Thursday, May 31, 2012

Optical forces on cylinders near subwavelength slits: effects of extraordinary transmission and excitation of Mie resonances

F. J. Valdivia-Valero and M. Nieto-Vesperinas

We study the optical forces on particles, either dielectric or metallic, in or out their Mie resonances, near a subwavelength slit in extraordinary transmission regime. Calculations are two-dimensional, so that those particles are infinite cylinders. Illumination is with p-polarization. We show that the presence of the slit enhances by two orders of magnitude the transversal forces of optical tweezers from a beam alone. In addition, a drastically different effect of these particle resonances on the optical forces that they experience; namely, we demonstrate an enhancement of these forces, also of binding nature, at plasmon resonance wavelengths on metallic nanocylinders, whereas dielectric cylinders experience optical forces that decrease at wavelengths exciting their whispering gallery modes. Particles located at the entrance of the slit are easily bound to apertures due to the coincidence in the forward direction of scattering and gradient forces, but those particles at the exit of the slit suffer a competition between forward scattering force components and backward gradient forces which make more complex the bonding or antibonding nature of the resulting mechanical action.


Tuning the detection sensitivity: a model for axial backfocal plane interferometric tracking

Lars Friedrich and Alexander Rohrbach

Backfocal plane (BFP) interferometry is a single particle tracking technique that allows one to measure minutedisplacements of a microscopic particle from the center of a beam’s focus in three dimensions. In this Letter, we present a Fourier optics model to describe the interference effects that allow one to track the position of a particle moving along the optical axis. A detection numerical aperture is derived theoretically and confirmed experimentally, within which the interference intensity has a positive correlation with the axial position of the scatterer. For larger detection angles, the correlation is negative. The model helps to understand previously reported measurements and to optimize BFP interferometric tracking.


Monday, May 28, 2012

Electrostrictive counterforce on fluid microdroplet in short laser pulse

S. Å. Ellingsen and I. Brevik

When a micrometer-sized fluid droplet is illuminated by a laser pulse, there is a fundamental distinction between two cases. If the pulse is short in comparison with the transit time for sound across the droplet, the disruptive optical Abraham–Minkowski radiation force is countered by electrostriction, and the net stress is compressive. In contrast, if the pulse is long on this scale, electrostriction is cancelled by elastic pressure and the surviving term of the electromagnetic force, the Abraham–Minkowski force, is disruptive and deforms the droplet. Ultrashort laser pulses are routinely used in modern experiments, and impressive progress has moreover been made on laser manipulation of liquid surfaces in recent times, making a theory for combining the two pertinent. We analyze the electrostrictive contribution analytically and numerically for a spherical droplet.


Sunday, May 27, 2012

Optical manipulation of particle ensembles in air

Vladlen G. Shvedov, Cyril Hnatovsky, Natalia Shostka, Andrei V. Rode, and Wieslaw Krolikowski

We demonstrate that airborne light-absorbing particles can be photophoretically trapped and moved inside an optical lattice formed by multiple-beam interference. This technique allows simultaneous three-dimensional manipulation of multiple micro-objects in gases.


Saturday, May 26, 2012

Nascent RNA structure modulates the transcriptional dynamics of RNA polymerases

Bradley Zamft, Lacramioara Bintu, Toyotaka Ishibashi, and Carlos Bustamante

RNA polymerase pausing represents an important mechanism of transcriptional regulation. In this study, we use a single-molecule transcription assay to investigate the effect of template base-pair composition on pausing by RNA polymerase II and the evolutionarily distinct mitochondrial polymerase Rpo41. For both enzymes, pauses are shorter and less frequent on GC-rich templates. Significantly, incubation with RNase abolishes the template dependence of pausing. A kinetic model, wherein the secondary structure of the nascent RNA poses an energetic barrier to pausing by impeding backtracking along the template, quantitatively predicts the pause densities and durations observed. The energy barriers extracted from the data correlate well with RNA folding energies obtained from cotranscriptional folding simulations. These results reveal that RNA secondary structures provide a cis-acting mechanism by which sequence modulates transcriptional elongation.


