Monday, September 28, 2015

Manipulation of dielectric Rayleigh particles using highly focused elliptically polarized vector fields

Bing Gu, Danfeng Xu, Guanghao Rui, Meng Lian, Yiping Cui, and Qiwen Zhan

Generation of vectorial optical fields with arbitrary polarization distribution is of great interest in areas where exotic optical fields are desired. In this work, we experimentally demonstrate the versatile generation of linearly polarized vector fields, elliptically polarized vector fields, and circularly polarized vortex beams through introducing attenuators in a common-path interferometer. By means of Richards–Wolf vectorial diffraction method, the characteristics of the highly focused elliptically polarized vector fields are studied. The optical force and torque on a dielectric Rayleigh particle produced by these tightly focused vector fields are calculated and exploited for the stable trapping of dielectric Rayleigh particles. It is shown that the additional degree of freedom provided by the elliptically polarized vector field allows one to control the spatial structure of polarization, to engineer the focusing field, and to tailor the optical force and torque on a dielectric Rayleigh particle.


Manipulation of metallic nanoparticle with evanescent vortex Bessel beam

Guanghao Rui, Xiaoyan Wang, and Yiping Cui

In this work, we propose a novel strategy to optically trap and manipulate metallic nanoparticles using evanescent vortex Bessel beam (EVBB). A versatile method is presented to generate evanescent Bessel beam with tunable optical angular momentum by focusing a radially polarized vortex beam onto a one-dimensional photonics band gap structure. The behavior of a metallic nanoparticle in the EVBB is numerically studied. We show that such particle can be stably trapped near the surface. The orbital angular momentum drives the metallic nanoparticle to orbit around the beam axis, and the direction of the orbital motion is controlled by the handedness of the helical phase front. The technique demonstrated in this work may open up new avenues for optical manipulation, and the non-contact tunable orbiting dynamics of the trapped particle may find important applications in higher resolution imaging techniques.


Freestyle 3D laser traps: tools for studying light-driven particle dynamics and beyond

José A. Rodrigo and Tatiana Alieva

We show that a freestyle laser trap, including high-intensity and phase gradient forces along arbitrary curves, is able to confine multiple particles and drive their motion with the ability to speed them up or slow them down. This Letter reports, for first time, to the best of our knowledge, how such a trap can be experimentally created deep within the sample to construct rotating colloidal motors and study collective particle dynamics in distinct configurations. This new laser tool opens up promising perspectives in the study of hydrodynamics and optofluidics at the microscale.


Friday, September 25, 2015

Fabrication and Operation of a Nano-Optical Conveyor Belt

Jason Ryan, Yuxin Zheng, Paul Hansen, Lambertus Hesselink

The technique of using focused laser beams to trap and exert forces on small particles has enabled many pivotal discoveries in the nanoscale biological and physical sciences over the past few decades. The progress made in this field invites further study of even smaller systems and at a larger scale, with tools that could be distributed more easily and made more widely available. Unfortunately, the fundamental laws of diffraction limit the minimum size of the focal spot of a laser beam, which makes particles smaller than a half-wavelength in diameter hard to trap and generally prevents an operator from discriminating between particles which are closer together than one half-wavelength. This precludes the optical manipulation of many closely-spaced nanoparticles and limits the resolution of optical-mechanical systems. Furthermore, manipulation using focused beams requires beam-forming or steering optics, which can be very bulky and expensive. To address these limitations in the system scalability of conventional optical trapping our lab has devised an alternative technique which utilizes near-field optics to move particles across a chip. Instead of focusing laser beams in the far-field, the optical near field of plasmonic resonators produces the necessary local optical intensity enhancement to overcome the restrictions of diffraction and manipulate particles at higher resolution. Closely-spaced resonators produce strong optical traps which can be addressed to mediate the hand-off of particles from one to the next in a conveyor-belt-like fashion. Here, we describe how to design and produce a conveyor belt using a gold surface patterned with plasmonic C-shaped resonators and how to operate it with polarized laser light to achieve super-resolution nanoparticle manipulation and transport. The nano-optical conveyor belt chip can be produced using lithography techniques and easily packaged and distributed.


Measurement of 3D-forces on a Micro Particle in Acoustofluidic Devices Using an Optical Trap

Andreas Lamprecht, Stefan Lakämper, Iwan A.T. Schaap, Jurg Dual

Here, we use a calibrated high gradient laser trap to directly measure the total time-averaged 3D force on a dielectric silica parti- cle in the regime of an ultrasonic standing wave. Acoustic radiation and acoustic streaming apply forces on an optically trapped particle within an acoustofluidic device. From measuring the induced displacements from the laser trap center in three dimen- sions the acoustic forces (0.2-50pN) can be calculated in dependence of the particle position and excitation frequency. Thus, the real pressure distributions within acoustofluidic devices can be mapped out. The three dimensional direct measurement, as pre- sented here, opens up the possibility to quantify so far inaccessible small scale phenomena such as the effects of: a.) local and global acoustic streaming, and b.) boundaries or close-by objects.


Nonequilibrium Fluctuations in Biological Strands, Machines, and Cells

Shoichi Toyabe, and Masaki Sano

Can physics provide a quantitative methodology and unified view to elucidate rich and diverse biological phenomena? Nonequilibrium fluctuations are key quantities. These fluctuations have universal symmetries, convey essential information about systems’ behaviors, and are experimentally accessible in most systems. We review experimental developments to extract information from the nonequilibrium fluctuations of biological systems. In particular, we focus on the three major hierarchies in small scales: strands, molecular machines, and cells.


