit is a fundamental barrier in optical science and engineering. It limits the minimum feature size in surface patterning technologies, such as lithography and laser direct writing. It also restricts the resolution for optical imaging, which includes different kinds of microscopes. Microspheres have been demonstrated as a powerful platform to challenge the diffraction limit. Microspheres can manipulate the light in a novel way that conventional optical components cannot achieve. In this review, we summarize the fundamental physical mechanisms and the related applications of microspheres in two primary research directions: first, to focus light energy on the sample surface, which leads to nano-patterning and achieves a sub-100 nm feature size and second, to manipulate light reflected back from the sample surface, which forms the foundation of super-resolution optical imaging to observe nano-structures. We also analyze key features, development, limitation, and opportunities of the nano-patterning and nano-imaging systems based on the microsphere.
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Optimal reconfiguration with collision avoidance for a granular spacecraft using laser pressure
Kunpeng Zhang, Yao Zhang
Granular spacecraft, as a new type of distributed spacecraft system, “are complex multibody systems composed of a spatially disordered distribution of a large number of elements” (Quadrelli et al., 2013), designed and controlled to perform certain desired functions. To achieve the collision-free reconfiguration of granular spacecraft, an optimal reconfiguration algorithm with collision avoidance for a granular spacecraft using laser pressure is presented in this paper. The three-dimensional model of granular spacecraft using laser pressure is established first, and the reconfiguration problem statement is presented. The optimal reconfiguration plan is designed based on optimal transport, and the collision-free trajectories for all the particles are generated based on Voronoi partitioning, with the motion constraints under the control of laser pressure. Finally, the numerical simulations are provided to demonstrate the effectiveness of the proposed reconfiguration algorithm and the feasibility of using laser actuators for the reconfiguration of a granular spacecraft.
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Cascaded Plasmon-Enhanced Emission from a Single Upconverting Nanocrystal
Amirhossein Alizadehkhaledi, Adriaan L. Frencken, Mohsen Kamandar Dezfouli, Stephen Hughes, Frank C. J. M. van Veggel, Reuven Gordon
Plasmonics has been used to enhance light–matter interaction at the extreme subwavelength scale. Intriguingly, it is possible to achieve multiple plasmonic resonances from a single nanostructure, and these can be used in combination to provide cascaded enhanced interactions. Here, we demonstrate three distinct plasmon resonances for enhanced upconversion emission from a single upconverting nanocrystal trapped in a metal nanoaperture optical tweezer. For apertures where the plasmonic resonances occur at the emission wavelengths only, a moderate enhancement of a factor of 4 is seen. However, by tuning the aperture to enhance the excitation laser as well, an additional factor of 100 enhancement in the emission is achieved. Since lanthanide-doped nanocrystals are stable emitters, this approach of using multiple subwavelength resonances can improve applications including photovoltaics, photocatalysis, and imaging. The nanocrystals can also contain only single ions, allowing for studying quantum emitter properties and applications to single-photon sources.
DOI
Plasmonics has been used to enhance light–matter interaction at the extreme subwavelength scale. Intriguingly, it is possible to achieve multiple plasmonic resonances from a single nanostructure, and these can be used in combination to provide cascaded enhanced interactions. Here, we demonstrate three distinct plasmon resonances for enhanced upconversion emission from a single upconverting nanocrystal trapped in a metal nanoaperture optical tweezer. For apertures where the plasmonic resonances occur at the emission wavelengths only, a moderate enhancement of a factor of 4 is seen. However, by tuning the aperture to enhance the excitation laser as well, an additional factor of 100 enhancement in the emission is achieved. Since lanthanide-doped nanocrystals are stable emitters, this approach of using multiple subwavelength resonances can improve applications including photovoltaics, photocatalysis, and imaging. The nanocrystals can also contain only single ions, allowing for studying quantum emitter properties and applications to single-photon sources.
DOI
Wednesday, June 19, 2019
Harvesting Mechanical Work From Folding-Based Protein Engines: From Single-Molecule Mechanochemical Cycles to Macroscopic Devices
Linglan Fu, Han Wang, and Hongbin Li
Mechanochemical coupling cycles underlie the work-generation mechanisms of biological systems and are realized by highly regulated conformational changes of the protein machineries. However, it has been challenging to utilize protein conformational changes to do mechanical work at the macroscopic level in biomaterials, and it remains elusive to construct macroscopic mechanochemical devices based on molecular-level mechanochemical coupling systems. Here, the authors demonstrate that protein folding can be utilized to realize protein’s mechanochemical cycles at both single-molecule and macroscopic levels. Our results demonstrate, for the first time, the successful harnessing of mechanical work generated by protein folding in a macroscopic protein hydrogel device, and the work generated by protein folding compares favorably with the energy output of molecular motors. Our work bridges a gap between single-molecule and macroscopic levels, and paves the way to utilizing proteins as building blocks to design protein-based artificial muscles and soft actuators.
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Mechanochemical coupling cycles underlie the work-generation mechanisms of biological systems and are realized by highly regulated conformational changes of the protein machineries. However, it has been challenging to utilize protein conformational changes to do mechanical work at the macroscopic level in biomaterials, and it remains elusive to construct macroscopic mechanochemical devices based on molecular-level mechanochemical coupling systems. Here, the authors demonstrate that protein folding can be utilized to realize protein’s mechanochemical cycles at both single-molecule and macroscopic levels. Our results demonstrate, for the first time, the successful harnessing of mechanical work generated by protein folding in a macroscopic protein hydrogel device, and the work generated by protein folding compares favorably with the energy output of molecular motors. Our work bridges a gap between single-molecule and macroscopic levels, and paves the way to utilizing proteins as building blocks to design protein-based artificial muscles and soft actuators.
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Optical Enantioseparation of Racemic Emulsions of Chiral Microparticles
Nina Kravets, Artur Aleksanyan, Hamza Chraïbi, Jacques Leng, and Etienne Brasselet
We report on the experimental demonstration of chirality-selective mechanical separation of randomly distributed assemblies of right-handed and left-handed chiral microparticles by optical means. Chiral-resolution experiments are made using two-dimensional emulsions of chiral-liquid-crystal droplets under the action of circularly polarized laser beams and do not require information on the initial location of the particles. Also, we numerically identify that the cooperative effects of hydrodynamic interactions mediated by the viscous fluid surrounding the particles can speed up the enantioseparation process.
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We report on the experimental demonstration of chirality-selective mechanical separation of randomly distributed assemblies of right-handed and left-handed chiral microparticles by optical means. Chiral-resolution experiments are made using two-dimensional emulsions of chiral-liquid-crystal droplets under the action of circularly polarized laser beams and do not require information on the initial location of the particles. Also, we numerically identify that the cooperative effects of hydrodynamic interactions mediated by the viscous fluid surrounding the particles can speed up the enantioseparation process.
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Stochastic Control for Orientation and Transportation of Microscopic Objects Using Multiple Optically Driven Robotic Fingertips
Quang Minh Ta; Chien Chern Cheah
The effect of Brownian motion on maneuvering of micro-objects in a fluid medium is one of the fundamental differences between micro-manipulation and robotic manipulators in the physical world. Besides, due to the limitation of feasible sensors and actuators in micro-manipulation, current control techniques for manipulation of micro-objects or cells are mostly dependent on the physical properties of target micro-objects or cells. In this paper, we propose the first stochastic control technique to achieve simultaneous orientation and transportation of micro-objects with Brownian perturbations. Several micro-particles which are optically trapped and driven by laser beams are utilized as fingertips to first grasp a target micro-object. Cooperative control of robot-assisted stage and the fingertips is then performed to achieve the control objective, in which the target micro-object is transported toward a desired position by using the robot-assisted stage, and at the same time, it is oriented toward a desired angular position by using the fingertips. This paper provides a stochastic control framework for simultaneous orientation and transportation of micro-objects with arbitrary types in the micro-world, and thus bringing micro-manipulation using optical tweezers closer to robotic manipulation in the physical world. Rigorous mathematical formulation and stability analysis for simultaneous orientation and positioning feedback control of micro-objects in the presence of the stochastic perturbations are derived, and experimental results are also presented.
DOI
The effect of Brownian motion on maneuvering of micro-objects in a fluid medium is one of the fundamental differences between micro-manipulation and robotic manipulators in the physical world. Besides, due to the limitation of feasible sensors and actuators in micro-manipulation, current control techniques for manipulation of micro-objects or cells are mostly dependent on the physical properties of target micro-objects or cells. In this paper, we propose the first stochastic control technique to achieve simultaneous orientation and transportation of micro-objects with Brownian perturbations. Several micro-particles which are optically trapped and driven by laser beams are utilized as fingertips to first grasp a target micro-object. Cooperative control of robot-assisted stage and the fingertips is then performed to achieve the control objective, in which the target micro-object is transported toward a desired position by using the robot-assisted stage, and at the same time, it is oriented toward a desired angular position by using the fingertips. This paper provides a stochastic control framework for simultaneous orientation and transportation of micro-objects with arbitrary types in the micro-world, and thus bringing micro-manipulation using optical tweezers closer to robotic manipulation in the physical world. Rigorous mathematical formulation and stability analysis for simultaneous orientation and positioning feedback control of micro-objects in the presence of the stochastic perturbations are derived, and experimental results are also presented.
DOI
Harnessing optical nonlinearity to control reversal of trapping force under pulsed excitation: a theoretical investigation
Anita Devi and Arijit K De
The dramatic influence of optical Kerr effect on the nature of trapping force/potential under pulsed excitation has recently been explored, particularly in the context of trapping of dielectric nanoparticles (Devi and De 2016 Opt. Express 24 21485–96, Devi and De 2017 Phys. Rev. A 96 023856). However, the utility of such effect has yet to be fully understood, which we discuss here. For a variety of nanoparticles (core, core/shell, and hollow-core), we theoretically show how optical force/potential depend on the nature of the material under pulsed excitation and, most importantly, how the force/potential reverses from repulsive to attractive for certain hollow-core nanoparticles made of high nonlinear refractive index material.
DOI
The dramatic influence of optical Kerr effect on the nature of trapping force/potential under pulsed excitation has recently been explored, particularly in the context of trapping of dielectric nanoparticles (Devi and De 2016 Opt. Express 24 21485–96, Devi and De 2017 Phys. Rev. A 96 023856). However, the utility of such effect has yet to be fully understood, which we discuss here. For a variety of nanoparticles (core, core/shell, and hollow-core), we theoretically show how optical force/potential depend on the nature of the material under pulsed excitation and, most importantly, how the force/potential reverses from repulsive to attractive for certain hollow-core nanoparticles made of high nonlinear refractive index material.