Friday, May 25, 2012

Equilibrium orientations and positions of non-spherical particles in optical traps

Yongyin Cao, Alexander B Stilgoe, Lixue Chen, Timo A Nieminen, and Halina Rubinsztein-Dunlop

Dynamic simulation is a powerful tool to observe the behavior of arbitrary shaped particles trapped in a focused laser beam. Here we develop a method to find equilibrium positions and orientations using dynamic simulation. This general method is applied to micro- and nano-cylinders as a demonstration of its predictive power. Orientation landscapes for particles trapped with beams of differing polarisation are presented. The torque efficiency of micro-cylinders at equilibrium in a plane is also calculated as a function of tilt angle. This systematic investigation elucidates in both the function and properties of micro- and nano-cylinders trapped in optical tweezers.


Thursday, May 24, 2012

Biomimetic three-dimensional microenvironment for controlling stem cell fate

Hu Zhang, Sheng Dai, Jingxiu Bi and Kuo-Kang Liu

Stem cell therapy is an emerging technique which is being translated into treatment of degenerated tissues. However, the success of translation relies on the stem cell lineage commitment in the degenerated regions of interest. This commitment is precisely controlled by the stem cell microenvironment. Engineering a biomimetic three-dimensional microenvironment enables a thorough understanding of the mechanisms of governing stem cell fate. We review the individual microenvironment components, including soluble factors, extracellular matrix, cell–cell interaction and mechanical stimulation. The perspectives in creating the biomimetic microenvironments are discussed with emerging techniques.


Wednesday, May 23, 2012

Enhancement of Mechanical Q Factors by Optical Trapping

K.-K. Ni, R. Norte, D. J. Wilson, J. D. Hood, D. E. Chang, O. Painter, and H. J. Kimble

The quality factor of a mechanical resonator is an important figure of merit for various sensing applications and for observing quantum behavior. Here, we demonstrate a technique to push the quality factor of a micromechanical resonator beyond conventional material and fabrication limits by using an optical field to stiffen or trap a particular motional mode. Optical forces increase the oscillation frequency by storing most of the mechanical energy in a nearly lossless optical potential, thereby strongly diluting the effect of material dissipation. By placing a 130 nm thick SiO2 pendulum in an optical standing wave, we achieve an increase in the pendulum center-of-mass frequency from 6.2 to 145 kHz. The corresponding quality factor increases 50-fold from its intrinsic value to a final value of Q=5.8(1.1)×105, representing more than an order of magnitude improvement over the conventional limits of SiO2 for this geometry. Our technique may enable new opportunities for mechanical sensing and facilitate observations of quantum behavior in this class of mechanical systems.


Speeding Up Particle Trajectory Simulations Under Moving Force Fields using Graphic Processing Units

Robert Patro, John P. Dickerson, Sujal Bista, Satyandra K. Gupta, Amitabh Varshney

In this paper, we introduce a graphic processing unit (GPU)-based framework for simulating particle trajectories under both static and dynamic force fields. By exploiting the highly parallel nature of the problem and making efficient use of the available hardware, our simulator exhibits a significant speedup over its CPU-based analog. We apply our framework to a specific experimental simulation: the computation of trapping probabilities associated with micron-sized silica beads in optical trapping workbenches. When evaluating large numbers of trajectories (4096), we see approximately a 356 times speedup of the GPU-based simulator over its CPU-based counterpart.


Monday, May 21, 2012

Sustained glycolytic oscillations in individual, isolated yeast cells

Anna-Karin Gustavsson, David D. van Niekerk, Caroline B. Adiels, Franco du Preez, Mattias Goksor, Jacky L. Snoep

Yeast glycolytic oscillations have been studied since the 1950s in cell free extracts and in intact cells. Thus far, for intact cells sustained oscillations have only been observed at the population level, i.e. for synchronized cultures at high biomass concentrations. Using optical tweezers to position yeast cells in a microuidic chamber, we were able to observe sustained oscillations in individual, isolated cells. With a detailed kinetic model for the cellular reactions we could simulate the heterogeneity in the response of the individual cells assuming small diferences in a single internal parameter. This is the first time that sustained limit cycle oscillations are shown in isolated yeast cells.