Towards high-throughput microfluidic Raman-activated cell sorting

Qiang Zhang, Peiran Zhang, Honglei Gou, Chunbo Mou, Wei E. Huang, Menglong Yang, Jian Xu and Bo Ma

Raman-activated cell sorting (RACS) is a promising single-cell analysis technology that is able to identify and isolate individual cells of targeted type, state or environment from an isogenic population or complex consortium of cells, in a label-free and non-invasive manner. However, compared with those widely used yet labeling-required or staining-dependent cell sorting technologies such as FACS and MACS, the weak Raman signal greatly limits the further development of the existing RACS systems to achieve higher throughput. Strategies that can tackle this bottleneck include, first, improvement of Raman-acquisition efficiency and quality based on advanced Raman spectrometers and enhanced Raman techniques; second, development of novel microfluidic devices for cell sorting followed by integration into a complete RACS system. Exploiting these strategies, prototypes for a new generation of RACS have been demonstrated, such as flow-based OT-RACS, DEP-RACS, and SERS/CARS flow cytometry. Such high-throughput microfluidic RACS can provide biologists with a powerful single-cell analysis tool to explore the scientific questions or applications that have been beyond the reach of FACS and MACS.


Transformation optics beyond the manipulation of light trajectories

Vincent Ginis, Philippe Tassin

Since its inception in 2006, transformation optics has become an established tool to understand and design electromagnetic systems. It provides a geometrical perspective into the properties of light waves without the need for a ray approximation. Most studies have focused on modifying the trajectories of light rays, e.g. beam benders, lenses, invisibility cloaks, etc. In this contribution, we explore transformation optics beyond the manipulation of light trajectories. With a few well-chosen examples, we demonstrate that transformation optics can be used to manipulate electromagnetic fields up to an unprecedented level. In the first example, we introduce an electromagnetic cavity that allows for deep subwavelength confinement of light. The cavity is designed with transformation optics even though the concept of trajectory ceases to have any meaning in a structure as small as this cavity. In the second example, we show that the properties of Cherenkov light emitted in a transformation-optical material can be understood and modified from simple geometric considerations. Finally, we show that optical forces—a quadratic function of the fields—follow the rules of transformation optics too. By applying a folded coordinate transformation to a pair of waveguides, optical forces can be enhanced just as if the waveguides were closer together. With these examples, we open up an entirely new spectrum of devices that can be conceived using transformation optics.


Wednesday, September 23, 2015

Enhancement of optical forces using slow light in a photonic crystal waveguide

Mark G. Scullion, Yoshihiko Arita, Thomas F. Krauss, and Kishan Dholakia

The paradigm of slow light in photonic crystal waveguides has already led to startling advances in nonlinear interactions and optical switching. Importantly, as slow light implies a highly reduced group velocity, this also leads to an original route for the enhancement of optical forces by appropriate tuning of the waveguide properties. Here, we demonstrate the use of slow light to enhance the guiding of submicrometer dielectric particles on a photonic crystal waveguide. Studies are based on a range of particle sizes, and we observe a four-fold enhancement in guiding velocity simply by changing the wavelength of the exciting laser within the slow light region. The particle velocity is therefore seen to be dependent upon the group velocity of light in the waveguide in agreement with force simulations. Finally, the enhancement of the lateral trap stiffness transverse to the waveguide axis further confirms the benefit of slow light for particle manipulation.


Extracting physical chemistry from mechanics: a new approach to investigate DNA interactions with drugs and proteins in single molecule experiments

M. S. Rocha

In this review we focus on the idea of establishing connections between the mechanical properties of DNA–ligand complexes and the physical chemistry of DNA–ligand interactions. This type of connection is interesting because it opens the possibility of performing a robust characterization of such interactions by using only one experimental technique: single molecule stretching. Furthermore, it also opens new possibilities in comparing results obtained by very different approaches, in particular when comparing single molecule techniques to ensemble-averaging techniques. We start the manuscript reviewing important concepts of DNA mechanics, from the basic mechanical properties to the Worm-Like Chain model. Next we review the basic concepts of the physical chemistry of DNA–ligand interactions, revisiting the most important models used to analyze the binding data and discussing their binding isotherms. Then, we discuss the basic features of the single molecule techniques most used to stretch DNA–ligand complexes and to obtain “force × extension” data, from which the mechanical properties of the complexes can be determined. We also discuss the characteristics of the main types of interactions that can occur between DNA and ligands, from covalent binding to simple electrostatic driven interactions. Finally, we present a historical survey of the attempts to connect mechanics to physical chemistry for DNA–ligand systems, emphasizing a recently developed fitting approach useful to connect the persistence length of DNA–ligand complexes to the physicochemical properties of the interaction. Such an approach in principle can be used for any type of ligand, from drugs to proteins, even if multiple binding modes are present.


Numerical study of sensitivity enhancement in a photonic crystal microcavity biosensor due to optical forces

Adam T. Heiniger, Benjamin L. Miller, and Philippe M. Fauchet

Photonic crystal microcavity biosensors can detect single biomolecules, but reliance on diffusion from microfluidic flow for particle delivery limits the minimum detectable particle concentration. Here the particle equation of motion is solved to find the sensitivity enhancement due to optical forces. The enhancement is examined for a range of parameters, including input optical power, fluid flow rate, device quality factor, and particle size.


Kinesin’s Front Head Is Gated by the Backward Orientation of Its Neck Linker

Merve Yusra Dogan, Sinan Can, Frank B. Cleary, Vedud Purde, Ahmet Yildiz

Kinesin-1 is a two-headed motor that takes processive 8-nm hand-over-hand steps and transports intracellular cargos toward the plus-end of microtubules. Processive motility requires a gating mechanism to coordinate the mechanochemical cycles of the two heads. Kinesin gating involves neck linker (NL), a short peptide that interconnects the heads, but it remains unclear whether gating is facilitated by the NL orientation or tension. Using optical trapping, we measured the force-dependent microtubule release rate of kinesin monomers under different nucleotide conditions and pulling geometries. We find that pulling NL in the backward direction inhibits nucleotide binding and subsequent release from the microtubule. This inhibition is independent of the magnitude of tension (2–8 pN) exerted on NL. Our results provide evidence that the front head of a kinesin dimer is gated by the backward orientation of its NL until the rear head releases from the microtubule.