DOI
Intracavity optical trapping of microscopic particles in a ring-cavity fiber laser
Fatemeh Kalantarifard, Parviz Elahi, Ghaith Makey, Onofrio M. Maragò, F. Ömer Ilday & Giovanni Volpe
Standard optical tweezers rely on optical forces arising when a focused laser beam interacts with a microscopic particle: scattering forces, pushing the particle along the beam direction, and gradient forces, attracting it towards the high-intensity focal spot. Importantly, the incoming laser beam is not affected by the particle position because the particle is outside the laser cavity. Here, we demonstrate that intracavity nonlinear feedback forces emerge when the particle is placed inside the optical cavity, resulting in orders-of-magnitude higher confinement along the three axes per unit laser intensity on the sample. This scheme allows trapping at very low numerical apertures and reduces the laser intensity to which the particle is exposed by two orders of magnitude compared to a standard 3D optical tweezers. These results are highly relevant for many applications requiring manipulation of samples that are subject to photodamage, such as in biophysics and nanosciences.
DOI
Standard optical tweezers rely on optical forces arising when a focused laser beam interacts with a microscopic particle: scattering forces, pushing the particle along the beam direction, and gradient forces, attracting it towards the high-intensity focal spot. Importantly, the incoming laser beam is not affected by the particle position because the particle is outside the laser cavity. Here, we demonstrate that intracavity nonlinear feedback forces emerge when the particle is placed inside the optical cavity, resulting in orders-of-magnitude higher confinement along the three axes per unit laser intensity on the sample. This scheme allows trapping at very low numerical apertures and reduces the laser intensity to which the particle is exposed by two orders of magnitude compared to a standard 3D optical tweezers. These results are highly relevant for many applications requiring manipulation of samples that are subject to photodamage, such as in biophysics and nanosciences.
DOI
Is the Zero Reynolds Number Approximation Valid for Ciliary Flows?
Da Wei, Parviz Ghoddoosi Dehnavi, Marie-Eve Aubin-Tam, and Daniel Tam
Stokes equations are commonly used to model the hydrodynamic flow around cilia on the micron scale. The validity of the zero Reynolds number approximation is investigated experimentally with a flow velocimetry approach based on optical tweezers, which allows the measurement of periodic flows with high spatial and temporal resolution. We find that beating cilia generate a flow, which fundamentally differs from the stokeslet field predicted by Stokes equations. In particular, the flow velocity spatially decays at a faster rate and is gradually phase delayed at increasing distances from the cilia. This indicates that the quasisteady approximation and use of Stokes equations for unsteady ciliary flow are not always justified and the finite timescale for vorticity diffusion cannot be neglected. Our results have significant implications in studies of synchronization and collective dynamics of microswimmers.
DOI
Stokes equations are commonly used to model the hydrodynamic flow around cilia on the micron scale. The validity of the zero Reynolds number approximation is investigated experimentally with a flow velocimetry approach based on optical tweezers, which allows the measurement of periodic flows with high spatial and temporal resolution. We find that beating cilia generate a flow, which fundamentally differs from the stokeslet field predicted by Stokes equations. In particular, the flow velocity spatially decays at a faster rate and is gradually phase delayed at increasing distances from the cilia. This indicates that the quasisteady approximation and use of Stokes equations for unsteady ciliary flow are not always justified and the finite timescale for vorticity diffusion cannot be neglected. Our results have significant implications in studies of synchronization and collective dynamics of microswimmers.
DOI
Efficient Optical Trapping and Detection of Nanoparticle Via Plasmonic Bowtie Notch
Yi-Chang Lin; Po-Tsung Lee
For manipulating nanoparticles, we propose a nanotweezer named plasmonic bowtie notch (PBN) by adding a thin metal film to the bottom of plasmonic bowtie aperture. The PBN can exert a large trapping force on nanoparticle because the enhanced field of resonance mode can be much accessed by nanoparticle. The optical properties and factors influencing trapping force of PBN are fully investigated and discussed. The optimized PBN shows an excellent trapping capability with ultralow threshold excitation intensity of 0.64 mW/μm 2 for stable trapping of a 100 nm polystyrene particle. In addition, it provides a label-free detection of the trapped particle by observing the extinction spectrum. The sensitivity to trapped target size of 71 pm redshift in peak wavelength per 1 nm increase of particle size is relatively high compared with those obtained from most near-field tweezers. Furthermore, the footprint of PBN is only 200 nm × 220 nm. The capabilities of the proposed design show a great potential in the application of nanoparticle trapping and sensing.
DOI
For manipulating nanoparticles, we propose a nanotweezer named plasmonic bowtie notch (PBN) by adding a thin metal film to the bottom of plasmonic bowtie aperture. The PBN can exert a large trapping force on nanoparticle because the enhanced field of resonance mode can be much accessed by nanoparticle. The optical properties and factors influencing trapping force of PBN are fully investigated and discussed. The optimized PBN shows an excellent trapping capability with ultralow threshold excitation intensity of 0.64 mW/μm 2 for stable trapping of a 100 nm polystyrene particle. In addition, it provides a label-free detection of the trapped particle by observing the extinction spectrum. The sensitivity to trapped target size of 71 pm redshift in peak wavelength per 1 nm increase of particle size is relatively high compared with those obtained from most near-field tweezers. Furthermore, the footprint of PBN is only 200 nm × 220 nm. The capabilities of the proposed design show a great potential in the application of nanoparticle trapping and sensing.
DOI
Monday, June 17, 2019
Extreme mechanical diversity of human telomeric DNA revealed by fluorescence-force spectroscopy
Jaba Mitra, Monika A. Makurath, Thuy T. M. Ngo, Alice Troitskaia, Yann R. Chemla, and Taekjip Ha
G-quadruplexes (GQs) can adopt diverse structures and are functionally implicated in transcription, replication, translation, and maintenance of telomere. Their conformational diversity under physiological levels of mechanical stress, however, is poorly understood. We used single-molecule fluorescence-force spectroscopy that combines fluorescence resonance energy transfer with optical tweezers to measure human telomeric sequences under tension. Abrupt GQ unfolding with K+ in solution occurred at as many as four discrete levels of force. Added to an ultrastable state and a gradually unfolding state, there were six mechanically distinct structures. Extreme mechanical diversity was also observed with Na+, although GQs were mechanically weaker. Our ability to detect small conformational changes at low forces enabled the determination of refolding forces of about 2 pN. Refolding was rapid and stochastically redistributed molecules to mechanically distinct states. A single guanine-to-thymine substitution mutant required much higher ion concentrations to display GQ-like unfolding and refolded via intermediates, contrary to the wild type. Contradicting an earlier proposal, truncation to three hexanucleotide repeats resulted in a single-stranded DNA-like mechanical behavior under all conditions, indicating that at least four repeats are required to form mechanically stable structures.
DOI
G-quadruplexes (GQs) can adopt diverse structures and are functionally implicated in transcription, replication, translation, and maintenance of telomere. Their conformational diversity under physiological levels of mechanical stress, however, is poorly understood. We used single-molecule fluorescence-force spectroscopy that combines fluorescence resonance energy transfer with optical tweezers to measure human telomeric sequences under tension. Abrupt GQ unfolding with K+ in solution occurred at as many as four discrete levels of force. Added to an ultrastable state and a gradually unfolding state, there were six mechanically distinct structures. Extreme mechanical diversity was also observed with Na+, although GQs were mechanically weaker. Our ability to detect small conformational changes at low forces enabled the determination of refolding forces of about 2 pN. Refolding was rapid and stochastically redistributed molecules to mechanically distinct states. A single guanine-to-thymine substitution mutant required much higher ion concentrations to display GQ-like unfolding and refolded via intermediates, contrary to the wild type. Contradicting an earlier proposal, truncation to three hexanucleotide repeats resulted in a single-stranded DNA-like mechanical behavior under all conditions, indicating that at least four repeats are required to form mechanically stable structures.
DOI
Quo vadis, plasmonic optical tweezers?
Kenneth B. Crozier
Conventional optical tweezers based on traditional optical microscopes are subject to the diffraction limit, making the precise trapping and manipulation of very small particles challenging. Plasmonic optical tweezers can surpass this constraint, but many potential applications would benefit from further enhanced performance and/or expanded functionalities. In this Perspective, we discuss trends in plasmonic tweezers and describe important opportunities presented by its interdisciplinary combination with other techniques in nanoscience. We furthermore highlight several open questions concerning fundamentals that are likely to be important for many potential applications.
DOI
Conventional optical tweezers based on traditional optical microscopes are subject to the diffraction limit, making the precise trapping and manipulation of very small particles challenging. Plasmonic optical tweezers can surpass this constraint, but many potential applications would benefit from further enhanced performance and/or expanded functionalities. In this Perspective, we discuss trends in plasmonic tweezers and describe important opportunities presented by its interdisciplinary combination with other techniques in nanoscience. We furthermore highlight several open questions concerning fundamentals that are likely to be important for many potential applications.
DOI
Electromagnetic radiation force on a perfect electromagnetic conductor (PEMC) circular cylinder
F.G. Mitri
Unlike isotropic dielectric objects, the interaction of incident electromagnetic (EM) or optical waves with a material allowing rotary polarization produces coupled polarized internal and scattered fields. The aim of this investigation is to examine theoretically a novel physical effect, which concerns the contributions of the co-polarized and cross-polarized fields to the radiation force (per-length) experienced by an infinitely long perfect electromagnetic conductor (PEMC) cylinder having a circular cross-section and illuminated by TM-polarized plane progressive waves propagating perpendicularly to its axis. The multipole partial-wave series expansion method in cylindrical coordinates is used to derive exact series expansions for the co-polarized and cross-polarized components of the longitudinal radiation force per-length (i.e. acting along the direction of wave propagation). In contrast with the perfect electric or magnetic conductors (PECs or PMCs), or the dielectric cylinder case, numerical illustrative results for the radiation force function (which is the radiation force per unit energy density and cross-sectional surface) clearly demonstrate the contribution of the cross-polarized component of the radiation force for a PEMC cylinder allowing rotary polarization. The results show that the cross-polarized component of the radiation force function can be positive or negative as the dimensionless frequency parameter ka varies (where k is the wavenumber in the medium of wave propagation and a is the radius of the cylinder). Moreover, it vanishes for ka = 0.67946 regardless of the admittance of the PEMC cylinder. Notice that the total force (i.e. the sum of the co-polarized and cross-polarized components) is always repulsive (i.e., positive). It is also verified that the results are in complete agreement with the law of energy conservation applied to scattering. The present analysis generalizes the classical radiation force investigations by introducing extra new terms in the series expansion for the longitudinal radiation force function for cylindrical materials allowing rotary polarization.