Saturday, May 19, 2012

Optical tweezers: light for manipulating microscopic world

Raktim Dasgupta

Optical tweezers make use of a tightly focused laser beam to trap, move, guide, rotate and even sort microscopic objects solely with light. Although the basic laser tweezers, making use of a TEM00 laser beam to create a single trap point, have proved to be useful for any applications in areas ranging from physics to biology, a major breakthrough in this field came as the use of computer generated holograms enabled researchers to create multiple trap sites from single laser source (holographic optical tweezers). Coupled with microfluidic techniques, holographic optical tweezers have promised development of optical techniques for high throughput sorting of different cell types under a single micro-chip platform. The holographic methods have also helped the use of specialized laser beams like Laguerre-Gaussian beams instead of the conventional laser beam for interesting applications like orienting/rotating the trapped objects or trapping cells with minimum photodamage. Further, combining optical tweezers with Raman spectroscopy is becoming increasingly popular for studying single cell biochemistry as use of optical forces to immobilize the cells under investigations not only avoids the negative effects of fixing the cells onto substrate but also improve the quality of the recorded spectra. These advanced optical trapping techniques as outlined above along with some illustrative biophotonics applications have been explored.


Dynamic antibody binding properties in the pathogenesis of HIT

Bruce S. Sachais, Rustem I. Litvinov, Serge V. Yarovoi, Lubica Rauova, Jillian L. Hinds, Ann H. Rux, Gowthami M. Arepally, Mortimer Poncz, Adam Cuker, John W. Weisel, and Douglas B. Cines

Rapid laboratory assessment of heparin-induced thrombocytopenia (HIT) is important for disease recognition and management. The utility of contemporary immunoassays to detect anti-platelet factor 4 (PF4)/heparin antibodies is hindered by identification of antibodies unassociated with disease. To begin to distinguish properties of pathogenic anti-PF4/heparin antibodies, we compared isotype-matched monoclonal antibodies that bind to different epitopes: KKO causes thrombocytopenia in an in vivo model of HIT, while RTO does not. KKO binding to PF4/heparin is specifically inhibited by human HIT-antibodies that activate platelets, while inhibition of RTO binding is not differentially affected. Heparin increased the avidity of KKO binding to PF4 without affecting RTO, but did not increase total-binding or binding to non-tetrameric PF4K50E. Single-molecule forced unbinding demonstrated KKO was 8-fold more reactive towards PF4 tetramers and formed stronger complexes than RTO, but not to PF4K50Edimers. KKO, but not RTO, promoted oligomerization of PF4, but not PF4K50E. These studies reveal differences in the properties of anti-PF4 antibodies that cause thrombocytopenia not revealed by ELISA that correlate with oligomerization of PF4 and sustained high-avidity interactions that may simulate transient antibody-antigen interactions in vivo. These differences suggest the potential importance of epitope specificity in the pathogenesis of HIT. 

Characterization of Bessel beams generated by polymeric microaxicons

F Merola, S Coppola, V Vespini, S Grilli and P Ferraro

We present a quick, simple and accurate digital holographic characterization of the Bessel beams produced by polymeric microaxicons. This technique allows the numerical reconstruction of both intensity and phase of the beam at whichever point starting from a single acquired hologram. From these data, it is possible to go back to the axicon structure, and to gather information about their characteristics. In particular, the focal length and the depth of focus of the axicon lens are experimentally measured, and the full width at half maximum of the beam is obtained too. The depth of focus, very large for a Bessel beam with respect to a Gaussian one, is successfully exploited for optical trapping of micrometric objects.