Monday, September 21, 2015

Optofluidic Near-Field Optical Microscopy: Near-Field Mapping of a Silicon Nanocavity Using Trapped Microbeads

Christophe Pin, Benoît Cluzel, Claude Renaut, Emmanuel Picard, David Peyrade, Emmanuel Hadji, and Frédérique de Fornel

By analyzing the thermal motion of fluorescent dielectric microbeads trapped in the near-field of a silicon nanocavity, we investigate the influence of the bead’s size and the trapping laser power on the shape of the optical trap and the “effective” trap stiffness. We demonstrate that the trapping potential is proportional to the subwavelength patterns of the electromagnetic near-field intensity distribution for unexpectedly large Mie particle sizes. More especially, we show that mapping the trapping potential experienced by a 500 nm diameter bead reveals the nanopatterns of the cavity resonant mode. This result highlights how photonic force microscopy in nanotweezers can provide an elegant way to image evanescent fields at the nanoscale via the thermal motion of optically trapped fluorescent microprobes.


Enhanced optical manipulation of cells using anti-reflection coated microparticles

Derek Craig, Alison McDonald, Michael Mazilu, Helen Rendall, Frank Gunn-Moore, and Kishan Dholakia

We demonstrate the use of anti-reflection (AR) coated microparticles for the enhanced optical manipulation of cells. Specifically, we incubate CHO-K1, HL60 and NMuMG cell lines with AR coated titania microparticles and subsequently performed drag force measurements using optical trapping. Direct comparisons were performed between native, polystyrene microparticle and AR microparticle tagged cells. The optical trapping efficiency was recorded by measuring the Q value in a drag force experiment. CHO-K1 cells incubated with AR microparticles show an increase in the Q value of nearly 220% versus native cells. With the inclusion of AR microparticles, cell velocities exceeding 50μm/s were recorded for only 33mW of laser trapping power. Cell viability was confirmed with fluorescent dyes and cells expressing a fluorescent ubiquitination-based cell cycle protein (FUCCI) which verified no disruption to the cell cycle in the presence of AR microparticles.


Bacteria-based branched structures for bionanophotonics

Hongbao Xin, Yuchao Li and Baojun Li

Branched photonic structures have served as paramount important components for nanophotonic integration and circuitry. However, these structures are generally constructed with photonic and plasmonic nanowires, which are nonbiomaterials and often need to be specially engineered to interface with cells and biological system. For bionanophotonics, photonic components assembled with self-adaptive biomaterials are highly desirable to be directly interfaced with the dynamic biological system. In this work, branched structures for bionanophotonics assembled with natural living biomaterials, i.e., nanorod-shaped Escherichia coli bacteria are reported. The E. coli cells were orderly trapped using a specially desired tapered optical fiber, forming structures with different branches and lengths. Light-propagation performances along these branched structures were investigated, and the robustness property of the structures were demonstrated. The results show that the bacteria-based branched structures provide different promising self-sustainable and evolvable components, such as multidirectional waveguides and beam splitters, for bionanophotonics by connecting the biological and optical worlds with a seamless interface.


Recent advances in optical fiber devices for microfluidics integration

Robert Blue and Deepak Uttamchandani

Detection and trapping of molecules can be achieved with optical fibers directly located within the fluidic microchannel. This paper examines the recent emergence of miniaturized optical fiber based sensing and actuating devices that have been successfully integrated into fluidic microchannels that are part of microfluidic and lab-on-chip systems. Fluidic microsystems possess the advantages of reduced sample volumes, faster and more sensitive biological assays, multi-sample and parallel analysis, and are seen as the de facto bioanalytical platform of the future. This paper considers the cases where the optical fiber is not merely used as a simple light guide delivering light across a microchannel, but where the fiber itself is engineered to create a new sensor or tool for use within the environment of the fluidic microchannel.


Thursday, September 17, 2015

A ruthenium dimer complex with a flexible linker slowly threads between DNA bases in two distinct steps

Meriem Bahira, Micah J. McCauley, Ali A. Almaqwashi, Per Lincoln, Fredrik Westerlund, Ioulia Rouzina and Mark C. Williams

Several multi-component DNA intercalating small molecules have been designed around ruthenium-based intercalating monomers to optimize DNA binding properties for therapeutic use. Here we probe the DNA binding ligand [μ-C4(cpdppz)2(phen)4Ru2]4+, which consists of two Ru(phen)2dppz2+ moieties joined by a flexible linker. To quantify ligand binding, double-stranded DNA is stretched with optical tweezers and exposed to ligand under constant applied force. In contrast to other bis-intercalators, we find that ligand association is described by a two-step process, which consists of fast bimolecular intercalation of the first dppz moiety followed by ∼10-fold slower intercalation of the second dppz moiety. The second step is rate-limited by the requirement for a DNA-ligand conformational change that allows the flexible linker to pass through the DNA duplex. Based on our measured force-dependent binding rates and ligand-induced DNA elongation measurements, we are able to map out the energy landscape and structural dynamics for both ligand binding steps. In addition, we find that at zero force the overall binding process involves fast association (∼10 s), slow dissociation (∼300 s), and very high affinity (Kd ∼10 nM). The methodology developed in this work will be useful for studying the mechanism of DNA binding by other multi-step intercalating ligands and proteins.


Trapping of intense light in hollow shell

Shixia Luan, Wei Yu, M. Y. Yu, Suming Weng, Jingwei Wang, Han Xu, Hongbin Zhuo and A. Y. Wong

A small hollow shell for trapping laser light is proposed. Two-dimensional particle-in-cell simulation shows that under appropriate laser and plasma conditions a part of the radiation fields of an intense short laser pulse can enter the cavity of a small shell through an over-critical density plasma in an adjacent guide channel and become trapped. The trapped light evolves into a circulating radial wave pattern until its energy is dissipated.