DOI
Unlike isotropic dielectric objects, the interaction of incident electromagnetic (EM) or optical waves with a material allowing rotary polarization produces coupled polarized internal and scattered fields. The aim of this investigation is to examine theoretically a novel physical effect, which concerns the contributions of the co-polarized and cross-polarized fields to the radiation force (per-length) experienced by an infinitely long perfect electromagnetic conductor (PEMC) cylinder having a circular cross-section and illuminated by TM-polarized plane progressive waves propagating perpendicularly to its axis. The multipole partial-wave series expansion method in cylindrical coordinates is used to derive exact series expansions for the co-polarized and cross-polarized components of the longitudinal radiation force per-length (i.e. acting along the direction of wave propagation). In contrast with the perfect electric or magnetic conductors (PECs or PMCs), or the dielectric cylinder case, numerical illustrative results for the radiation force function (which is the radiation force per unit energy density and cross-sectional surface) clearly demonstrate the contribution of the cross-polarized component of the radiation force for a PEMC cylinder allowing rotary polarization. The results show that the cross-polarized component of the radiation force function can be positive or negative as the dimensionless frequency parameter ka varies (where k is the wavenumber in the medium of wave propagation and a is the radius of the cylinder). Moreover, it vanishes for ka = 0.67946 regardless of the admittance of the PEMC cylinder. Notice that the total force (i.e. the sum of the co-polarized and cross-polarized components) is always repulsive (i.e., positive). It is also verified that the results are in complete agreement with the law of energy conservation applied to scattering. The present analysis generalizes the classical radiation force investigations by introducing extra new terms in the series expansion for the longitudinal radiation force function for cylindrical materials allowing rotary polarization.
DOI
Multimaterial Manufacture Through Combining Optical Tweezers with Multiphoton Fabrication
M. Askari, C. J. Tuck, Q. Hu, R. J. M. Hague and R. D. Wildman
Multi-Photon Polymerization (MPP) is a technique used to fabricate complex micro-scale 3D structures using ultra-short laser pulses. Typically, MPP is used to manufacture micron-scale components in photopolymer materials. However, the development of micron scale processes that can produce components from multiple materials within a single manufacturing step would be advantageous. This would allow the inclusion of particles that are manipulated and embedded within structures with sub-micron feature sizes. To achieve this, an MPP system was combined with an optical trapping (OT) setup in order to independently manipulate microparticles in the x, y and z planes. Particles were transported into the fabrication site using the OT and encapsulated using the MPP laser. Here it is shown that combining the OT capabilities with an additive manufacturing technique enables the production of complex multi-material artifacts.
DOI
Multi-Photon Polymerization (MPP) is a technique used to fabricate complex micro-scale 3D structures using ultra-short laser pulses. Typically, MPP is used to manufacture micron-scale components in photopolymer materials. However, the development of micron scale processes that can produce components from multiple materials within a single manufacturing step would be advantageous. This would allow the inclusion of particles that are manipulated and embedded within structures with sub-micron feature sizes. To achieve this, an MPP system was combined with an optical trapping (OT) setup in order to independently manipulate microparticles in the x, y and z planes. Particles were transported into the fabrication site using the OT and encapsulated using the MPP laser. Here it is shown that combining the OT capabilities with an additive manufacturing technique enables the production of complex multi-material artifacts.
DOI
Using Single-Molecule Chemo-Mechanical Unfolding to Simultaneously Probe Multiple Structural Parameters in Protein Folding
Emily J. Guinn, and Susan Marqusee
While single-molecule force spectroscopy has greatly advanced the study of protein folding, there are limitations to what can be learned from studying the effect of force alone. We developed a novel technique, chemo-mechanical unfolding, that combines multiple perturbants—force and chemical denaturant—to more fully characterize the folding process by simultaneously probing multiple structural parameters—the change in end-to-end distance, and solvent accessible surface area. Here, we describe the theoretical background, experimental design, and data analysis for chemo-mechanical unfolding experiments probing protein folding thermodynamics and kinetics. This technique has been applied to characterize parallel protein folding pathways, the protein denatured state, protein folding on the ribosome, and protein folding intermediates.
DOI
While single-molecule force spectroscopy has greatly advanced the study of protein folding, there are limitations to what can be learned from studying the effect of force alone. We developed a novel technique, chemo-mechanical unfolding, that combines multiple perturbants—force and chemical denaturant—to more fully characterize the folding process by simultaneously probing multiple structural parameters—the change in end-to-end distance, and solvent accessible surface area. Here, we describe the theoretical background, experimental design, and data analysis for chemo-mechanical unfolding experiments probing protein folding thermodynamics and kinetics. This technique has been applied to characterize parallel protein folding pathways, the protein denatured state, protein folding on the ribosome, and protein folding intermediates.
DOI
Position detection using differential signals of the coupling light in a tapered-lensed dual-beam optical fiber trap
Wei Xiong, Guangzong Xiao, Ying Zhang, Xiang Han, Xinlin Chen and Hui Luo
A technique for position detection in a tapered-lensed dual-beam optical fiber trap based on differential measurements is proposed. The difference in the coupling power of two tapered fibers is used to sense the position of the trapped particle. The detecting resolution can reach 45 nm. The fiber coupling theory is used to analyze the coupling efficiency of the fibers. Results show that tapered fibers are more beneficial than cleaved fibers in the differential measurements system. The influences of the radius of the taper end and the fiber separations are discussed in detail.
DOI
A technique for position detection in a tapered-lensed dual-beam optical fiber trap based on differential measurements is proposed. The difference in the coupling power of two tapered fibers is used to sense the position of the trapped particle. The detecting resolution can reach 45 nm. The fiber coupling theory is used to analyze the coupling efficiency of the fibers. Results show that tapered fibers are more beneficial than cleaved fibers in the differential measurements system. The influences of the radius of the taper end and the fiber separations are discussed in detail.
DOI
Tuning Nanoparticle Electrodynamics by an Optical-Matter-Based Laser Beam Shaper
Fan Nan, Zijie Yan
Spatially modulated optical fields provide the perspective of tuning nanoparticle (NP) dynamics in a colloidal suspension. Here, it is shown that the lateral interferometric optical field created by a chain of optically bound Au NPs (i.e., optical matter) can tailor the electrodynamic interactions among more Au NPs. The free-standing NP chain, which is assembled and confined by an auxiliary optical line, shapes the main trapping beam and guides the self-organization of Au NPs under an optimized polarization direction. We find that the NP chain can largely enhance the anisotropic optical binding interaction of two nearby NPs but suppress the anisotropic interaction of multiple NPs, leading to isotropic self-organization. The dynamics and structural transitions of the NPs are well-reproduced in a simulation by using a coupled finite-difference time-domain (FDTD)-Langevin dynamics approach. Our work provides a new dual-beam optical trapping and in situ laser beam shaping approach to study and control interparticle electrodynamic interactions among colloidal NPs.
DOI
Spatially modulated optical fields provide the perspective of tuning nanoparticle (NP) dynamics in a colloidal suspension. Here, it is shown that the lateral interferometric optical field created by a chain of optically bound Au NPs (i.e., optical matter) can tailor the electrodynamic interactions among more Au NPs. The free-standing NP chain, which is assembled and confined by an auxiliary optical line, shapes the main trapping beam and guides the self-organization of Au NPs under an optimized polarization direction. We find that the NP chain can largely enhance the anisotropic optical binding interaction of two nearby NPs but suppress the anisotropic interaction of multiple NPs, leading to isotropic self-organization. The dynamics and structural transitions of the NPs are well-reproduced in a simulation by using a coupled finite-difference time-domain (FDTD)-Langevin dynamics approach. Our work provides a new dual-beam optical trapping and in situ laser beam shaping approach to study and control interparticle electrodynamic interactions among colloidal NPs.
DOI
Friday, June 14, 2019
Optical gradient force for tuning, actuation, and manipulation of nonlinearity in graphene nanomechanical resonator
Aneesh Dash, Chandan Samanta, Praveen Ranganath, S K Selvaraja and A K Naik
Graphene nano-mechanical resonators integrated over waveguides provide a powerful sensing platform based on the interaction of graphene with the evanescent wave. An integrated actuation scheme that does not compromise this interaction is required for optimal usage of the ultra-sensitive platform. Conventional electrical and optical actuation techniques are not favorable towards efficient utilization of the near-field interaction. We propose tuning and actuation of these resonators using on-chip optical gradient force due to the guided wave as an alternative to these conventional techniques. We have used the fundamental quasi-TM optical mode in a silicon waveguide in a finite-element model. We obtain a force–distribution that is spatially correlated with the fundamental mechanical mode of the graphene nano-mechanical resonator. We demonstrate that for an evanescent continuous-wave (CW) optical power of 8 μW, the resonant frequency of the device can be tuned by about 24.5%. With an intensity-modulated optical power ≤0.1 μW, the mechanical mode can be driven to nonlinearity. We also demonstrate cancellation of the Duffing nonlinearity at a CW power of 5.4 μW, which can be used to improve the linear dynamic range of vibration. The distributed optical gradient force can produce linear resonant amplitudes that are 50% higher than those obtained using conventional actuation schemes. This actuation scheme is robust against fluctuations in the evanescent optical power and in the refractive index of the side-cladding of the waveguide. This ensures minimal cross-talk from the optical mode to the mechanical mode in nano-mechanical sensing applications.
DOI
Graphene nano-mechanical resonators integrated over waveguides provide a powerful sensing platform based on the interaction of graphene with the evanescent wave. An integrated actuation scheme that does not compromise this interaction is required for optimal usage of the ultra-sensitive platform. Conventional electrical and optical actuation techniques are not favorable towards efficient utilization of the near-field interaction. We propose tuning and actuation of these resonators using on-chip optical gradient force due to the guided wave as an alternative to these conventional techniques. We have used the fundamental quasi-TM optical mode in a silicon waveguide in a finite-element model. We obtain a force–distribution that is spatially correlated with the fundamental mechanical mode of the graphene nano-mechanical resonator. We demonstrate that for an evanescent continuous-wave (CW) optical power of 8 μW, the resonant frequency of the device can be tuned by about 24.5%. With an intensity-modulated optical power ≤0.1 μW, the mechanical mode can be driven to nonlinearity. We also demonstrate cancellation of the Duffing nonlinearity at a CW power of 5.4 μW, which can be used to improve the linear dynamic range of vibration. The distributed optical gradient force can produce linear resonant amplitudes that are 50% higher than those obtained using conventional actuation schemes. This actuation scheme is robust against fluctuations in the evanescent optical power and in the refractive index of the side-cladding of the waveguide. This ensures minimal cross-talk from the optical mode to the mechanical mode in nano-mechanical sensing applications.
DOI
Single-Crystal Rutile TiO2 Nanocylinders are Highly Effective Transducers of Optical Force and Torque
Seungkyu Ha, Ying Tang, Maarten M. van Oene, Richard Janissen, Roland M. Dries, Belen Solano, Aurèle J. L. Adam, Nynke H. Dekker
Optical trapping of (sub)micron-sized particles is broadly employed in nanoscience and engineering. The materials commonly employed for these particles, however, have physical properties that limit the transfer of linear or angular momentum (or both). This reduces the magnitude of forces and torques, and the spatiotemporal resolution, achievable in linear and angular traps. Here, we overcome these limitations through the use of single-crystal rutile TiO2, which has an exceptionally large optical birefringence, a high index of refraction, good chemical stability, and is amenable to geometric control at the nanoscale. We show that rutile TiO2 nanocylinders form powerful joint force and torque transducers in aqueous environments by using only moderate laser powers to apply nN·nm torques at kHz rotational frequencies to tightly trapped particles. In doing so, we demonstrate how rutile TiO2 nanocylinders outperform other materials and offer unprecedented opportunities to expand the control of optical force and torque at the nanoscale.