Thursday, May 17, 2012

Microassembly of complex and three-dimensional microstructures using holographic optical tweezers

R Ghadiri, T Weigel, C Esen and A Ostendorf

In this paper we investigate a flexible method for the fabrication of complex microstructures using binding microparticles. Utilizing optical forces, micro-objects are caught, positioned and used as building blocks to form defined structures, analogous to assembling processes in the macroscopic world. Durable linkage between the individual particles is realized using biomolecules with high affinities applied as particle coatings. Planar structures can be assembled employing optical manipulation as well as three-dimensional patterns by stacking the generated layers. Even the properties of the generated structures can be locally designed as desired by using building blocks from diverse materials exhibiting different properties. This method benefits from its simplicity and the potential extensibility of the fabricated structure at any time of the experiment.


Optimized back-focal-plane interferometry directly measures forces of optically trapped particles

Arnau Farré, Ferran Marsà, and Mario Montes-Usategui

Back-focal-plane interferometry is used to measure displacements of optically trapped samples with very high spatial and temporal resolution. However, the technique is closely related to a method that measures the rate of change in light momentum. It has long been known that displacements of the interference pattern at the back focal plane may be used to track the optical force directly, provided that a considerable fraction of the light is effectively monitored. Nonetheless, the practical application of this idea has been limited to counter-propagating, low-aperture beams where the accurate momentum measurements are possible. Here, we experimentally show that the connection can be extended to single-beam optical traps. In particular, we show that, in a gradient trap, the calibration product κ·β (where κ is the trap stiffness and 1/β is the position sensitivity) corresponds to the factor that converts detector signals into momentum changes; this factor is uniquely determined by three construction features of the detection instrument and does not depend, therefore, on the specific conditions of the experiment. Then, we find that force measurements obtained from back-focal-plane displacements are in practice not restricted to a linear relationship with position and hence they can be extended outside that regime. Finally, and more importantly, we show that these properties are still recognizable even when the system is not fully optimized for light collection. These results should enable a more general use of back-focal-plane interferometry whenever the ultimate goal is the measurement of the forces exerted by an optical trap.


Optical shield: measuring viscosity of turbid fluids using optical tweezers

M. P. Lee, A. Curran, G. M. Gibson, M. Tassieri, N. R. Heckenberg, and M. J. Padgett

The viscosity of a fluid can be measured by tracking the motion of a suspended micron-sized particle trapped by optical tweezers. However, when the particle density is high, additional particles entering the trap compromise the tracking procedure and degrade the accuracy of the measurement. In this work we introduce an additional Laguerre–Gaussian, i.e. annular, beam surrounding the trap, acting as an optical shield to exclude contaminating particles.


Plasmonic optical trap having very large active volume realized with nano-ring structure

Zhiwen Kang, Haixi Zhang, Haifei Lu, Jianbin Xu, Hock-Chun Ong, Ping Shum, and Ho-Pui Ho

The feasibility of using gold nano-rings as plasmonic nano-optical tweezers is investigated. We found that at a resonant wavelength of λ=785  nm, the nano-ring produces a maximum trapping potential of ∼32kBT on gold nanoparticles. The existence of multiple potential wells results in a very large active volume of ∼106  nm3 for trapping the target particles. The report nano-ring design provides an effective approach for manipulating nano-objects in very low concentration into the high-field region and is well suited for integration with microfluidics for lab-on-a-chip applications.


Monday, May 14, 2012

Active DNA unwinding dynamics during processive DNA replication

José A. Morin, Francisco J. Cao, José M. Lázaro, J. Ricardo Arias-Gonzalez, José M. Valpuesta, José L. Carrascosa, Margarita Salas, and Borja Ibarra

Duplication of double-stranded DNA (dsDNA) requires a fine-tuned coordination between the DNA replication and unwinding reactions. Using optical tweezers, we probed the coupling dynamics between these two activities when they are simultaneously carried out by individual Phi29 DNA polymerase molecules replicating a dsDNA hairpin. We used the wild-type and an unwinding deficient polymerase variant and found that mechanical tension applied on the DNA and the DNA sequence modulate in different ways the replication, unwinding rates, and pause kinetics of each polymerase. However, incorporation of pause kinetics in a model to quantify the unwinding reaction reveals that both polymerases destabilize the fork with the same active mechanism and offers insights into the topological strategies that could be used by the Phi29 DNA polymerase and other DNA replication systems to couple unwinding and replication reactions.