Electromagnetic force on structured metallic surfaces

Andrew H. Velzen and Kevin J. Webb

We present a method by which the relatively weak electromagnetic force exerted on a surface can be dramatically enhanced. By structuring a metal surface at the nanoscale, we show that the force can be substantially increased over that on the planar metallic surface. The basis for this effect is found to be cavity-enhanced fields and the excitation of surface waves, and results are related to theory. In practice, this force enhancement could be expanded to other materials in various frequency regimes. This increased electromagnetic force should facilitate an expansion of applications related to optomechanics.


Stability and instability for low refractive-index-contrast particle trapping in a dual-beam optical trap

Alison Huff, Charles N. Melton, Linda S. Hirst, and Jay E. Sharping

A dual-beam optical trap is used to trap and manipulate dielectric particles. When the refractive index of these particles is comparable to that of the surrounding medium, equilibrium trapping locations within the system shift from stable to unstable depending on fiber separation and particle size. This is due to to the relationship between gradient and scattering forces. We experimentally and computationally study the transitions between stable and unstable trapping of poly(methyl methacrylate) beads for a range of parameters relevant to experimental setups involving giant unilamellar vesicles. We present stability maps for various fiber separations and particle sizes, and find that careful attention to particle size and configuration is necessary to obtain reproducible quantitative results for soft matter stretching experiments.


Tuesday, September 15, 2015

Deinococcus radiodurans RecA nucleoprotein filaments characterized at the single-molecule level with optical tweezers

Georgii Pobegalov, Galina Cherevatenko, Aleksandr Alekseev, Anton Sabantsev, Oksana Kovaleva, Alexey Vedyaykin, Natalia Morozova, Dmitrii Baitin, Mikhail Khodorkovskii

Deinococcus radiodurans can survive extreme doses of ionizing radiation due to the very efficient DNA repair mechanisms that are able to cope even with hundreds of double-strand breaks. RecA, the critical protein of homologous recombination in bacteria, is one of the key components of the DNA-repair system. Repair of double-strand breaks requires RecA binding to DNA and assembly of the RecA nucleoprotein helical filaments. The Escherichia coli RecA protein (EcRecA) and its interactions with DNA have been extensively studied using various approaches including single-molecule techniques, while the D. radiodurans RecA (DrRecA) remains much less characterized. However, DrRecA shows some remarkable differences from E. coli homolog. Here we combine microfluidics and single-molecule DNA manipulation with optical tweezers to follow the binding of DrRecA to long double-stranded DNA molecules and probe the mechanical properties of DrRecA nucleoprotein filaments at physiological pH. Our data provide a direct comparison of DrRecA and EcRecA binding to double-stranded DNA under identical conditions. We report a significantly faster filaments assembly as well as lower values of persistence length and contour length for DrRecA nucleoprotein filaments compared to EcRecA. Our results support the existing model of DrRecA forming more frequent and less continuous filaments relative to those of EcRecA.


Parametric force analysis for measurement of arbitrary optical forces on particles trapped in air or vacuum

Haesung Park and Thomas W. LeBrun

We demonstrate a new method to measure arbitrary optical forces on particles trapped in gaseous or vacuum environments using the ring down of a trapped particle following electrostatic excitation of particle motion in the trap. The method is not limited to the common constraints of linear forces for small oscillations or conservative forces, allows for a wide displacement range, and measures forces directly from trajectories in near-real time. We use transient response analysis to model the nearly ideal response for small oscillations, and illustrate the more general case by demonstrating a nonlinear response to impulse excitation at a displacement where the optical force is linear. Simulations verify the applicability to nonlinear forces from a general potential, and comparison to traditional thermodynamic measures shows excellent agreement. Combined with in-situ microscopy to measure the particle diameter, this allows for the estimation of all system parameters assuming only the manufacturer’s value for the particle density.


Stability of particle propulsion by waveguide modes in the regimes where resonant states are formed

A. V. Maslov

Optical forces acting on dielectric particles inside waveguides are studied. The investigation is carried out within the framework of the two-dimensional model: a cylinder inside a parallel-plate waveguide with perfect metal walls. It is shown that although the appearance of resonant states can lead to a significant increase of backscattering and, therefore, the propelling force, the transverse force can either keep the particle in the location of the efficient propulsion or push it away. The propulsion and trapping regimes are related to the change of the resonant wavelength with particle location. Besides the geometrical and material parameters, the polarization of the incident mode is shown to significantly affect the particle dynamics. The relation of the resonant-state formation and Wood's anomalies in periodic gratings is also discussed.


Dynamic clustering of driven colloidal particles on a circular path

Shogo Okubo, Syuhei Shibata, Yuriko Sassa Kawamura, Masatoshi Ichikawa, and Yasuyuki Kimura

We studied the collective motion of particles forced to move along a circular path in water by utilizing an optical vortex. Their collective motion, including the spontaneous formation of clusters and their dissociation, was observed. The observed temporal patterns depend on the number of particles on the path and the variation of their sizes. The addition of particles with different sizes suppresses the dynamic formation and dissociation of clusters and promotes the formation of specific stationary clusters. These experimental findings are reproduced by numerical simulations that take into account the hydrodynamic interaction between the particles and the radial trapping force confining the particles to the circular path. A transition between stationary and nonstationary clustering of the particles was observed by varying their size ratio in the binary-size systems. Our simulation reveals that the transition can be either continuous or discontinuous depending on the number of different-size particles. This result suggests that the size distribution of particles has a significant effect on the collective behavior of self-propelled particles in viscous fluids.