Optical trapping of (sub)micron-sized particles is broadly employed in nanoscience and engineering. The materials commonly employed for these particles, however, have physical properties that limit the transfer of linear or angular momentum (or both). This reduces the magnitude of forces and torques, and the spatiotemporal resolution, achievable in linear and angular traps. Here, we overcome these limitations through the use of single-crystal rutile TiO2, which has an exceptionally large optical birefringence, a high index of refraction, good chemical stability, and is amenable to geometric control at the nanoscale. We show that rutile TiO2 nanocylinders form powerful joint force and torque transducers in aqueous environments by using only moderate laser powers to apply nN·nm torques at kHz rotational frequencies to tightly trapped particles. In doing so, we demonstrate how rutile TiO2 nanocylinders outperform other materials and offer unprecedented opportunities to expand the control of optical force and torque at the nanoscale.
Optically Accessible MEMS Resonant Mass Sensor for Biological Applications
Ethan G. Keeler ; Chen Zou ; Lih Y. Lin
Resonant measurement of mass has emerged as a powerful tool for cellular characterization in biological and medical research. For application in clinical diagnostics and development, in pursuit of large volumes of sample data, microfluidics become an essential conveyor for serial measurement. The nature of channel fabrication within a resonant structure often prohibits optical characterization and manipulation methods within its inner volume due to the opacity of constituent materials. This perpetuates a lost opportunity for simultaneous investigation with important optical techniques, including laser trapping, fluorescent microscopy, flow cytometry, and many other critical approaches. In an attempt to unify these technologies, we seek to maintain the optical availability of samples as they undergo resonant mass measurement. To the best of the authors’ knowledge, the resulting device is the first optically-clear fluidic-enabled resonant structure that is scalable for large cellular study. As such, this paper describes the integrated sensor and its supporting system, accompanied by important specifications and metrics.
DOI
Resonant measurement of mass has emerged as a powerful tool for cellular characterization in biological and medical research. For application in clinical diagnostics and development, in pursuit of large volumes of sample data, microfluidics become an essential conveyor for serial measurement. The nature of channel fabrication within a resonant structure often prohibits optical characterization and manipulation methods within its inner volume due to the opacity of constituent materials. This perpetuates a lost opportunity for simultaneous investigation with important optical techniques, including laser trapping, fluorescent microscopy, flow cytometry, and many other critical approaches. In an attempt to unify these technologies, we seek to maintain the optical availability of samples as they undergo resonant mass measurement. To the best of the authors’ knowledge, the resulting device is the first optically-clear fluidic-enabled resonant structure that is scalable for large cellular study. As such, this paper describes the integrated sensor and its supporting system, accompanied by important specifications and metrics.
DOI
Thermally robust spin correlations between two 85Rb atoms in an optical microtrap
Pimonpan Sompet, Stuart S. Szigeti, Eyal Schwartz, Ashton S. Bradley & Mikkel F. Andersen
The complex collisional properties of atoms fundamentally limit investigations into a range of processes in many-atom ensembles. In contrast, the bottom-up assembly of few- and many-body systems from individual atoms offers a controlled approach to isolating and studying such collisional processes. Here, we use optical tweezers to individually assemble pairs of trapped 85Rb atoms, and study the spin dynamics of the two-body system in a thermal state. The spin-2 atoms show strong pair correlation between magnetic sublevels on timescales exceeding one second, with measured relative number fluctuations 11.9 ± 0.3 dB below quantum shot noise, limited only by detection efficiency. Spin populations display relaxation dynamics consistent with simulations and theoretical predictions for 85Rb spin interactions, and contrary to the coherent spin waves witnessed in finite-temperature many-body experiments and zero-temperature two-body experiments. Our experimental approach offers a versatile platform for studying two-body quantum dynamics and may provide a route to thermally robust entanglement generation.
DOI
The complex collisional properties of atoms fundamentally limit investigations into a range of processes in many-atom ensembles. In contrast, the bottom-up assembly of few- and many-body systems from individual atoms offers a controlled approach to isolating and studying such collisional processes. Here, we use optical tweezers to individually assemble pairs of trapped 85Rb atoms, and study the spin dynamics of the two-body system in a thermal state. The spin-2 atoms show strong pair correlation between magnetic sublevels on timescales exceeding one second, with measured relative number fluctuations 11.9 ± 0.3 dB below quantum shot noise, limited only by detection efficiency. Spin populations display relaxation dynamics consistent with simulations and theoretical predictions for 85Rb spin interactions, and contrary to the coherent spin waves witnessed in finite-temperature many-body experiments and zero-temperature two-body experiments. Our experimental approach offers a versatile platform for studying two-body quantum dynamics and may provide a route to thermally robust entanglement generation.
DOI
Optical traps and anti-traps for glass nanoplates in hollow waveguides
M. C. Günendi, S. Xie, D. Novoa, and P. StJ. Russell
We study theoretically the optical forces acting on glass nanoplates introduced into hollow waveguides, and show that, depending on the sign of the laser detuning relative to the nanoplate resonance, optomechanical back-action between nanoplate and hollow waveguide can create both traps and anti-traps at intensity nodes and anti-nodes in the supermode field profile, behaving similarly to those experienced by cold atoms when the laser frequency is red or blue detuned of an atomic resonance. This arises from dramatic distortions to the mode profile in the hollow waveguide when the nanoplate is off-resonant, producing gradient forces that vary strongly with nanoplate position. In a planar system, we show that when the nanoplate is constrained by an imaginary mechanical spring, its position exhibits strong bistability as the base position is varied. We then treat a two-dimensional system consisting of an anti-resonant nanoplate in the hollow core of a photonic crystal fiber, and predict the stable dark trapping of nanoplate at core center against both translational and rotational motion. The results show that spatial and angular position of nano-scale objects in hollow waveguides can be optically controlled by launching beams with appropriately synthesized transverse field profiles.
DOI
We study theoretically the optical forces acting on glass nanoplates introduced into hollow waveguides, and show that, depending on the sign of the laser detuning relative to the nanoplate resonance, optomechanical back-action between nanoplate and hollow waveguide can create both traps and anti-traps at intensity nodes and anti-nodes in the supermode field profile, behaving similarly to those experienced by cold atoms when the laser frequency is red or blue detuned of an atomic resonance. This arises from dramatic distortions to the mode profile in the hollow waveguide when the nanoplate is off-resonant, producing gradient forces that vary strongly with nanoplate position. In a planar system, we show that when the nanoplate is constrained by an imaginary mechanical spring, its position exhibits strong bistability as the base position is varied. We then treat a two-dimensional system consisting of an anti-resonant nanoplate in the hollow core of a photonic crystal fiber, and predict the stable dark trapping of nanoplate at core center against both translational and rotational motion. The results show that spatial and angular position of nano-scale objects in hollow waveguides can be optically controlled by launching beams with appropriately synthesized transverse field profiles.
DOI
Evaluation of Luminescence Properties of Single Hydrophilic Upconversion Nanoparticles by Optical Trapping
Ya-Feng Kang, Bei Zheng, Chong-Yang Song, Cheng-Yu Li, Zhi-Liang Chen, Qiong-Shui Wu, Yi Yang, Dai-Wen Pang, Hong-Wu Tang
By using multifunctional optical tweezers (OT) equipped with a 980 nm continuous-wave single-mode diode laser and multiple detectors, we are able to trap single upconversion nanoparticles stably as well as synchronously analyze their luminescence properties. Successive trapping of individual octylamine-modified poly(acrylic acid) encapsulated UCNPs (OPA-UCNPs) is proved by real-time monitoring of forward scattering (FSC), luminescence intensity and spectra, and the results verify that these nanoparticles possess excellent colloidal stability and uniform luminescence properties. Besides, ligand/solvent-dependent surface quenching effect of single UCNPs is investigated, and the results show that OPA-UCNPs by hydrophobic encapsulation strategy possess outstanding luminescence properties and the property of OPA to reduce the quenching effect of ligands and water molecules are well identified. Upconversion luminescence decay lifetime also explains the mechanism that the presence of OPA molecules reduces surface quenching effect, thus OPA-UCNPs exhibit longer luminescence decay lifetime. Therefore, we not only provide a new method to evaluate the luminescence properties of single nanoparticles by using multifunctional OT but also prove that encapsulating hydrophobic UCNPs with amphiphilic molecules is an alternative strategy to prepare monodisperse hydrophilic UCNPs while significantly maintaining their luminescence properties.
DOI
By using multifunctional optical tweezers (OT) equipped with a 980 nm continuous-wave single-mode diode laser and multiple detectors, we are able to trap single upconversion nanoparticles stably as well as synchronously analyze their luminescence properties. Successive trapping of individual octylamine-modified poly(acrylic acid) encapsulated UCNPs (OPA-UCNPs) is proved by real-time monitoring of forward scattering (FSC), luminescence intensity and spectra, and the results verify that these nanoparticles possess excellent colloidal stability and uniform luminescence properties. Besides, ligand/solvent-dependent surface quenching effect of single UCNPs is investigated, and the results show that OPA-UCNPs by hydrophobic encapsulation strategy possess outstanding luminescence properties and the property of OPA to reduce the quenching effect of ligands and water molecules are well identified. Upconversion luminescence decay lifetime also explains the mechanism that the presence of OPA molecules reduces surface quenching effect, thus OPA-UCNPs exhibit longer luminescence decay lifetime. Therefore, we not only provide a new method to evaluate the luminescence properties of single nanoparticles by using multifunctional OT but also prove that encapsulating hydrophobic UCNPs with amphiphilic molecules is an alternative strategy to prepare monodisperse hydrophilic UCNPs while significantly maintaining their luminescence properties.
DOI
Photonic tractor beams: a review
Weiqiang Ding; Tongtong Zhu; Lei-Ming Zhou; Cheng-Wei Qiu
Usually, an unfocused light beam, such as a paraxial Gaussian beam, can exert a force on an object along the direction of light propagation, which is known as light pressure. Recently, however, it was found that an unfocused light beam can also exert an optical pulling force (OPF) on an object toward the source direction; the beam is accordingly named an optical tractor beam. In recent years, this intriguing force has attracted much attention and a huge amount of progress has been made both in theory and experiment. We briefly review recent progress achieved on this topic. We classify the mechanisms to achieve an OPF into four different kinds according to the dominant factors. The first one is tailoring the incident beam. The second one is engineering the object’s optical parameters. The third one is designing the structured material background, in which the light–matter interaction occurs, and the fourth one is utilizing the indirect photophoretic force, which is related to the thermal effect of light absorption. For all the methods, we analyze the basic principles and review the recent achievements. Finally, we also give a brief conclusion and an outlook on the future development of this field.