Optical Manipulation in Liquid Crystals: Effects of Tight Focusing on Nonlinear Optical Reorientation

F. Simoni, F. Aieta, F. Bracalente, L. Criante & L. Lucchetti

We report a detailed analysis of the problem of nonlinear optical reorientation induced in a nematic liquid crystal by a tightly focused Gaussian beam under experimental conditions that prevent the effect of conventional trapping originated by optical gradient forces. Novel features arise under these conditions: no threshold effect; reduced reorientation if compared the one induced by a paraxial Gaussian beam, orientational singularity at the focal waist.


Saturday, May 12, 2012

Direct Manipulation of Malaria Parasites with Optical Tweezers Reveals Distinct Functions of Plasmodium Surface Proteins

Stephan Hegge, Kai Uhrig, Martin Streichfuss, Gisela Kynast-Wolf, Kai Matuschewski, Joachim P. Spatz, and Friedrich Frischknecht

Plasmodium sporozoite motility is essential for establishing malaria infections. It depends on initial adhesion to a substrate as well as the continuous turnover of discrete adhesion sites. Adhesion and motility are mediated by a dynamic actin cytoskeleton and surface proteins. The mode of adhesion formation and the integration of adhesion forces into fast and continuous forward locomotion remain largely unknown. Here, we use optical tweezers to directly trap individual parasites and probe adhesion formation. We find that sporozoites lacking the surface proteins TRAP and S6 display distinct defects in initial adhesion. trap(-) sporozoites adhere preferentially with their front end, while s6(-) sporozoites show no such preference. The cohesive strength of the initial adhesion site is differently affected by actin filament depolymerization at distinct adhesion sites along the parasite for trap(-) and s6(-) sporozoites. These spatial differences between TRAP and S6 in their functional interaction with actin filaments show that these proteins have non-redundant roles during adhesion and motility. We suggest that complex protein-protein interactions and signaling events govern the regulation of parasite gliding at different sites along the parasite. Investigating how these events are coordinated will be essential for our understanding of sporozoite gliding motility, which is crucial for malaria infection. Laser tweezers will be an valuable part of the toolset.


Friday, May 11, 2012

Optical trapping of gold nanoparticles by cylindrical vector beam

Lu Huang, Honglian Guo, Jiafang Li, Lin Ling, Baohua Feng, and Zhi-Yuan Li

Optical trapping of gold nanoparticles is experimentally demonstrated using radially and azimuthally polarized beams. The transverse optical trapping stiffness of gold nanoparticles is measured. The radially polarized beam exhibits a higher trapping efficiency than the azimuthally polarized beam and the Gaussian beam. The transverse stiffness of particles with different diameters is measured experimentally and calculated via the discrete-dipole approximation method, and good agreement between theory and experiment is found.


Microfluidic sorting with blinking optical traps

R. Dasgupta, R. S. Verma, and P. K. Gupta

It is shown that by appropriately choosing the periodicity of a blinking optical trap only larger sized colloidal spheres can be selectively trapped out of a mixed population. This happens because smaller sized, more agile, spheres escape out of the trap volume during the off period of the trap beam. Therefore, by scanning an array of blinking traps over a mixed sample, bigger spheres can be forced to move with the traps and eventually could be taken to the output side. Experimental demonstration of sorting between 1 µm and 2 µm diameter silica spheres is presented.


Tuesday, May 8, 2012

Manipulation of gold nanorods with dual-optical tweezers for surface plasmon resonance control

Lin Ling, Hong-Lian Guo, Xiao-Lan Zhong, Lu Huang, Jia-Fang Li, Lin Gan and Zhi-Yuan Li

Gold nanorods are too tiny to be manipulated using conventional mechanical methods. In this paper, we demonstrate the trapping, transferring, positioning and patterning of gold nanorods with dual-optical tweezers. The convenient manipulations are achieved by taking advantage of the longitudinal surface plasmon resonance of gold nanorods and the anisotropic optical trapping forces formed by two linearly polarized Gaussian beams. The trapped gold nanoparticles are positioned extremely firmly and quickly on a substrate compared with randomly dispersed ones. It is observed that gold nanorods show advantages over gold nanospheres with regard to positioning speed and stability. More importantly, versatile plasmon coupling effects have been achieved in some patterned nanorods.


