Sudipta K. Bera, Avinash Kumar, Souvik Sil, Tushar Kanti Saha, Tanumoy Saha, and Ayan Banerjee
We trap absorbing micro-particles in air by photophoretic forces generated using a single loosely focused Gaussian trapping beam. We measure a component of the radial Brownian motion of a trapped particle cluster and determine the power spectral density, mean squared displacement, and normalized position and velocity autocorrelation functions to characterize the photophoretic body force in a quantitative fashion for the first time. The trapped particles also undergo spontaneous rotation due to the action of this force. This is evident from the spectral density that displays clear peaks at the rotation and the particles’ inertial resonance frequencies. We fit the spectral density to the well-known analytical function derived from the Langevin equation, measure the resonance and rotation frequencies, and determine the values for particle mass that we verify at different trapping laser powers with reasonable accuracy.
DOI
Tuesday, September 20, 2016
Monday, September 19, 2016
Waveguide-based dielectric resonance structure: a theoretical analysis to change cell–substrate separation by a repulsive optical force
Abdollah Hassanzadeh and Darya Azami
A waveguide-based dielectric resonance structure is introduced to enhance the optical pressure on a well-spread and attached cell. To calculate the change in cell–substrate separation a three-layered dielectric film, which is considered as a model for a well-spread and attached cell to its substrate, is connected to the substrate by springs. Each spring represents a single adhesion bound. The enhanced optical pressure on the sample, the changes in the cell–substrate separation distance, and strain on the cell are found. The obtained results are compared with those of both total internal reflection and interference reflection microscopes. Then, the penetration depth of the evanescent field and the enhancement factor for various modes are obtained. The results show that the enhancement factor and the optical pressure in the proposed resonance structure are 3 orders of magnitude higher than the conventional structure and the penetration depth of the evanescent wave is increased by 30 percent. We show that a measurable change in the cell–substrate distance (around 6 nm) can occur under the applied optical force. If this waveguide-based resonance structure is used in a waveguide evanescent field microscopy setup it is possible to simultaneously image and apply an effective optical pressure on cells and also to reduce the imaging time. Furthermore, there is no metal in the resonance structure to be worried about the heating and damaging biological samples.
DOI
A waveguide-based dielectric resonance structure is introduced to enhance the optical pressure on a well-spread and attached cell. To calculate the change in cell–substrate separation a three-layered dielectric film, which is considered as a model for a well-spread and attached cell to its substrate, is connected to the substrate by springs. Each spring represents a single adhesion bound. The enhanced optical pressure on the sample, the changes in the cell–substrate separation distance, and strain on the cell are found. The obtained results are compared with those of both total internal reflection and interference reflection microscopes. Then, the penetration depth of the evanescent field and the enhancement factor for various modes are obtained. The results show that the enhancement factor and the optical pressure in the proposed resonance structure are 3 orders of magnitude higher than the conventional structure and the penetration depth of the evanescent wave is increased by 30 percent. We show that a measurable change in the cell–substrate distance (around 6 nm) can occur under the applied optical force. If this waveguide-based resonance structure is used in a waveguide evanescent field microscopy setup it is possible to simultaneously image and apply an effective optical pressure on cells and also to reduce the imaging time. Furthermore, there is no metal in the resonance structure to be worried about the heating and damaging biological samples.
DOI
Opto-electro-fluidics and tip coax conical surface plasmons
Touvia Miloh
The concept of electromagnetic energy enhancement and nanofocusing phenomena near the tip of a metaconical conducting tip by means of a surface plasmon-polaritons mechanism is discussed theoretically. In particular, we consider conical metallic structures with small apex angles and derive the corresponding dispersion relation under optimal (maximal field enhancement) operating conditions. It is demonstrated analytically that the aforementioned conditions can induce large dielectrophoretic forces near the conical tip, which can be harnessed for sorting and controlling nanoparticles in a manner similar to optical tweezers. Similarly, by considering Joule heating effects in the metal and heat conduction in the surrounding solute, it is shown that a considerable (dc) flow convection and mixing can be generated in the aqueous phase near the tip by such ac incited optical means (including common low-input lasers operating in the visible and near-infrared spectrum ranges). Analytic near-field expressions are also obtained for the opto-electro-thermo-induced flow and vorticity distributions in the electrolyte exhibiting a singular behavior near the rounded tip. Using a coax conical metastructure composed of two noble metals, surface-plasmon field enhancement is a technique for the optimal manipulation of dielectric and polarizable nanoparticles as well as for inducing indirect mixing in the liquid around the tip by generating microvortices.