Monday, September 14, 2015

Ray-optics model for optical force and torque on a spherical metal-coated Janus microparticle

Jing Liu, Chao Zhang, Yiwu Zong, Honglian Guo, and Zhi-Yuan Li

In this paper, we develop a theoretical method based on ray optics to calculate the optical force and torque on a metallo-dielectric Janus particle in an optical trap made from a tightly focused Gaussian beam. The Janus particle is a 2.8 μm diameter polystyrene sphere half-coated with gold thin film several nanometers in thickness. The calculation result shows that the focused beam will push the Janus particle away from the center of the trap, and the equilibrium position of the Janus particle, where the optical force and torque are both zero, is located in a circular orbit surrounding the laser beam axis. The theoretical results are in good agreement qualitatively and quantitatively with our experimental observation. As the ray-optics model is simple in principle, user friendly in formalism, and cost effective in terms of computation resources and time compared with other usual rigorous electromagnetics approaches, the developed theoretical method can become an invaluable tool for understanding and designing ways to control the mechanical motion of complicated microscopic particles in various optical tweezers.


DNA visualization in single molecule studies carried out with optical tweezers: Covalent versus non-covalent attachment of fluorophores

Sandy Suei, Allan Raudsepp, Lisa M. Kent, Stephen A.J. Keen, Vyacheslav V. Filichev, Martin A.K. Williams

In this study, we investigated the use of the covalent attachment of fluorescent dyes to double-stranded DNA (dsDNA) stretched between particles using optical tweezers (OT) and compared the mechanical properties of the covalently-functionalized chain to that of unmodified DNA and to DNA bound to a previously uncharacterized groove-binder, SYBR-gold. Modified DNA species were obtained by covalently linking azide-functionalized organic fluorophores onto the backbone of DNA chains via the alkyne moieties of modified bases that were incorporated during PCR. These DNA molecules were then constructed into dumbbells by attaching polystyrene particles to the respective chain ends via biotin or digoxigenin handles that had been pre-attached to the PCR primers which formed the ends of the synthesized molecule. Using the optical tweezers, the DNA was stretched by separating the two optically trapped polystyrene particles. Displacements of the particles were measured in 3D using an interpolation-based normalized cross-correlation method and force-extension curves were calculated and fitted to the worm-like chain model to parameterize the mechanical properties of the DNA. Results showed that both the contour and persistence length of the covalently-modified dsDNAs were indistinguishable from that of the unmodified dsDNA, whereas SYBR–gold binding perturbed the contour length of the chain in a force-dependent manner.


Friday, September 11, 2015

Cavity-Assisted Manipulation of Freely Rotating Silicon Nanorods in High Vacuum

Stefan Kuhn, Peter Asenbaum, Alon Kosloff, Michele Sclafani, Benjamin A. Stickler, Stefan Nimmrichter, Klaus Hornberger, Ori Cheshnovsky, Fernando Patolsky, and Markus Arndt

Optical control of nanoscale objects has recently developed into a thriving field of research with far-reaching promises for precision measurements, fundamental quantum physics and studies on single-particle thermodynamics. Here, we demonstrate the optical manipulation of silicon nanorods in high vacuum. Initially, we sculpture these particles into a silicon substrate with a tailored geometry to facilitate their launch into high vacuum by laser-induced mechanical cleavage. We manipulate and trace their center-of-mass and rotational motion through the interaction with an intense intracavity field. Our experiments show that the anisotropy of the nanorotors leads to optical forces that are three times stronger than on silicon nanospheres of the same mass. The optical torque experienced by the spinning rods will enable cooling of the rotational motion and torsional optomechanics in a dissipation-free environment.


Cdc42 and RhoA reveal different spatio-temporal dynamics upon local stimulation with Semaphorin-3A

Federico Iseppon, Luisa M. R. Napolitano, Vincent Torre and Dan Cojoc

Small RhoGTPases, such as Cdc42 and RhoA, are key players in integrating external cues and intracellular signaling pathways that regulate growth cone (GC) motility. Indeed, Cdc42 is involved in actin polymerization and filopodia formation, whereas RhoA induces GC collapse and neurite retraction through actomyosin contraction. In this study we employed Förster Resonance Energy Transfer (FRET) microscopy to study the spatio-temporal dynamics of Cdc42 and RhoA in GCs in response to local Semaphorin-3A (Sema3A) stimulation obtained with lipid vesicles filled with Sema3A and positioned near the selected GC using optical tweezers. We found that Cdc42 and RhoA were activated at the leading edge of NG108-15 neuroblastoma cells during spontaneous cycles of protrusion and retraction, respectively. The release of Sema3A brought to a progressive activation of RhoA within 30 s from the stimulus in the central region of the GC that collapsed and retracted. In contrast, the same stimulation evoked waves of Cdc42 activation propagating away from the stimulated region. A more localized stimulation obtained with Sema3A coated beads placed on the GC, led to Cdc42 active waves that propagated in a retrograde manner with a mean period of 70 s, and followed by GC retraction. Therefore, Sema3A activates both Cdc42 and RhoA with a complex and different spatial-temporal dynamics.


Thursday, September 10, 2015

Nanofiber-excited plasmonic manipulation of polystyrene nanospheres

Y. Li, Y. J. Hu and Q. Wu

This paper reports optical nanofiber-excited plasmonic manipulation of polystyrene nanospheres. Gold nanoparticles (200 nm in diameter) are deposited on the surface of a nanofiber, and their local surface plasmon resonance (LSPR) is excited by the evanescent wave around the nanofiber. Theoretical results indicate that, the field enhancement resulting from the LSPR excitation can generate an enhanced gradient and scattering forces acting on the spheres, and an accelerated propulsion occurred. To verify the prediction, experiments were performed to trap and transport polystyrene nanospheres along a 500 nm diameter fiber decorated with gold nanoparticles, the average enhancement factor of the velocity of spheres for the LSPR case is found to be about 5 times with respect to the nanofiber case.