Mechanism of Homology Recognition in DNA Recombination from Dual-Molecule Experiments

Iwijn De Vlaminck, Marijn T.J. van Loenhout, Ludovit Zweifel, Johan den Blanken, Koen Hooning, Susanne Hage, Jacob Kerssemakers, Cees Dekker

In E. coli homologous recombination, a filament of RecA protein formed on DNA searches and pairs a homologous sequence within a second DNA molecule with remarkable speed and fidelity. Here, we directly probe the strength of the two-molecule interactions involved in homology search and recognition using dual-molecule manipulation, combining magnetic and optical tweezers. We find that the filament's secondary DNA-binding site interacts with a single strand of the incoming double-stranded DNA during homology sampling. Recognition requires opening of the helix and is strongly promoted by unwinding torsional stress. Recognition is achieved upon binding of both strands of the incoming dsDNA to each of two ssDNA-binding sites in the filament. The data indicate a physical picture for homology recognition in which the fidelity of the search process is governed by the distance between the DNA-binding sites.


Principles of single-molecule manipulation and its application in biological physics


Recent advances in nanoscale manipulation and piconewton force detection provide a unique tool for studying the mechanical and thermodynamic properties of biological molecules and complexes at the single-molecule level. Detailed equilibrium and dynamics information on proteins and DNA have been revealed by single-molecule manipulation and force detection techniques. The atomic force microscope (AFM) and optical tweezers have been widely used to quantify the intra- and inter-molecular interactions of many complex biomolecular systems. In this article, we describe the background, analysis, and applications of these novel techniques. Experimental procedures that can serve as a guide for setting up a single-molecule manipulation system using the AFM are also presented.


Friday, May 4, 2012

Stability analysis and thermal motion of optically trapped nanowires

S H Simpson and S Hanna

We investigate the stability and thermal motion of optically trapped nanowires, with aspect ratios in the range 10–100. A simple analytical model is used to determine qualitative features of the system, assuming that the nanowire is weakly scattering and the incident beam is paraxial. As expected, the model predicts that the nanowire will align with the beam axis. In this configuration the translational stiffness coefficients of the trap approach their limiting values for long nanowires like 𝒪(L−3), where L is the nanowire length, the limit for the stiffness parallel to the beam axis being zero. The rotational stiffness coefficients vary more slowly, according to 𝒪(L−1). Also, it is predicted that defocusing decreases the translational stiffness perpendicular to the beam, while increasing rotational stiffness. These findings are reinforced by comparison with rigorous electromagnetic calculations which additionally reveal the effects of radiation pressure and finite scattering. A strong polarization effect is observed in the numerical simulations and coupled translational and rotational motions arise which influence the trap stability. The use of nanowire traps for force sensing is discussed.


Hollow-core photonic crystal fiber based multifunctional optical system for trapping, position sensing, and detection of fluorescent particles

V. K. Shinoj and V. M. Murukeshan

We demonstrate a novel multifunctional optical system that is capable of trapping, imaging, position sensing, and fluorescence detection of micrometer-sized fluorescent test particles using hollow-core photonic crystal fiber (HC-PCF). This multifunctional optical system for trapping, position sensing, and fluorescent detection is designed such that a near-IR laser light is used to create an optical trap across a liquid-filled HC-PCF, and a 473 nm laser is employed as a source for fluorescence excitation. This proposed system and the obtained results are expected to significantly enable an efficient integrated trapping platform employing HC-PCF for diagnostic biomedical applications.