DOI
The concept of electromagnetic energy enhancement and nanofocusing phenomena near the tip of a metaconical conducting tip by means of a surface plasmon-polaritons mechanism is discussed theoretically. In particular, we consider conical metallic structures with small apex angles and derive the corresponding dispersion relation under optimal (maximal field enhancement) operating conditions. It is demonstrated analytically that the aforementioned conditions can induce large dielectrophoretic forces near the conical tip, which can be harnessed for sorting and controlling nanoparticles in a manner similar to optical tweezers. Similarly, by considering Joule heating effects in the metal and heat conduction in the surrounding solute, it is shown that a considerable (dc) flow convection and mixing can be generated in the aqueous phase near the tip by such ac incited optical means (including common low-input lasers operating in the visible and near-infrared spectrum ranges). Analytic near-field expressions are also obtained for the opto-electro-thermo-induced flow and vorticity distributions in the electrolyte exhibiting a singular behavior near the rounded tip. Using a coax conical metastructure composed of two noble metals, surface-plasmon field enhancement is a technique for the optimal manipulation of dielectric and polarizable nanoparticles as well as for inducing indirect mixing in the liquid around the tip by generating microvortices.
DOI
Femtosecond scalpel-optical tweezers: efficient tool for assisted hatching and trophectoderm biopsy
D S Sitnikov, I V Ilina, Yu V Khramova, M A Filatov and M L Semenova
Ultrashort laser pulses have enabled highly precise and delicate processing of biological specimens. We present the results of using femtosecond (fs) laser pulses for dissection of zona pellucida (ZP) in mouse embryos during assisted hatching procedure and for trophectoderm biopsy as well. We studied the effects of application of fs laser radiation in the infrared (1028 nm) and visible (514 nm) wavelength ranges. Laser irradiation parameters were optimized so as not to compromise the viability of the treated embryos. Embryo biopsy was carried out in late-stage mouse preimplantation embryos. Femtosecond laser pulses were applied to detach the desired amount of trophectoderm cells from the blastocyst, while the optical tweezers trapped the cells and moved them out of the embryo. The parameters of laser radiation were optimized so as to efficiently perform embryo biopsy and preserve the viability of the treated embryos. The thermal effects can be significantly lower when fs lasers are used as compared to CW or long-pulse lasers. It is crucial when dealing with living cells or organisms.
DOI
Ultrashort laser pulses have enabled highly precise and delicate processing of biological specimens. We present the results of using femtosecond (fs) laser pulses for dissection of zona pellucida (ZP) in mouse embryos during assisted hatching procedure and for trophectoderm biopsy as well. We studied the effects of application of fs laser radiation in the infrared (1028 nm) and visible (514 nm) wavelength ranges. Laser irradiation parameters were optimized so as not to compromise the viability of the treated embryos. Embryo biopsy was carried out in late-stage mouse preimplantation embryos. Femtosecond laser pulses were applied to detach the desired amount of trophectoderm cells from the blastocyst, while the optical tweezers trapped the cells and moved them out of the embryo. The parameters of laser radiation were optimized so as to efficiently perform embryo biopsy and preserve the viability of the treated embryos. The thermal effects can be significantly lower when fs lasers are used as compared to CW or long-pulse lasers. It is crucial when dealing with living cells or organisms.
DOI
Trapping analyte molecules in hotspots with modified free-standing silver bowtie nanostructures for SERS detection
Daren Xu, Lingxiao Liu, Fei Teng, Feifei Wu and Nan Lu
In this paper, we present a method to trap analyte molecules in hotspots by fabricating free-standing silver bowtie nanostructures with supporting bridges. The existence of supporting bridges makes more analyte molecules in hotspots, which efficiently enhance the Raman intensity. An enhancement factor of 2.4 × 108 is obtained for the structures, and a low concentration of analyte (10−10 M) can be detected. Furthermore, the relative standard deviation for Raman measurement is down to 5.91% for the structures. This method may be applied for SERS measurements.
DOI
In this paper, we present a method to trap analyte molecules in hotspots by fabricating free-standing silver bowtie nanostructures with supporting bridges. The existence of supporting bridges makes more analyte molecules in hotspots, which efficiently enhance the Raman intensity. An enhancement factor of 2.4 × 108 is obtained for the structures, and a low concentration of analyte (10−10 M) can be detected. Furthermore, the relative standard deviation for Raman measurement is down to 5.91% for the structures. This method may be applied for SERS measurements.