DOI
Usually, an unfocused light beam, such as a paraxial Gaussian beam, can exert a force on an object along the direction of light propagation, which is known as light pressure. Recently, however, it was found that an unfocused light beam can also exert an optical pulling force (OPF) on an object toward the source direction; the beam is accordingly named an optical tractor beam. In recent years, this intriguing force has attracted much attention and a huge amount of progress has been made both in theory and experiment. We briefly review recent progress achieved on this topic. We classify the mechanisms to achieve an OPF into four different kinds according to the dominant factors. The first one is tailoring the incident beam. The second one is engineering the object’s optical parameters. The third one is designing the structured material background, in which the light–matter interaction occurs, and the fourth one is utilizing the indirect photophoretic force, which is related to the thermal effect of light absorption. For all the methods, we analyze the basic principles and review the recent achievements. Finally, we also give a brief conclusion and an outlook on the future development of this field.
DOI
Dynamics of angular momentum-torque conversion in silicon waveguides
Wenjia Li, Jianlong Liu, Yang Gao, Keya Zhou, and Shutian Liu
We present a refined theoretical analysis on the relationship between the optical total angular momenta (TAM) and the optical torque (OT) in a birefringent silicon waveguide. By using the vector angular spectrum method, we demonstrate the dynamic evolutions of the OT, TAM, spin angular momentum (SAM), and orbital angular momentum (OAM). The SAM and OAM coexist and evolve simultaneously in the propagation. The ratio between the OAM and TAM is related to the incident wavelength and the size of waveguide. Moreover, we design a three-layer waveguide structure to convert the light chirality and generate high torque. The performance of such torque-generator is analyzed numerically in detail.
DOI
We present a refined theoretical analysis on the relationship between the optical total angular momenta (TAM) and the optical torque (OT) in a birefringent silicon waveguide. By using the vector angular spectrum method, we demonstrate the dynamic evolutions of the OT, TAM, spin angular momentum (SAM), and orbital angular momentum (OAM). The SAM and OAM coexist and evolve simultaneously in the propagation. The ratio between the OAM and TAM is related to the incident wavelength and the size of waveguide. Moreover, we design a three-layer waveguide structure to convert the light chirality and generate high torque. The performance of such torque-generator is analyzed numerically in detail.
DOI
Thursday, June 13, 2019
Optical Conveyor Belts for Chiral Discrimination: Influence of De-Phasing Parameter
Luis Carretero, Pablo Acebal and Salvador Blaya
A numerical analysis is carried out of the influence of the de-phasing parameter of an optical conveyor belt in the enantiomeric separation. The optical conveyor belt is obtained by the interference of a Laguerre Gaussian and a Gaussian beam with different beam waists, which are temporally de-phased. In order to obtain the maximum separation distance between enantiomers, we calculate the optimum range of values of the de-phasing parameter.
DOI
A numerical analysis is carried out of the influence of the de-phasing parameter of an optical conveyor belt in the enantiomeric separation. The optical conveyor belt is obtained by the interference of a Laguerre Gaussian and a Gaussian beam with different beam waists, which are temporally de-phased. In order to obtain the maximum separation distance between enantiomers, we calculate the optimum range of values of the de-phasing parameter.
DOI
Re‐evaluation of physical interaction between plant peroxisomes and other organelles using live‐cell imaging techniques
Kazusato Oikawa, Makoto Hayashi, Yasuko Hayashi, Mikio Nishimura
The dynamic behavior of organelles is essential for plant survival under various environmental conditions. Plant organelles, with various functions, migrate along actin filaments and contact other types of organelles, leading to physical interactions at a specific site called the membrane contact site. Recent studies have revealed the importance of physical interactions in maintaining efficient metabolite flow between organelles. In this review, we first summarize peroxisome function under different environmental conditions and growth stages to understand organelle interactions. We then discuss current knowledge regarding the interactions between peroxisome and other organelles, i.e., the oil bodies, chloroplast, and mitochondria from the perspective of metabolic and physiological regulation, with reference to various organelle interactions and techniques for estimating organelle interactions occurring in plant cells.
DOI
The dynamic behavior of organelles is essential for plant survival under various environmental conditions. Plant organelles, with various functions, migrate along actin filaments and contact other types of organelles, leading to physical interactions at a specific site called the membrane contact site. Recent studies have revealed the importance of physical interactions in maintaining efficient metabolite flow between organelles. In this review, we first summarize peroxisome function under different environmental conditions and growth stages to understand organelle interactions. We then discuss current knowledge regarding the interactions between peroxisome and other organelles, i.e., the oil bodies, chloroplast, and mitochondria from the perspective of metabolic and physiological regulation, with reference to various organelle interactions and techniques for estimating organelle interactions occurring in plant cells.
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Interactions, topology and photonic properties of liquid crystal colloids and dispersions
Igor Muševič
Liquid crystal-based colloids demonstrate new type of colloidal interaction between the particles, which is of elastic origin, has very long range and is extremely strong, with pair-binding elastic potential exceeding several thousands of kBT. This gives rise to novel and strong colloidal assembly mechanisms, which can be used to assemble 2D and 3D colloidal crystals. This mini-review discusses colloidal pair interaction mechanisms and topological aspects of colloidal interaction. Topological defects, which are responsible for elastic forces between colloidal particles show an amazing diversity of different structures, including colloidal entanglement, knotting and linking of particles. An introduction is given to dispersions of liquid crystal droplets in an immiscible fluid, which represent a new class of photonic micro-devices, including micro-lasers, micro-fibers and optical micro-cavities made of tuneable liquid crystals.
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Liquid crystal-based colloids demonstrate new type of colloidal interaction between the particles, which is of elastic origin, has very long range and is extremely strong, with pair-binding elastic potential exceeding several thousands of kBT. This gives rise to novel and strong colloidal assembly mechanisms, which can be used to assemble 2D and 3D colloidal crystals. This mini-review discusses colloidal pair interaction mechanisms and topological aspects of colloidal interaction. Topological defects, which are responsible for elastic forces between colloidal particles show an amazing diversity of different structures, including colloidal entanglement, knotting and linking of particles. An introduction is given to dispersions of liquid crystal droplets in an immiscible fluid, which represent a new class of photonic micro-devices, including micro-lasers, micro-fibers and optical micro-cavities made of tuneable liquid crystals.
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Comparison of Approaches for Measuring and Predicting the Viscosity of Ternary Component Aerosol Particles
Grazia Rovelli, Young-Chul Song, Adrian M. Maclean, David O. Topping, Allan K. Bertram, Jonathan P. Reid
Measurements of the water activity-dependent viscosity of aerosol particles from two techniques are compared, specifically from the coalescence of two droplets in holographic optical tweezers (HOT) and poke-and-flow experiments on particles deposited onto a glass substrate. These new data are also compared with the fitting of dimer coagulation, isolation, and coalescence (DCIC) measurements. The aerosol system considered in this work are ternary mixtures of sucrose-citric acid-water and sucrose-NaNO3-water, at varying solute mass ratios. Results from HOT and poke-and-flow are in excellent agreement over their overlapping range of applicability (∼103–107 Pa s); fitted curves from DCIC data show variable agreement with the other two techniques because of the sensitivity of the applied modeling framework to the representation of water content in the particles. Further, two modeling approaches for the predictions of the water activity-dependent viscosity of these ternary systems are evaluated. We show that it is possible to represent their viscosity with relatively simple mixing rules applied to the subcooled viscosity values of each component or to the viscosity of the corresponding binary mixtures.
DOI
Measurements of the water activity-dependent viscosity of aerosol particles from two techniques are compared, specifically from the coalescence of two droplets in holographic optical tweezers (HOT) and poke-and-flow experiments on particles deposited onto a glass substrate. These new data are also compared with the fitting of dimer coagulation, isolation, and coalescence (DCIC) measurements. The aerosol system considered in this work are ternary mixtures of sucrose-citric acid-water and sucrose-NaNO3-water, at varying solute mass ratios. Results from HOT and poke-and-flow are in excellent agreement over their overlapping range of applicability (∼103–107 Pa s); fitted curves from DCIC data show variable agreement with the other two techniques because of the sensitivity of the applied modeling framework to the representation of water content in the particles. Further, two modeling approaches for the predictions of the water activity-dependent viscosity of these ternary systems are evaluated. We show that it is possible to represent their viscosity with relatively simple mixing rules applied to the subcooled viscosity values of each component or to the viscosity of the corresponding binary mixtures.
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Study on the Optical Field Distribution of Asymmetric Nanostructures Based on Surface Plasmon
Leijun Zhang; Jiao Jiao; Jisheng Tong; Chunguang Ma; Qing Zhao
Optical force, which is known as a powerful effect of the light on all objects and is too weak to be perceived directly. While the momentum transfer between light and matter can be greatly improved at the nanometer scale. In this paper, we mainly designed an asymmetric deep subwavelength metal nanostructure to generate a strong optical field gradient force by utilizing the resonant interaction between light and nanostructures as well as coupled excitation of surface plasmon, which can promote the movement of the nanoparticles. After simulating and optimizing, we learned that the intensity of optical field excited by the stepped trapezoidal nanostructure we designed in this paper is 140 times higher than that of incident light. At last, we fabricated and tested the sample of the designed nanostructures with the method of electron-beam lithography (EBL), and get the data of the speckle distribution curve and scattering curve of the sample. We discovered that in the wavelength range from 300 nm to 1100 nm, the sample will be excited to a strong plasma. It can be inferred that the devices cascaded by such structural units can accelerate and emit a large number of nanoparticles to generate considerable thrust, which has a great application prospects for the precise positioning of spacecraft.
DOI
Optical force, which is known as a powerful effect of the light on all objects and is too weak to be perceived directly. While the momentum transfer between light and matter can be greatly improved at the nanometer scale. In this paper, we mainly designed an asymmetric deep subwavelength metal nanostructure to generate a strong optical field gradient force by utilizing the resonant interaction between light and nanostructures as well as coupled excitation of surface plasmon, which can promote the movement of the nanoparticles. After simulating and optimizing, we learned that the intensity of optical field excited by the stepped trapezoidal nanostructure we designed in this paper is 140 times higher than that of incident light. At last, we fabricated and tested the sample of the designed nanostructures with the method of electron-beam lithography (EBL), and get the data of the speckle distribution curve and scattering curve of the sample. We discovered that in the wavelength range from 300 nm to 1100 nm, the sample will be excited to a strong plasma. It can be inferred that the devices cascaded by such structural units can accelerate and emit a large number of nanoparticles to generate considerable thrust, which has a great application prospects for the precise positioning of spacecraft.