Thursday, May 3, 2012

Optical Tweezers with Assistance of Sub-Microsecond-Duration Pulse Laser Beam

Saki Maeda, Tadao Sugiura, and Kotaro Minato

We report optical tweezers with the assistance of a strong instantaneous force generated by a focused pulse laser beam of sub-microsecond duration. A strong instantaneous force is required in biological applications. A suitable pulse duration for pulse assistance in water is derived on the order of 100 to 1000 ns from motion analysis of a micrometer-sized particle. We performed optical tweezers experiments with a focused pulse beam of 160 ns duration coaxially incident with a CW laser beam. From the experiments on particle extraction from a glass surface, the required energy for extraction is smaller than the case of 150 µs duration by a factor of 60.


Two examples of using physical mechanics approach to evaluate colloidal stability

ZhiWei Sun and ShengHua Xu

Since Mr. Tsien brought up his idea of physical mechanics, as a new field in engineering science, to public attention in the early 50’s of the 20th century, innumerable application examples of physical mechanics approach in diverse fields have manifested its strong vitality increasingly. One of important aspects in applications of physical mechanics is to appropriately choose the microscopic quantity for the system in consideration and build a bridge to connect its relevant microscopic information to its desired macroscopic properties. We present two unique cases of using the physical mechanics approach to study colloidal stability. In the first case we measured the outcomes from artificially induced collisions at individual particle levels, by means of directly observing artificially induced collisions with the aid of optical tweezers. In the second case, by using T-matrix method, the microscopic quantity extinction cross section of the doublet can be accurately evaluated and therefore the measurement range and accuracy of the turbidity methodology for determining the CRC are greatly improved.


Torque Generation of Kinesin Motors Is Governed by the Stability of the Neck Domain

Melanie Brunnbauer, Renate Dombi, Thi-Hieu Ho, Manfred Schliwa, Matthias Rief, Zeynep Ökten

In long-range transport of cargo, prototypical kinesin-1 steps along a single protofilament on the microtubule, an astonishing behavior given the number of theoretically available binding sites on adjacent protofilaments. Using a laser trap assay, we analyzed the trajectories of several representatives from the kinesin-2 class on freely suspended microtubules. In stark contrast to kinesin-1, these motors display a wide range of left-handed spiraling around microtubules and thus generate torque during cargo transport. We provide direct evidence that kinesin's neck region determines the torque-generating properties. A model system based on kinesin-1 corroborates this result: disrupting the stability of the neck by inserting flexible peptide stretches resulted in pronounced left-handed spiraling. Mimicking neck stability by crosslinking significantly reduced the spiraling of the motor up to the point of protofilament tracking. Finally, we present a model that explains the physical basis of kinesin's spiraling around the microtubule. 

Circular motion of particles suspended in a Gaussian beam with circular polarization validates the spin part of the internal energy flow

O. V. Angelsky, A. Ya. Bekshaev, P. P. Maksimyak, A. P. Maksimyak, I. I. Mokhun, S. G. Hanson, C. Yu. Zenkova, and A. V. Tyurin

Non-spherical dielectric microparticles were suspended in a water-filled cell and exposed to a coherent Gaussian light beam with controlled state of polarization. When the beam polarization is linear, the particles were trapped at certain off-axial position within the beam cross section. After switching to the right (left) circular polarization, the particles performed spinning motion in agreement with the angular momentum imparted by the field, but they were involved in an orbital rotation around the beam axis as well, which in previous works [Y. Zhao et al, Phys. Rev. Lett. 99, 073901 (2007)] was treated as evidence for the spin-to orbital angular momentum conversion. Since in our realization the moderate focusing of the beam excluded the possibility for such a conversion, we consider the observed particle behavior as a demonstration of the macroscopic “spin energy flow” predicted by the theory of inhomogeneously polarized paraxial beams [A. Bekshaev et al, J. Opt. 13, 053001 (2011)].