Escape forces and trajectories in optical tweezers and their effect on calibration

Ann A. M. Bui, Alexander B. Stilgoe, Nima Khatibzadeh, Timo A. Nieminen, Michael W. Berns, and Halina Rubinsztein-Dunlop

Whether or not an external force can make a trapped particle escape from optical tweezers can be used to measure optical forces. Combined with the linear dependence of optical forces on trapping power, a quantitative measurement of the force can be obtained. For this measurement, the particle is at the edge of the trap, away from the region near the equilbrium position where the trap can be described as a linear spring. This method provides the ability to measure higher forces for the same beam power, compared with using the linear region of the trap, with lower risk of optical damage to trapped specimens. Calibration is typically performed by using an increasing fluid flow to exert an increasing force on a trapped particle until it escapes. In this calibration technique, the particle is usually assumed to escape along a straight line in the direction of fluid-flow. Here, we show that the particle instead follows a curved trajectory, which depends on the rate of application of the force (i.e., the acceleration of the fluid flow). In the limit of very low acceleration, the particle follows the surface of zero axial optical force during the escape. The force required to produce escape depends on the trajectory, and hence the acceleration. This can result in variations in the escape force of a factor of two. This can have a major impact on calibration to determine the escape force efficiency. Even when calibration measurements are all performed in the low acceleration regime, variations in the escape force efficiency of 20% or more can still occur. We present computational simulations using generalized Lorenz–Mie theory and experimental measurements to show how the escape force efficiency depends on rate of increase of force and trapping power, and discuss the impact on calibration.


Calibrating oscillation response of a piezo-stage using optical tweezers

Jin-Hua Zhou, Di Li, Xin-Yao Hu, Min-Cheng Zhong, Zi-Qiang Wang, Lei Gong, Wei-Wei Liu, and Yin-Mei Li

In optical tweezers, a piezo-stage (PZT) is widely used to precisely position samples for force clamp, calibrating optical trap and stretching DNA. For a trapped bead in solution, the oscillation response of PZT is vital for all kinds of applications. A coupling ratio, actual amplitude to nominal amplitude, can be calibrated by power spectral density during sinusoidal oscillations. With oscillation frequency increasing, coupling ratio decreases in both x- and y-directions, which is also confirmed by the calibration with light scattering of scanning two aligned beads on slide. Those oscillation responses are related with deformability of chamber and the intrinsic characteristics of PZT. If we take nominal amplitude as actual amplitude for sinusoidal oscillations at 50 Hz, the amplitude is overestimated ~2 times in x-direction and ~3 times in y-direction. That will lead to huge errors for subsequent calibrations.


Hairpins under tension: RNA versus DNA

Mathilde Bercy and Ulrich Bockelmann

We use optical tweezers to control the folding and unfolding of individual DNA and RNA hairpins by force. Four hairpin molecules are studied in comparison: two DNA and two RNA ones. We observe that the conformational dynamics is slower for the RNA hairpins than for their DNA counterparts. Our results indicate that structures made of RNA are dynamically more stable. This difference might contribute to the fact that DNA and RNA play fundamentally different biological roles in spite of chemical similarity.


Micromechanics of cellularized biopolymer networks

Christopher A. R. Jones, Matthew Cibula, Jingchen Feng, Emma A. Krnacik, David H. McIntyre, Herbert Levine, and Bo Sun

Collagen gels are widely used in experiments on cell mechanics because they mimic the extracellular matrix in physiological conditions. Collagen gels are often characterized by their bulk rheology; however, variations in the collagen fiber microstructure and cell adhesion forces cause the mechanical properties to be inhomogeneous at the cellular scale. We study the mechanics of type I collagen on the scale of tens to hundreds of microns by using holographic optical tweezers to apply pN forces to microparticles embedded in the collagen fiber network. We find that in response to optical forces, particle displacements are inhomogeneous, anisotropic, and asymmetric. Gels prepared at 21 °C and 37 °C show qualitative difference in their micromechanical characteristics. We also demonstrate that contracting cells remodel the micromechanics of their surrounding extracellular matrix in a strain- and distance-dependent manner. To further understand the micromechanics of cellularized extracellular matrix, we have constructed a computational model which reproduces the main experiment findings.


Wednesday, September 9, 2015

Pushing, pulling, and squeezing our way to understanding mechanotransduction

Michael J. Siedlika, Victor D. Varnera, Celeste M. Nelson

Mechanotransduction is often described in the context of force-induced changes in molecular conformation, but molecular-scale mechanical stimuli arise in vivo in the context of complex, multicellular tissue structures. For this reason, we highlight and review experimental methods for investigating mechanotransduction across multiple length scales. We begin by discussing techniques that probe the response of individual molecules to applied force. We then move up in length scale to highlight techniques aimed at uncovering how cells transduce mechanical stimuli into biochemical activity. Finally, we discuss approaches for determining how these stimuli arise in multicellular structures. We expect that future work will combine techniques across these length scales to provide a more comprehensive understanding of mechanotransduction.


Optically induced rotation of Rayleigh particles by vortex beams with different states of polarization

Manman Li, Shaohui Yan, Baoli Yao, Yansheng Liang, Ming Lei, Yanlong Yang

Optical vortex beams carry optical orbital angular momentum (OAM) and can induce an orbital motion of trapped particles in optical trapping. We show that the state of polarization (SOP) of vortex beams will affect the details of this optically induced orbital motion to some extent. Numerical results demonstrate that focusing the vortex beams with circular, radial or azimuthal polarizations can induce a uniform orbital motion on a trapped Rayleigh particle, while in the focal field of the vortex beam with linear polarization the particle experiences a non-uniform orbital motion. Among the formers, the vortex beam with circular polarization induces a maximum optical torque on the particle. Furthermore, by varying the topological charge of the vortex beams, the vortex beam with circular polarization gives rise to an optimum torque superior to those given by the other three vortex beams. These facts suggest that the circularly polarized vortex beam is more suitable for rotating particles.