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Gray-Molasses Optical-Tweezer Loading: Controlling Collisions for Scaling Atom-Array Assembly
M. O. Brown, T. Thiele, C. Kiehl, T.-W. Hsu, and C. A. Regal
To isolate individual neutral atoms in microtraps, experimenters have long harnessed molecular photoassociation to make atom distributions sub-Poissonian. While a variety of approaches have used a combination of attractive (red-detuned) and repulsive (blue-detuned) molecular states, to date all experiments have been predicated on red-detuned cooling. In our work, we present a shifted perspective—namely, the efficient way to capture single atoms is to eliminate red-detuned light in the loading stage and use blue-detuned light that both cools the atoms and precisely controls trap loss through the amount of energy released during atom-atom collisions in the photoassociation process. Subsequent application of red-detuned light then assures the preparation of maximally one atom in the trap. Using Λ-enhanced gray-molasses for loading, we study and model the molecular processes and find we can trap single atoms with 90% probability even in a very shallow optical tweezer. Using 100 traps loaded with 80% probability, we demonstrate one example of the power of enhanced loading by assembling a grid of 36 atoms using only a single move of rows and columns in 2D. Our insight is key in scaling the number of particles in a bottom-up quantum simulation and computation with atoms, or even molecules.
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Axial position detection for optical tweezers based on Moiré Deflectometry
Ali AkbarKhorshad, S. Nader S.Reihani
Optical tweezers are indispensable instruments for applying and measuring Pico-Newton range forces. The magnitude of the exerted force is determined by measuring the displacement of the trapped bead from the center of the trap. Recently, we developed a new detection system for optical tweezers based on Moiré Deflectometry (MD). In this work, we show, both theoretically and experimentally, that the introduced method can be used for detection in the axial direction, as well, with a significantly larger sensitivity compared to the commonly used QPD method. As an example, for a polystyrene bead this method could provide a sensitivity 285% larger than that of the QPD method.
DOI
Optical tweezers are indispensable instruments for applying and measuring Pico-Newton range forces. The magnitude of the exerted force is determined by measuring the displacement of the trapped bead from the center of the trap. Recently, we developed a new detection system for optical tweezers based on Moiré Deflectometry (MD). In this work, we show, both theoretically and experimentally, that the introduced method can be used for detection in the axial direction, as well, with a significantly larger sensitivity compared to the commonly used QPD method. As an example, for a polystyrene bead this method could provide a sensitivity 285% larger than that of the QPD method.
DOI
Wednesday, June 12, 2019
Investigation of multiple-dynein transport of melanosomes by non-invasive force measurement using fluctuation unit χ
Shin Hasegawa, Takashi Sagawa, Kazuho Ikeda, Yasushi Okada & Kumiko Hayashi
Pigment organelles known as melanosomes disperse or aggregate in a melanophore in response to hormones. These movements are mediated by the microtubule motors kinesin-2 and cytoplasmic dynein. However, the force generation mechanism of dynein, unlike that of kinesin, is not well understood. In this study, to address this issue, we investigated the dynein-mediated aggregation of melanosomes in zebrafish melanophores. We applied the fluctuation theorem of non-equilibrium statistical mechanics to estimate forces acting on melanosomes during transport by dynein, given that the energy of a system is related to its fluctuation. Our results demonstrate that multiple force-producing units cooperatively transport a single melanosome. Since the force is generated by dynein, this suggests that multiple dyneins carry a single melanosome. Cooperative transport has been reported for other organelles; thus, multiple-motor transport may be a universal mechanism for moving organelles within the cell.
DOI
Pigment organelles known as melanosomes disperse or aggregate in a melanophore in response to hormones. These movements are mediated by the microtubule motors kinesin-2 and cytoplasmic dynein. However, the force generation mechanism of dynein, unlike that of kinesin, is not well understood. In this study, to address this issue, we investigated the dynein-mediated aggregation of melanosomes in zebrafish melanophores. We applied the fluctuation theorem of non-equilibrium statistical mechanics to estimate forces acting on melanosomes during transport by dynein, given that the energy of a system is related to its fluctuation. Our results demonstrate that multiple force-producing units cooperatively transport a single melanosome. Since the force is generated by dynein, this suggests that multiple dyneins carry a single melanosome. Cooperative transport has been reported for other organelles; thus, multiple-motor transport may be a universal mechanism for moving organelles within the cell.
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Mutual interaction of red blood cells influenced by nanoparticles
Tatiana Avsievich, Alexey Popov, Alexander Bykov & Igor Meglinski
Despite extensive studies on different types of nanoparticles as potential drug carriers, the application of red blood cells (RBCs) as natural transport agents for systemic drug delivery is considered a new paradigm in modern medicine and possesses great potential. There is a lack of studies on the influence of drug carriers of different compositions on RBCs, especially regarding their potential impact on human health. Here, we apply conventional microscopy to observe the formation of RBC aggregates and optical tweezers to quantitatively assess the mutual interaction of RBCs incubated with inorganic and polymeric nanoparticles. Scanning electron microscopy is utilized for direct observation of nanoparticle localization on RBC membranes. The experiments are performed in a platelet-free blood plasma mimicking the RBC natural environment. We show that nanodiamonds influence mutual RBC interactions more antagonistically than other nanoparticles, resulting in higher aggregation forces and the formation of larger cell aggregates. In contrast, polymeric particles do not cause anomalous RBC aggregation. The results emphasize the application of optical tweezers for the direct quantitative assessment of the mutual interaction of RBCs influenced by nanomaterials.
DOI
Despite extensive studies on different types of nanoparticles as potential drug carriers, the application of red blood cells (RBCs) as natural transport agents for systemic drug delivery is considered a new paradigm in modern medicine and possesses great potential. There is a lack of studies on the influence of drug carriers of different compositions on RBCs, especially regarding their potential impact on human health. Here, we apply conventional microscopy to observe the formation of RBC aggregates and optical tweezers to quantitatively assess the mutual interaction of RBCs incubated with inorganic and polymeric nanoparticles. Scanning electron microscopy is utilized for direct observation of nanoparticle localization on RBC membranes. The experiments are performed in a platelet-free blood plasma mimicking the RBC natural environment. We show that nanodiamonds influence mutual RBC interactions more antagonistically than other nanoparticles, resulting in higher aggregation forces and the formation of larger cell aggregates. In contrast, polymeric particles do not cause anomalous RBC aggregation. The results emphasize the application of optical tweezers for the direct quantitative assessment of the mutual interaction of RBCs influenced by nanomaterials.
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Energy landscape of colloidal dumbbells in a periodic distribution of light
E. Sarmiento-Gómez, J. A. Rivera-Morán and J. L. Arauz-Lara
Using a ray tracing calculation, the energy landscape of dumbbells, made of spherical colloidal particles, interacting with a periodic distribution of light is calculated. As shown previously [E. Sarmiento-Gomez, J. A. Rivera-Moran and J. L. Aruaz-Lara, Soft Matter, 2018, 14, 3684], planar aggregates of spherical particles adopt discrete configurations in such light distribution. Here we focus on the case of colloidal dumbbells both symmetric and asymmetric from an experimental and theoretical point of view. It has been shown that the direct calculation using the ray tracing approximation is in excellent agreement with the experiment in spite of the fact that the particles size and the wavelength of the trapping light are comparable. We also corroborate, at least for the more simple case of a single particle in a parabolic light distribution, that the simple method used here provides the same results as the more complex and general Lorenz–Mie approach giving a more simple yet reliable method for the calculation of the energy landscape of colloidal aggregates in periodic light distributions.
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Using a ray tracing calculation, the energy landscape of dumbbells, made of spherical colloidal particles, interacting with a periodic distribution of light is calculated. As shown previously [E. Sarmiento-Gomez, J. A. Rivera-Moran and J. L. Aruaz-Lara, Soft Matter, 2018, 14, 3684], planar aggregates of spherical particles adopt discrete configurations in such light distribution. Here we focus on the case of colloidal dumbbells both symmetric and asymmetric from an experimental and theoretical point of view. It has been shown that the direct calculation using the ray tracing approximation is in excellent agreement with the experiment in spite of the fact that the particles size and the wavelength of the trapping light are comparable. We also corroborate, at least for the more simple case of a single particle in a parabolic light distribution, that the simple method used here provides the same results as the more complex and general Lorenz–Mie approach giving a more simple yet reliable method for the calculation of the energy landscape of colloidal aggregates in periodic light distributions.
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Generating digital drug cocktails via optical manipulation of drug-containing particles and photo-patterning of hydrogels
Yi-Sin Chen, Ko-Chin Chung, Wen-Yen Huang, Wen-Bin Lee, Chien-Yu Fu, Chih-Hung Wang and Gwo-Bin Lee
An integrated microfluidic system combining 1) an optically-induced-dielectrophoresis (ODEP) module for manipulation of drug-containing particles and 2) an ultraviolet (UV) “direct writing” module capable of patterning hydrogels was established herein for automatic formulation of customized digital drug cocktails. Using the ODEP module, the drug-containing particles were assembled by using moving light patterns generated from a digital projector. The hydrogel, poly(ethylene glycol) diacrylate (PEGDA), was used as the medium in the ODEP module such that the assembled drug-containing particles could be UV-cured and consequently encapsulated in “pills” of specific sizes and shapes by using the UV direct writing module. At an optimal ODEP force of 335 pN, which was achieved in a solution of 15% PEGDA in 0.2 M sucrose, it was possible to manipulate and UV-cure the drug-containing particles. Furthermore, with a digital micromirror device inside the UV direct writing module, different UV patterns could be designed and projected, allowing for the digital drug cocktails to be packaged into different shapes in <60 s. As a demonstration, emulsion droplets containing two different anti-cancer drugs were further tested to show the capability of the developed device. This represents an automatic digital drug cocktail formulating device which stands to revolutionize personalized medicine.
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An integrated microfluidic system combining 1) an optically-induced-dielectrophoresis (ODEP) module for manipulation of drug-containing particles and 2) an ultraviolet (UV) “direct writing” module capable of patterning hydrogels was established herein for automatic formulation of customized digital drug cocktails. Using the ODEP module, the drug-containing particles were assembled by using moving light patterns generated from a digital projector. The hydrogel, poly(ethylene glycol) diacrylate (PEGDA), was used as the medium in the ODEP module such that the assembled drug-containing particles could be UV-cured and consequently encapsulated in “pills” of specific sizes and shapes by using the UV direct writing module. At an optimal ODEP force of 335 pN, which was achieved in a solution of 15% PEGDA in 0.2 M sucrose, it was possible to manipulate and UV-cure the drug-containing particles. Furthermore, with a digital micromirror device inside the UV direct writing module, different UV patterns could be designed and projected, allowing for the digital drug cocktails to be packaged into different shapes in <60 s. As a demonstration, emulsion droplets containing two different anti-cancer drugs were further tested to show the capability of the developed device. This represents an automatic digital drug cocktail formulating device which stands to revolutionize personalized medicine.