DOI
Near-field levitated quantum optomechanics with nanodiamonds
M. L. Juan, G. Molina-Terriza, T. Volz, and O. Romero-Isart
We theoretically show that the dipole force of an ensemble of quantum emitters embedded in a dielectric nanosphere can be exploited to achieve near-field optical levitation. The key ingredient is that the polarizability from the ensemble of embedded quantum emitters can be larger than the bulk polarizability of the sphere, thereby enabling the use of repulsive optical potentials and consequently the levitation using optical near fields. In levitated cavity quantum optomechanics, this could be used to boost the single-photon coupling by combining larger polarizability to mass ratio, larger field gradients, and smaller cavity volumes while remaining in the resolved sideband regime and at room temperature. A case study is done with a nanodiamond containing a high density of silicon-vacancy color centers that is optically levitated in the evanescent field of a tapered nanofiber and coupled to a high-finesse microsphere cavity.
DOI
We theoretically show that the dipole force of an ensemble of quantum emitters embedded in a dielectric nanosphere can be exploited to achieve near-field optical levitation. The key ingredient is that the polarizability from the ensemble of embedded quantum emitters can be larger than the bulk polarizability of the sphere, thereby enabling the use of repulsive optical potentials and consequently the levitation using optical near fields. In levitated cavity quantum optomechanics, this could be used to boost the single-photon coupling by combining larger polarizability to mass ratio, larger field gradients, and smaller cavity volumes while remaining in the resolved sideband regime and at room temperature. A case study is done with a nanodiamond containing a high density of silicon-vacancy color centers that is optically levitated in the evanescent field of a tapered nanofiber and coupled to a high-finesse microsphere cavity.
DOI
Friday, September 16, 2016
Relationship between the Einstein-Laub electromagnetic force and the Lorentz force on free charge
Kevin J. Webb
An electromagnetic force density expression that is consistent with a development attributed to Einstein and Laub appears to be able to describe optical force experiments done to date with homogenized media. However, a major question that has persisted for about one century relates to the apparent discrepancy with the usual interpretation of the force description due to Lorentz in magnetized media. Specifically, it had appeared that the Einstein and Laub force density incorporated only the free-space permeability in relation to the force on the electric current density. It is shown here that the Einstein and Laub force density is consistent with the Lorentz picture in the static limit. This resolves a key impediment in establishing a unified force density description for electromagnetic waves interacting with matter.
DOI
An electromagnetic force density expression that is consistent with a development attributed to Einstein and Laub appears to be able to describe optical force experiments done to date with homogenized media. However, a major question that has persisted for about one century relates to the apparent discrepancy with the usual interpretation of the force description due to Lorentz in magnetized media. Specifically, it had appeared that the Einstein and Laub force density incorporated only the free-space permeability in relation to the force on the electric current density. It is shown here that the Einstein and Laub force density is consistent with the Lorentz picture in the static limit. This resolves a key impediment in establishing a unified force density description for electromagnetic waves interacting with matter.
DOI
An endosomal tether undergoes an entropic collapse to bring vesicles together
David H. Murray, Marcus Jahnel, Janelle Lauer, Mario J. Avellaneda, Nicolas Brouilly, Alice Cezanne, Hernán Morales-Navarrete, Enrico D. Perini, Charles Ferguson, Andrei N. Lupas, Yannis Kalaidzidis, Robert G. Parton, Stephan W. Grill & Marino Zerial
An early step in intracellular transport is the selective recognition of a vesicle by its appropriate target membrane, a process regulated by Rab GTPases via the recruitment of tethering effectors1, 2, 3, 4. Membrane tethering confers higher selectivity and efficiency to membrane fusion than the pairing of SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) alone5, 6, 7. Here we address the mechanism whereby a tethered vesicle comes closer towards its target membrane for fusion by reconstituting an endosomal asymmetric tethering machinery consisting of the dimeric coiled-coil protein EEA1 (refs 6, 7) recruited to phosphatidylinositol 3-phosphate membranes and binding vesicles harbouring Rab5. Surprisingly, structural analysis reveals that Rab5:GTP induces an allosteric conformational change in EEA1, from extended to flexible and collapsed. Through dynamic analysis by optical tweezers, we confirm that EEA1 captures a vesicle at a distance corresponding to its extended conformation, and directly measure its flexibility and the forces induced during the tethering reaction. Expression of engineered EEA1 variants defective in the conformational change induce prominent clusters of tethered vesicles in vivo. Our results suggest a new mechanism in which Rab5 induces a change in flexibility of EEA1, generating an entropic collapse force that pulls the captured vesicle towards the target membrane to initiate docking and fusion.