Dynamics and Mechanism of Laser Trapping-Induced Crystal Growth of Hen Egg White Lysozyme

Jing-Ru Tu, Ken-ichi Yuyama, Hiroshi Masuhara, and Teruki Sugiyama

We propose the dynamics and mechanism of laser trapping-induced crystal growth of hen egg-white lysozyme (HEWL). A continuous-wave near-infrared laser beam is used as a trapping light source and focused at a point 10 μm away from a target tetragonal HEWL crystal that is spontaneously generated in solution. Laser trapping of HEWL liquid-like clusters in solution increases local concentration in the focus, where the free motion and orientation of the clusters are strongly restricted, and the clusters show high rigidity and ordering. The cluster association and reorientation at the micrometer-sized focus is evolved to a large highly concentrated domain of the clusters, where the specific target crystal is grown. Initially, the high rigidity and ordering of the clusters strongly suppress the crystal grow rate compared to spontaneous crystal growth. Continuous laser trapping at the focus of the initially formed domain, however, leads to the transition to another domain with different concentration, rigidity, and ordering of the clusters, which surprisingly enhances the crystal growth rate. More interestingly, the clusters in both domains have anisotropic features reflecting the laser polarization direction, which also contributes to the crystal growth.


Robotic Cell Manipulation Using Optical Tweezers With Unknown Trapping Stiffness and Limited FOV

Xiang Li; Chien Chern Cheah

In existing control methods for optical tweezers, the trapping stiffness is usually assumed to be constant and known exactly. However, the stiffness varies according to the size of the trapped particle and is also dependant on the distance between the center of the laser beam and the particle. It is, therefore, difficult to identify the exact model of the trapping stiffness. In addition, it is also assumed that the entire workspace is visible within the field of view (FOV) of the microscope. During trapping and manipulation, certain image features such as the desired position may leave the FOV, and therefore, visual feedback is not available. In this paper, a robotic setpoint control technique is proposed for optical manipulation with unknown trapping stiffness and limited FOV of the microscope. The proposed method allows the system to operate beyond the FOV and perform trapping and manipulation tasks without any knowledge of the trapping stiffness. The stability of the overall system is analyzed by using Lyapunov-like method, with consideration of the dynamics of both the cell and the manipulator of laser source. Experimental results are presented to illustrate the performance of the proposed method.


New Trends on Optical Fiber Tweezers

Rodrigues Ribeiro, R.S.; Soppera, O.; Gonzalez Oliva, A.; Guerreiro, A.; Jorge, P.A.S.

In the last few decades, optical trapping has played an unique role concerning contactless trapping and manipulation of biological specimens. More recently, optical fiber tweezers (OFTs) are emerging as a desirable alternative to bulk optical systems. In this paper, an overview of the state of the art of OFTs is presented, focusing on the main fabrication methods, their features and main achievements. In addition, new OFTs fabricated by guided wave photo polymerization are reported. Their theoretical and experimental characterization is given and results demonstrating its application in the manipulation of yeast cells and the organelles of plant cells are presented.


Tuesday, September 8, 2015

Bubble wrap for optical trapping and cell culturing

Craig McDonald and David McGloin

In this paper, we demonstrate that the bubbles of bubble wrap make ideal trapping chambers for integration with low-cost optical manipulation. The interior of the bubbles is sterile and gas permeable, allowing for the bubbles to be used to store and culture cells, while the flat side of the bubble wrap is of sufficient optical quality to allow for optical trapping inside the bubbles. Through the use of a 100 W bulb to cure hanging droplets of PDMS, a low-cost optical trapping system was constructed. Effector T cells were cultured in bubble wrap for 8 days and then trapped with the PDMS droplet based optical manipulation. These techniques further demonstrate the opportunities for biophysical analysis afforded through repurposing common materials in resource-limited settings.


Time-shared optical tweezers with a microlens array for dynamic microbead arrays

Yoshio Tanaka and Shin-ichi Wakida
Dynamic arrays of microbeads and cells offer great flexibility and potential as platforms for sensing and manipulation applications in various scientific fields, especially biology and medicine. Here, we present a simple method for assembling and manipulating dense dynamic arrays based on time-shared scanning optical tweezers with a microlens array. Three typical examples, including the dynamic and simultaneous bonding of microbeads in real-time, are demonstrated. The optical design and the hardware setup for our approach are also described.


Theory of radiation pressure on magneto–dielectric materials

Stephen M Barnett and Rodney Loudon

We present a classical linear response theory for a magneto–dielectric material and determine the polariton dispersion relations. The electromagnetic field fluctuation spectra are obtained and polariton sum rules for their optical parameters are presented. The electromagnetic field for systems with multiple polariton branches is quantized in three dimensions and field operators are converted to 1–dimensional forms appropriate for parallel light beams. We show that the field–operator commutation relations agree with previous calculations that ignored polariton effects. The Abraham (kinetic) and Minkowski (canonical) momentum operators are introduced and their corresponding single–photon momenta are identified. The commutation relations of these and of their angular analogues support the identification, in particular, of the Minkowski momentum with the canonical momentum of the light. We exploit the Heaviside–Larmor symmetry of Maxwell's equations to obtain, very directly, the Einsetin–Laub force density for action on a magneto–dielectric. The surface and bulk contributions to the radiation pressure are calculated for the passage of an optical pulse into a semi–infinite sample.


Enhanced optical trapping via structured scattering

Michael A. Taylor, Muhammad Waleed, Alexander B. Stilgoe, Halina Rubinsztein-Dunlop & Warwick P. Bowen

Interferometry can completely redirect light, providing the potential for strong and controllable optical forces. However, small particles do not naturally act like interferometric beamsplitters and the optical scattering from them is not generally thought to allow efficient interference. Instead, optical trapping is typically achieved via deflection of the incident field. Here, we show that a suitably structured incident field can achieve beamsplitter-like interactions with scattering particles. The resulting trap offers order-of-magnitude higher stiffness than the usual Gaussian trap in one axis, even when constrained to phase-only structuring. We demonstrate trapping of 3.5–10.0 μm silica spheres, achieving a stiffness up to 27.5 ± 4.1 times higher than was possible using Gaussian traps as well as a two-orders-of-magnitude higher measured signal-to-noise ratio. These results are highly relevant to many applications, including cellular manipulation, fluid dynamics, micro-robotics and tests of fundamental physics.