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Optical pulling at macroscopic distances
Xiao Li, Jun Chen, Zhifang Lin and Jack Ng
Optical tractor beams, proposed in 2011 and experimentally demonstrated soon after, offer the ability to pull particles against light propagation. It has attracted much research and public interest. Yet, its limited microscopic-scale range severely restricts its applicability. The dilemma is that a long-range Bessel beam, the most accessible beam for optical traction, has a small half-cone angle, θ0, making pulling difficult. Here, by simultaneously using several novel and compatible mechanisms, including transverse isotropy, Snell’s law, antireflection coatings (or impedance-matched metamaterials), and light interference, we overcome this dilemma and achieve long-range optical pulling at θ0 ≈ 1°. The range is estimated to be 14 cm when using ~1 W of laser power. Thus, macroscopic optical pulling can be realized in a medium or in a vacuum, with good tolerance of the half-cone angle and the frequency of the light.
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Optical tractor beams, proposed in 2011 and experimentally demonstrated soon after, offer the ability to pull particles against light propagation. It has attracted much research and public interest. Yet, its limited microscopic-scale range severely restricts its applicability. The dilemma is that a long-range Bessel beam, the most accessible beam for optical traction, has a small half-cone angle, θ0, making pulling difficult. Here, by simultaneously using several novel and compatible mechanisms, including transverse isotropy, Snell’s law, antireflection coatings (or impedance-matched metamaterials), and light interference, we overcome this dilemma and achieve long-range optical pulling at θ0 ≈ 1°. The range is estimated to be 14 cm when using ~1 W of laser power. Thus, macroscopic optical pulling can be realized in a medium or in a vacuum, with good tolerance of the half-cone angle and the frequency of the light.
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Optical trapping in vivo: theory, practice, and applications
Itia A. Favre-Bulle, Alexander B. Stilgoe, Ethan K. Scott, Halina Rubinsztein-Dunlop
Since the time of their introduction, optical tweezers (OTs) have grown to be a powerful tool in the hands of biologists. OTs use highly focused laser light to guide, manipulate, or sort target objects, typically in the nanoscale to microscale range. OTs have been particularly useful in making quantitative measurements of forces acting in cellular systems; they can reach inside living cells and be used to study the mechanical properties of the fluids and structures that they contain. As all the measurements are conducted without physically contacting the system under study, they also avoid complications related to contamination and tissue damage. From the manipulation of fluorescent nanodiamonds to chromosomes, cells, and free-swimming bacteria, OTs have now been extended to challenging biological systems such as the vestibular system in zebrafish. Here, we will give an overview of OTs, the complications that arise in carrying out OTs in vivo, and specific OT methods that have been used to address a range of otherwise inaccessible biological questions.
DOI
Since the time of their introduction, optical tweezers (OTs) have grown to be a powerful tool in the hands of biologists. OTs use highly focused laser light to guide, manipulate, or sort target objects, typically in the nanoscale to microscale range. OTs have been particularly useful in making quantitative measurements of forces acting in cellular systems; they can reach inside living cells and be used to study the mechanical properties of the fluids and structures that they contain. As all the measurements are conducted without physically contacting the system under study, they also avoid complications related to contamination and tissue damage. From the manipulation of fluorescent nanodiamonds to chromosomes, cells, and free-swimming bacteria, OTs have now been extended to challenging biological systems such as the vestibular system in zebrafish. Here, we will give an overview of OTs, the complications that arise in carrying out OTs in vivo, and specific OT methods that have been used to address a range of otherwise inaccessible biological questions.
DOI
Tuesday, June 11, 2019
Optical binding via surface plasmon polariton interference
Natalia Kostina, Mihail Petrov, Aliaksandra Ivinskaya, Sergey Sukhov, Andrey Bogdanov, Ivan Toftul, Manuel Nieto-Vesperinas, Pavel Ginzburg, and Alexander Shalin
Optical binding allows creation of mechanically stable nanoparticle configurations owing to formation of self-consistent optical trapping potentials. While the classical diffraction limit prevents achieving deeply subwavelength arrangements, auxiliary nanostructures enable tailoring optical forces via additional interaction channels. Here, a dimer configuration next to a metal surface was analyzed in detail and the contribution of surface plasmon polariton waves was found to govern the interaction dynamics. It is shown that the interaction channel, mediated by resonant surface waves, enables achieving subwavelength stable dimers. Furthermore, the vectorial structure of surface modes allows binding between two dipole nanoparticles along the direction of their dipole moments, contrary to vacuum binding, where a stable configuration is formed in the direction perpendicular to the polarization of the dipole moments. In addition, the enhancement by one order of magnitude of the optical binding stiffness is predicted owing to the surface plasmon polariton interaction channel. These phenomena pave the way for developing new flexible optical manipulators, allowing for control over a nanoparticle trajectory on subwavelength scales and opening opportunities for optical-induced anisotropic (i.e., with different periods along the field polarization as well as perpendicular to it) organization of particles on a plasmonic substrate.
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Optical sorting of small chiral particles by tightly focused vector beams
Manman Li, Shaohui Yan, Yanan Zhang, Yansheng Liang, Peng Zhang, and Baoli Yao
The identification and separation of substances by chirality has always been an important problem in biomedical research and industry. Light beams carry optical momentum, and can exert optical force on any object they impinge due to the transfer of momentum. Different chiral objects will experience different optical forces when illuminated by the same light beam. We demonstrate here, based on the dipolar approximation, that a tightly focused vector beam can selectively trap and rotate small chiral particles in the transverse plane via the chirality-tailored optical forces. The radial optical force can transversely trap the chiral particles off axis or push them away depending on the real part of the chirality parameter, while the lateral optical force manifesting as the azimuthal optical force can drive the trapped particles to orbitally rotate with opposite chiral absorption in opposite directions. The study reported here may find applications in discriminating and separating chiral objects with specified chirality.
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The identification and separation of substances by chirality has always been an important problem in biomedical research and industry. Light beams carry optical momentum, and can exert optical force on any object they impinge due to the transfer of momentum. Different chiral objects will experience different optical forces when illuminated by the same light beam. We demonstrate here, based on the dipolar approximation, that a tightly focused vector beam can selectively trap and rotate small chiral particles in the transverse plane via the chirality-tailored optical forces. The radial optical force can transversely trap the chiral particles off axis or push them away depending on the real part of the chirality parameter, while the lateral optical force manifesting as the azimuthal optical force can drive the trapped particles to orbitally rotate with opposite chiral absorption in opposite directions. The study reported here may find applications in discriminating and separating chiral objects with specified chirality.
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Force measurements show that uL4 and uL24 mechanically stabilize a fragment of 23S rRNA essential for ribosome assembly
Laurent Geffroy, Thierry Bizebard, Ryo Aoyama, Takuya Ueda and Ulrich Bockelmann
In vitro reconstitution studies have shown that ribosome assembly is highly cooperative and starts with the binding of a few ribosomal (r-) proteins to rRNA. It is unknown how these early binders act. Focusing on the initial stage of the assembly of the large subunit of the E. coli ribosome, we prepared a 79 nucleotide-long region of 23S rRNA encompassing the binding sites of the early binders uL4 and uL24. Force signals are measured in a DNA/RNA dumbbell configuration with a double optical tweezers setup. The rRNA fragment was stretched until unfolded, in the absence or in the presence of the r-proteins (either uL4, uL24 or both). We show that the r-proteins uL4 and uL24 individually stabilize the rRNA fragment, both acting as molecular clamps. Interestingly, this mechanical stabilization is enhanced when both proteins are bound simultaneously. Independently, we observe a cooperative binding of uL4 and uL24 to the rRNA fragment. These two aspects of r-proteins binding both contribute to the efficient stabilization of the 3D structure of the rRNA fragment under investigation. We finally consider implications of our results for large ribosomal subunit assembly.
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In vitro reconstitution studies have shown that ribosome assembly is highly cooperative and starts with the binding of a few ribosomal (r-) proteins to rRNA. It is unknown how these early binders act. Focusing on the initial stage of the assembly of the large subunit of the E. coli ribosome, we prepared a 79 nucleotide-long region of 23S rRNA encompassing the binding sites of the early binders uL4 and uL24. Force signals are measured in a DNA/RNA dumbbell configuration with a double optical tweezers setup. The rRNA fragment was stretched until unfolded, in the absence or in the presence of the r-proteins (either uL4, uL24 or both). We show that the r-proteins uL4 and uL24 individually stabilize the rRNA fragment, both acting as molecular clamps. Interestingly, this mechanical stabilization is enhanced when both proteins are bound simultaneously. Independently, we observe a cooperative binding of uL4 and uL24 to the rRNA fragment. These two aspects of r-proteins binding both contribute to the efficient stabilization of the 3D structure of the rRNA fragment under investigation. We finally consider implications of our results for large ribosomal subunit assembly.
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Nanostructured metals for light-based technologies
Reuven Gordon
The basic theoretical understanding of light interacting with nanostructured metals that has existed since the early 1900s has become more relevant in the last two decades, largely because of new approaches to structure metals down to the nanometer scale or smaller. Here, a broad overview of the concepts and applications of nanostructuring metals for light-based technologies is given. The theory of the response of metals to an applied oscillating field is given, including a discussion of nonlocal, nonlinear and quantum effects. Using this metal response, the guiding of electromagnetic (light) waves using metals is given, with a particular emphasis on the impact of nanostructured metals for tighter confinement and slower propagation. Similarly, the influence of metal nanostructures on light scattering by isolated metal structures, like nanoparticles and nanoantennas, is described, with basic results presented including plasmonic/circuit resonances, the single channel limit, directivity enhancement, the maximum power transfer theorem, limits on the magnetic response from kinetic inductance and the scaling of gap plasmons to the nanometer scale and smaller. A brief overview of nanofabrication approaches to creating metal nanostructures is given. Finally, existing and emerging light-based applications are presented, including those for sensing, spectroscopy (including local refractive index, Raman, IR absorption), detection (including Schottky detectors), switching (including terahertz photoconductive antennas), modulation, energy harvesting and photocatalysis, light emission (including lasers and tunneling based light emission), optical tweezing, nonlinear optics, subwavelength imaging and lithography and high density data storage.