DOI
An early step in intracellular transport is the selective recognition of a vesicle by its appropriate target membrane, a process regulated by Rab GTPases via the recruitment of tethering effectors1, 2, 3, 4. Membrane tethering confers higher selectivity and efficiency to membrane fusion than the pairing of SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) alone5, 6, 7. Here we address the mechanism whereby a tethered vesicle comes closer towards its target membrane for fusion by reconstituting an endosomal asymmetric tethering machinery consisting of the dimeric coiled-coil protein EEA1 (refs 6, 7) recruited to phosphatidylinositol 3-phosphate membranes and binding vesicles harbouring Rab5. Surprisingly, structural analysis reveals that Rab5:GTP induces an allosteric conformational change in EEA1, from extended to flexible and collapsed. Through dynamic analysis by optical tweezers, we confirm that EEA1 captures a vesicle at a distance corresponding to its extended conformation, and directly measure its flexibility and the forces induced during the tethering reaction. Expression of engineered EEA1 variants defective in the conformational change induce prominent clusters of tethered vesicles in vivo. Our results suggest a new mechanism in which Rab5 induces a change in flexibility of EEA1, generating an entropic collapse force that pulls the captured vesicle towards the target membrane to initiate docking and fusion.
DOI
Concentric Circular Grating Generated by the Patterning Trapping of Nanoparticles in an Optofluidic Chip
Hailang Dai, Zhuangqi Cao, Yuxing Wang, Honggen Li, Minghuang Sang, Wen Yuan, Fan Chen & Xianfeng Chen
Due to the field enhancement effect of the hollow-core metal-cladded optical waveguide chip, massive nanoparticles in a solvent are effectively trapped via exciting ultrahigh order modes. A concentric ring structure of the trapped nanoparticles is obtained since the excited modes are omnidirectional at small incident angle. During the process of solvent evaporation, the nanoparticles remain well trapped since the excitation condition of the optical modes is still valid, and a concentric circular grating consisting of deposited nanoparticles can be produced by this approach. Experiments via scanning electron microscopy, atomic force microscopy and diffraction of a probe laser confirmed the above hypothesis. This technique provides an alternative strategy to enable effective trapping of dielectric particles with low-intensity nonfocused illumination, and a better understanding of the correlation between the guided modes in an optical waveguide and the nanoparticles in a solvent.
DOI
Due to the field enhancement effect of the hollow-core metal-cladded optical waveguide chip, massive nanoparticles in a solvent are effectively trapped via exciting ultrahigh order modes. A concentric ring structure of the trapped nanoparticles is obtained since the excited modes are omnidirectional at small incident angle. During the process of solvent evaporation, the nanoparticles remain well trapped since the excitation condition of the optical modes is still valid, and a concentric circular grating consisting of deposited nanoparticles can be produced by this approach. Experiments via scanning electron microscopy, atomic force microscopy and diffraction of a probe laser confirmed the above hypothesis. This technique provides an alternative strategy to enable effective trapping of dielectric particles with low-intensity nonfocused illumination, and a better understanding of the correlation between the guided modes in an optical waveguide and the nanoparticles in a solvent.
DOI
Light-induced rotations of chiral birefringent microparticles in optical tweezers
M. G. Donato, A. Mazzulla, P. Pagliusi, A. Magazzù, R. J. Hernandez, C. Provenzano, P. G. Gucciardi, O. M. Maragò & G. Cipparrone
We study the rotational dynamics of solid chiral and birefringent microparticles induced by elliptically polarized laser light in optical tweezers. We find that both reflection of left circularly polarized light and residual linear retardance affect the particle dynamics. The degree of ellipticity of laser light needed to induce rotations is found. The experimental results are compared with analytical calculations of the transfer of angular moment from elliptically polarized light to chiral birefringent particles.
DOI
We study the rotational dynamics of solid chiral and birefringent microparticles induced by elliptically polarized laser light in optical tweezers. We find that both reflection of left circularly polarized light and residual linear retardance affect the particle dynamics. The degree of ellipticity of laser light needed to induce rotations is found. The experimental results are compared with analytical calculations of the transfer of angular moment from elliptically polarized light to chiral birefringent particles.
DOI
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