Single-molecule measurement of the effective temperature in non-equilibrium steady states

E. Dieterich, J. Camunas-Soler, M. Ribezzi-Crivellari, U. Seifert & F. Ritort

Temperature is a well-defined quantity for systems in equilibrium. For glassy systems, it has been extended to the non-equilibrium regime, showing up as an effective quantity in a modified version of the fluctuation–dissipation theorem. However, experimental evidence supporting this definition remains scarce. Here, we present the first direct experimental demonstration of the effective temperature by measuring correlations and responses in single molecules in non-equilibrium steady states generated under external random forces. We combine experiment, analytical theory and simulations for systems with different levels of complexity, ranging from a single bead in an optical trap to two-state and multiple-state DNA hairpins. From these data, we extract a unifying picture for the existence of an effective temperature based on the relative order of various timescales characterizing intrinsic relaxation and external driving. Our study thus introduces driven small systems as a fertile ground to address fundamental concepts in statistical physics, condensed-matter physics and biophysics.

Friday, September 4, 2015

Plasmonic Nanoparticle Aggregates in High-Intensity Laser Fields: Effect of Pulse Duration

A. E. Ershov, A. P. Gavrilyuk, S. V. Karpov

We use an optodynamic model to study the interaction of pulsed laser radiation of different duration with mono- and polydisperse dimers and trimers of plasmonic nanoparticles as resonant domains of colloid Ag multiparticle aggregates. A comparative analysis of the influence of pulse duration on the kinetic characteristics of domains accompanied by the change in their local structure was carried out taking into account the intensity of incident radiation. The obtained results explain the reasons for laser photochromic reactions in materials containing colloidal aggregates of plasmonic nanoparticles.


The interplay of optical force and ray-optic behavior between Luneburg lenses

Alireza Akbarzadeh, J. A. Crosse, Mohammad Danesh, Cheng-Wei Qiu, Aaron J. Danner, and Costas M. Soukoulis

The method of force tracing is employed to examine the optomechanical interaction between two and four Luneburg lenses. Using a simplified analytical model as well as a realistic numerical model, the dynamics of elastic and fully inelastic collisions between the lenses under the illumination of collimated beams are studied. It is shown that elastic collisions cause a pair of Luneburg lenses to exhibit oscillatory and translational motions simultaneously. The combination of these two motions can be used to optomechanically manipulate small particles. Additionally, it is addressed how fully inelastic collision of four Luneburg lenses can help us achieve full transparency as well as isolating space to trap particles.


Intrinsic heating in optically trapped Au nanoparticles measured by dark-field spectroscopy

Ana Andres-Arroyo, Fan Wang, Wen Jun Toe, and Peter Reece

Assessing the degree of heating present when a metal nanoparticle is trapped in an optical tweezers is critical for its appropriate use in biological applications as a nanoscale force sensor. Heating is necessarily present for trapped plasmonic particles because of the non-negligible extinction which contributes to an enhanced polarisability. We present a robust method for characterising the degree of heating of trapped metallic nanoparticles, using the intrinsic temperature dependence of the localised surface plasmon resonance (LSPR) to infer the temperature of the surrounding fluid at different incident laser powers. These particle specific measurements can be used to infer the rate of heating and local temperature of trapped nanoparticles. Our measurements suggest a considerable amount of a variability in the degree of heating, on the range of 414–673 K/W, for different 100 nm diameter Au nanoparticles, and we associated this with variations in the axial trapping position.


Radiation torques exerted on a sphere by focused Laguerre-Gaussian beams

Huachao Yu and Weilong She

Radiation torques exerted by focused Laguerre-Gaussian beams on a microsphere are investigated with the generalized Lorenz-Mie theory. It is shown that the spin and orbital torques, in particular, those exerted by circularly polarized beams, can be far larger than the net torque in magnitude, which suggests that they are not merely related to the turning effect on the sphere. It is found that they also include the contribution from the conversion between the spin and the orbital angular momentum induced by light scattering. Separations of the spin and orbital torques into mechanical and conversion components are presented. The dependences of these torque components on the beam's and sphere's parameters are also discussed.


Regulation of integrins in platelets

Joel S. Bennett

Blood platelets prevent bleeding after trauma by forming occlusive aggregates at sites of vascular injury. Platelet aggregation is mediated by the integrin heterodimer αIIbβ3 and occurs when platelet agonists generated at the injury site convert αIIbβ3 from its resting to its active conformation. Active αIIbβ3 is then able to bind macromolecular ligands such as fibrinogen that crosslink adjacent platelets into hemostatic aggregates. Platelets circulate in a plasma milieu containing high concentrations of the principal αIIbβ3 ligand fibrinogen. Thus, αIIbβ3 activity is tightly regulated to prevent the spontaneous formation of platelet aggregates. αIIbβ3 activity is regulated at least three levels. First, intramolecular interactions involving motifs located in the membrane-proximal stalk regions, transmembrane domains, and the membrane-proximal cytosolic tails of αIIb and β3 maintain αIIbβ3 in its inactive conformation. Transmembrane domain interactions appear particularly important because disrupting these interactions causes constitutive αIIbβ3 activation. Second, the agonist-stimulated binding of the cytosolic proteins talin and kindlin-3 to the β3 cytosolic tail rapidly causes αIIbβ3 activation by disrupting the intramolecular interactions constraining αIIbβ3 activity. Third, the strength of ligand binding to active αIIbβ3 seems to be allosterically regulated. Thus, αIIbβ3 exists in a minimum of three interconvertible states: an inactive (resting) state that does not interact with ligands and two active ligand binding states that differ in their affinity for fibrinogen and in the mechanical stability of fibrinogen complexes they form.