DOI
The basic theoretical understanding of light interacting with nanostructured metals that has existed since the early 1900s has become more relevant in the last two decades, largely because of new approaches to structure metals down to the nanometer scale or smaller. Here, a broad overview of the concepts and applications of nanostructuring metals for light-based technologies is given. The theory of the response of metals to an applied oscillating field is given, including a discussion of nonlocal, nonlinear and quantum effects. Using this metal response, the guiding of electromagnetic (light) waves using metals is given, with a particular emphasis on the impact of nanostructured metals for tighter confinement and slower propagation. Similarly, the influence of metal nanostructures on light scattering by isolated metal structures, like nanoparticles and nanoantennas, is described, with basic results presented including plasmonic/circuit resonances, the single channel limit, directivity enhancement, the maximum power transfer theorem, limits on the magnetic response from kinetic inductance and the scaling of gap plasmons to the nanometer scale and smaller. A brief overview of nanofabrication approaches to creating metal nanostructures is given. Finally, existing and emerging light-based applications are presented, including those for sensing, spectroscopy (including local refractive index, Raman, IR absorption), detection (including Schottky detectors), switching (including terahertz photoconductive antennas), modulation, energy harvesting and photocatalysis, light emission (including lasers and tunneling based light emission), optical tweezing, nonlinear optics, subwavelength imaging and lithography and high density data storage.
DOI
Cavity Cooling of a Levitated Nanosphere by Coherent Scattering
Uroš Delić, Manuel Reisenbauer, David Grass, Nikolai Kiesel, Vladan Vuletić, and Markus Aspelmeyer
We report three-dimensional (3D) cooling of a levitated nanoparticle inside an optical cavity. The cooling mechanism is provided by cavity-enhanced coherent scattering off an optical tweezer. The observed 3D dynamics and cooling rates are as theoretically expected from the presence of both linear and quadratic terms in the interaction between the particle motion and the cavity field. By achieving nanometer-level control over the particle location we optimize the position-dependent coupling and demonstrate axial cooling by two orders of magnitude at background pressures of 6×10−2 mbar. We also estimate a significant (>40dB) suppression of laser phase noise heating, which is a specific feature of the coherent scattering scheme. The observed performance implies that quantum ground state cavity cooling of levitated nanoparticles can be achieved for background pressures below 1×10−7mbar.
DOI
We report three-dimensional (3D) cooling of a levitated nanoparticle inside an optical cavity. The cooling mechanism is provided by cavity-enhanced coherent scattering off an optical tweezer. The observed 3D dynamics and cooling rates are as theoretically expected from the presence of both linear and quadratic terms in the interaction between the particle motion and the cavity field. By achieving nanometer-level control over the particle location we optimize the position-dependent coupling and demonstrate axial cooling by two orders of magnitude at background pressures of 6×10−2 mbar. We also estimate a significant (>40dB) suppression of laser phase noise heating, which is a specific feature of the coherent scattering scheme. The observed performance implies that quantum ground state cavity cooling of levitated nanoparticles can be achieved for background pressures below 1×10−7mbar.
DOI
Cavity-Based 3D Cooling of a Levitated Nanoparticle via Coherent Scattering
Dominik Windey, Carlos Gonzalez-Ballestero, Patrick Maurer, Lukas Novotny, Oriol Romero-Isart, and René Reimann
We experimentally realize cavity cooling of all three translational degrees of motion of a levitated nanoparticle in vacuum. The particle is trapped by a cavity-independent optical tweezer and coherently scatters tweezer light into the blue detuned cavity mode. For vacuum pressures around 10−5mbar , minimal temperatures along the cavity axis in the millikelvin regime are observed. Simultaneously, the center-of-mass (c.m.) motion along the other two spatial directions is cooled to minimal temperatures of a few hundred millikelvin. Measuring temperatures and damping rates as the pressure is varied, we find that the cooling efficiencies depend on the particle position within the intracavity standing wave. This data and the behavior of the c.m. temperatures as functions of cavity detuning and tweezer power are consistent with a theoretical analysis of the experiment. Experimental limits and opportunities of our approach are outlined.
DOI
We experimentally realize cavity cooling of all three translational degrees of motion of a levitated nanoparticle in vacuum. The particle is trapped by a cavity-independent optical tweezer and coherently scatters tweezer light into the blue detuned cavity mode. For vacuum pressures around 10−5mbar , minimal temperatures along the cavity axis in the millikelvin regime are observed. Simultaneously, the center-of-mass (c.m.) motion along the other two spatial directions is cooled to minimal temperatures of a few hundred millikelvin. Measuring temperatures and damping rates as the pressure is varied, we find that the cooling efficiencies depend on the particle position within the intracavity standing wave. This data and the behavior of the c.m. temperatures as functions of cavity detuning and tweezer power are consistent with a theoretical analysis of the experiment. Experimental limits and opportunities of our approach are outlined.
DOI
Photoinduced Atomic Force Spectroscopy and Imaging of Two-Dimensional Materials
T.U. Tumkur, M.A. Hurier, M.D. Pichois, M. Vomir, B. Donnio, J.L. Gallani, and M.V. Rastei
We report on the near-field imaging of atomically thin layers of two-dimensional (2D) materials using photoinduced force mapping. This is accomplished by modifying a traditional atomic force microscopy set up to detect optical forces between a nanoscale tip and a photoexcited sample. Our set up facilitates the imaging of few-layer flakes of MoS2 or WS2 and the simultaneous acquisition of optical force spectra, both in ambient and vacuum conditions. The evaluated force spectra in both samples exhibit the characteristic excitonic resonance peaks that are most typically observed in far-field absorption spectroscopy. We also show that nanoscale defect sites and flake edges can be distinguished from the crystalline surfaces with a high spectral resolution. Our results pave the way toward gaining a wholesome understanding of optical interactions and structure-property correlations in 2D materials and their heterostructures.
DOI
We report on the near-field imaging of atomically thin layers of two-dimensional (2D) materials using photoinduced force mapping. This is accomplished by modifying a traditional atomic force microscopy set up to detect optical forces between a nanoscale tip and a photoexcited sample. Our set up facilitates the imaging of few-layer flakes of MoS2 or WS2 and the simultaneous acquisition of optical force spectra, both in ambient and vacuum conditions. The evaluated force spectra in both samples exhibit the characteristic excitonic resonance peaks that are most typically observed in far-field absorption spectroscopy. We also show that nanoscale defect sites and flake edges can be distinguished from the crystalline surfaces with a high spectral resolution. Our results pave the way toward gaining a wholesome understanding of optical interactions and structure-property correlations in 2D materials and their heterostructures.
DOI
Friday, June 7, 2019
Analytical calculation of optical forces on spherical particles in optical tweezers: tutorial
Antonio Alvaro Ranha Neves and Carlos Lenz Cesar
Arthur Ashkin was awarded the 2018 Nobel Prize in Physics for the invention of optical tweezers. Since their first publication in 1986, optical tweezers have been used as a tool to measure forces and rheological properties of microscopic systems. For the calibration of these measurements, knowledge of the forces is fundamental. However, it is still common to deduce the optical forces from assumptions based on the particle size with respect to the trapping laser wavelength. This shows the necessity to develop a complete and accurate electromagnetic model that does not depend on early approximations of the force model. Furthermore, the model we have developed has several advantages, such as morphology-dependent resonances, size dependence for large spheres, and multipole effects from smaller particles, just to name a few. In this tutorial, we review and discuss the physical modeling of optical forces in optical tweezers, which are the resultant forces exerted by a trapping beam on a sphere of any size and composition.
DOI
Arthur Ashkin was awarded the 2018 Nobel Prize in Physics for the invention of optical tweezers. Since their first publication in 1986, optical tweezers have been used as a tool to measure forces and rheological properties of microscopic systems. For the calibration of these measurements, knowledge of the forces is fundamental. However, it is still common to deduce the optical forces from assumptions based on the particle size with respect to the trapping laser wavelength. This shows the necessity to develop a complete and accurate electromagnetic model that does not depend on early approximations of the force model. Furthermore, the model we have developed has several advantages, such as morphology-dependent resonances, size dependence for large spheres, and multipole effects from smaller particles, just to name a few. In this tutorial, we review and discuss the physical modeling of optical forces in optical tweezers, which are the resultant forces exerted by a trapping beam on a sphere of any size and composition.
DOI
Single-cell analysis reveals the effects of glutaraldehyde and formaldehyde on individual Nosema bombycis spores
Zhenbin Miao, Pengfei Zhang, Yu Zhang, Xuhua Huang, Junxian Liu and Guiwen Wang
Nosema bombycis (Nb) is the pathogen that causes pebrine in silkworms. Aldehydes are effective disinfectants commonly used in sericulture. However, the precise mechanism of their action on Nb spores remains unclear. Here, we used laser tweezers Raman spectroscopy to investigate the effects of glutaraldehyde and formaldehyde on individual Nb spores, as well as phase contrast microscopy imaging to monitor the germination dynamics of individual treated spores, to acquire a deeper understanding of the mechanism of action of aldehydes and to provide a theoretical reference for establishing an effective strategy for disease control in sericulture. The positions of the Raman peaks remained constant during treatment. The Raman intensity was enhanced and the germination rate of the spores significantly decreased with treatment time. Tlag, the time when individual spores begin to germinate, and Tgerm, the time for complete germination, increased with enhanced treatment. The germination time (ΔTgerm) showed no significant difference from that for untreated spores. Heterogeneity was shown, which is relevant to the resistance of Nb spores to aldehydes. The results indicate that glutaraldehyde and formaldehyde do not destroy the spore wall and plasma membrane, do not cause the leakage of intracellular components, and might not damage the extrusion apparatus. The effects of aldehydes on Nb spores are mainly on the spore coat. They may block the external factors that stimulate spore germination. Single-cell analysis based on novel optical techniques reveals the action of chemical sporicides on microsporidia spores in real time and explains the heterogeneity of cell stress resistance. These applications of new techniques offer new insight into traditional disinfectants.
DOI
Nosema bombycis (Nb) is the pathogen that causes pebrine in silkworms. Aldehydes are effective disinfectants commonly used in sericulture. However, the precise mechanism of their action on Nb spores remains unclear. Here, we used laser tweezers Raman spectroscopy to investigate the effects of glutaraldehyde and formaldehyde on individual Nb spores, as well as phase contrast microscopy imaging to monitor the germination dynamics of individual treated spores, to acquire a deeper understanding of the mechanism of action of aldehydes and to provide a theoretical reference for establishing an effective strategy for disease control in sericulture. The positions of the Raman peaks remained constant during treatment. The Raman intensity was enhanced and the germination rate of the spores significantly decreased with treatment time. Tlag, the time when individual spores begin to germinate, and Tgerm, the time for complete germination, increased with enhanced treatment. The germination time (ΔTgerm) showed no significant difference from that for untreated spores. Heterogeneity was shown, which is relevant to the resistance of Nb spores to aldehydes. The results indicate that glutaraldehyde and formaldehyde do not destroy the spore wall and plasma membrane, do not cause the leakage of intracellular components, and might not damage the extrusion apparatus. The effects of aldehydes on Nb spores are mainly on the spore coat. They may block the external factors that stimulate spore germination. Single-cell analysis based on novel optical techniques reveals the action of chemical sporicides on microsporidia spores in real time and explains the heterogeneity of cell stress resistance. These applications of new techniques offer new insight into traditional disinfectants.
DOI
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