Tuesday, May 1, 2012

Looking through the mirror: Optical microcavity-mirror image photonic interaction

Lei Shi, E. Xifré-Pérez, F. J. García de Abajo, and F. Meseguer

Although science fiction literature and art portray extraordinary stories of people interacting with their images behind a mirror, we know that they are not real and belong to the realm of fantasy. However, it is well known that charges or magnets near a good electrical conductor experience real attractive or repulsive forces, respectively, originating in the interaction with their images. Here, we show strong interaction between an optical microcavity and its image under external illumination. Specifically, we use silicon nanospheres whose high refractive index makes well-defined optical resonances feasible. The strong interaction produces attractive and repulsive forces depending on incident wavelength, cavity-metal separation and resonance mode symmetry. These intense repulsive photonic forces warrant a new kind of optical levitation that allows us to accurately manipulate small particles, with important consequences for microscopy, optical sensing and control of light by light at the nanoscale.


Spectroscopy of 3D-trapped particles inside a hollow-core microstructured optical fiber

Charithra Rajapakse, Fan Wang, Tiffany C. Y. Tang, Peter J. Reece, Sergio G. Leon-Saval, and Alexander Argyros

We report on the demonstration of three-dimensional optical trapping inside the core of a hollow-core microstructured optical fiber specifically designed and fabricated for this purpose. Optical trapping was achieved by means of an external tweezers beam incident transversely on the fiber and focused through the fiber cladding. Trapping was achieved for a range of particle sizes from 1 to 5 µm, and manipulation of the particles in three-dimensions through the entire cross-section of the fiber core was demonstrated. Spectroscopy was also performed on single fluorescent particles, with the fluorescence captured and guided in the fiber core. Video tracking methods allowed the optical traps to be characterized and photobleaching of single particles was also observed and characterized.


Fluctuations, linear response and heat flux of an aging system

J. R. Gomez-Solano, A. Petrosyan and S. Ciliberto

We measure the fluctuations of the position of a Brownian particle confined by an optical trap in an aging gelatin droplet after a fast quench. Its linear response to an external perturbation is also measured. We compute the spontaneous heat flux from the particle to the bath due to the non-equilibrium formation of the gel. We show that the mean heat flux is quantitatively related to the violation of the equilibrium fluctuation-dissipation theorem as a measure of the broken detailed balance during the aging process.


Effect of polarization on transport of particles in air by optical vortex beam

N O Eckerskorn, N Zeng, V G Shvedov, W Krolikowski and A V Rode

Experiments on transport of spherical particles in air by optical vortex beam show that the speed of transport depends drastically on light polarization. There is a clear correlation between the speed of particle transport in a pipeline formed by cross-polarized vortices: a horizontally polarized beam moves particles faster than a vertically polarized one. To elucidate this effect we demonstrate, both in theory and experiments, that a radial shift of particles away from the vortex axis due to gravity results in polarization dependence of the laser intensity absorbed by the particle and thus determines the speed of transport. The results demonstrate an additional degree of freedom to control particle transport by varying the polarization of the driving vortex beams.


Erythrocyte deformation in high-throughput optical stretchers

Ihab Sraj, Alex C. Szatmary, Sanjay A. Desai, David W. M. Marr, and Charles D. Eggleton
Optical stretchers can be used to quantify elastic and homeostatic properties of cells. Because they can apply forces to cells without requiring direct contact, they may noninvasively measure mechanical properties related to cell and membrane health. Present-day optical stretchers are, however, limited to measurements on individual stationary cells, limiting throughput. To overcome this limitation and allow study of variations in cell populations, we recently developed and tested a microfluidic chamber that measures optical stretching parameters for erythrocytes under dynamic flowing conditions. The method uses a single linear diode laser bar and permitted measurements at low flow rates and higher throughput. Here, we numerically investigate the feasibility of further increasing the measurement rates of the optical stretcher in parameter domains where hydrodynamic and optical forces are of comparable magnitude. To do this we couple a recently implemented dynamic optical ray-tracing technique with a fluid-structure interaction solver to simulate the deformation of osmotically swollen erythrocytes in fluid flow of variable rate. Our results demonstrate that a detectable steady-state stretch is induced at nominal optical powers and flow rates. In addition, we find that flow rates can be increased significantly with no major effect on net cell stretch showing the feasibility of application of this technique at greatly increased throughputs.