Dispersion and optical gradient force from high-order mode coupling between two hyperbolic metamaterial waveguides

Guanghui Wang, Weifeng Zhang, Jiahui Lu, Huijun Zhao

We analytically study dispersion properties and optical gradient forces of different-order transverse magnetic (TM) modes in two coupled hyperbolic metamaterial waveguides (HMMWs). According to Maxwell's equations, we obtain the dispersion relation of symmetric and antisymmetric modes, and calculate optical gradient forces of different-order modes by using Maxwell stress tensor. Numerical results show that the dispersion properties are dependent on the filling ratio, and the optical gradient forces of high-order TM modes are larger than the fundamental mode when the gap between two HMMWs is very narrow, but they weaken much faster than the case of low-order TM modes with the gap width increasing. In addition, the effects of the dielectric surrounding of waveguides on the coupling effect and optical gradient force are clarified. These properties offer an avenue for various optomechanical applications in optical sensors and actuators.

DOI

Knotting and unknotting of a protein in single molecule experiments

Fabian Ziegler, Nicole C. H. Lim, Soumit Sankar Mandal, Benjamin Pelz, Wei-Ping Ng, Michael Schlierf, Sophie E. Jackson, and Matthias Rief

Spontaneous folding of a polypeptide chain into a knotted structure remains one of the most puzzling and fascinating features of protein folding. The folding of knotted proteins is on the timescale of minutes and thus hard to reproduce with atomistic simulations that have been able to reproduce features of ultrafast folding in great detail. Furthermore, it is generally not possible to control the topology of the unfolded state. Single-molecule force spectroscopy is an ideal tool for overcoming this problem: by variation of pulling directions, we controlled the knotting topology of the unfolded state of the 52-knotted protein ubiquitin C-terminal hydrolase isoenzyme L1 (UCH-L1) and have therefore been able to quantify the influence of knotting on its folding rate. Here, we provide direct evidence that a threading event associated with formation of either a 31 or 52 knot, or a step closely associated with it, significantly slows down the folding of UCH-L1. The results of the optical tweezers experiments highlight the complex nature of the folding pathway, many additional intermediate structures being detected that cannot be resolved by intrinsic fluorescence. Mechanical stretching of knotted proteins is also of importance for understanding the possible implications of knots in proteins for cellular degradation. Compared with a simple 31 knot, we measure a significantly larger size for the 52 knot in the unfolded state that can be further tightened with higher forces. Our results highlight the potential difficulties in degrading a 52 knot compared with a 31 knot.

DOI

Photon momentum and optically induced force in matter derived from the eikonal equation

V.P. Torchigin, A.V. Torchigin

We show thatthe photon trajectory calculated on the basis ofthe Newton laws of mechanics coincides with that calculated on the basis of laws of the geometrical optics on assumption that the momentum of the photon in matter increases by n times as compared with that in free space. The force applied to the photon is calculated from the third Newton law as the force that is opposite to the net force produced by the electrical field of the photon in the matter. It is shown that there is no Lorentz force and the Maxwell force produced by an electrical filed in a dielectric is responsible for a change of the photon momentum.

DOI

Reflective Spin-Orbit Geometric Phase from Chiral Anisotropic Optical Media

Mushegh Rafayelyan, Georgiy Tkachenko, and Etienne Brasselet

We report on highly reflective spin-orbit geometric phase optical elements based on a helicity-preserving circular Bragg-reflection phenomenon. First, we present a dynamical geometric phase experiment using a flat chiral Bragg mirror. Then, we show that shaping such a geometric phase allows the efficient spin-orbit tailoring of light fields without the need to fulfill any condition on birefringent phase retardation, in contrast to the case of transmission spin-orbit optical elements. This is illustrated by optical vortex generation from chiral liquid crystal droplets in the Bragg regime that unveils spin-orbit consequences of the droplet’s curvature. Our results thus introduce a novel class of geometric phase elements—“Bragg-Berry” optical elements.

DOI

Reconfigurable optical assembly of nanostructures

Yunuen Montelongo, Ali K. Yetisen, Haider Butt & Seok-Hyun Yun

Arrangements of nanostructures in well-defined patterns are the basis of photonic crystals, metamaterials and holograms. Furthermore, rewritable optical materials can be achieved by dynamically manipulating nanoassemblies. Here we demonstrate a mechanism to configure plasmonic nanoparticles (NPs) in polymer media using nanosecond laser pulses. The mechanism relies on optical forces produced by the interference of laser beams, which allow NPs to migrate to lower-energy configurations. The resulting NP arrangements are stable without any external energy source, but erasable and rewritable by additional recording pulses. We demonstrate reconfigurable optical elements including multilayer Bragg diffraction gratings, volumetric photonic crystals and lenses, as well as dynamic holograms of three-dimensional virtual objects. We aim to expand the applications of optical forces, which have been mostly restricted to optical tweezers. Holographic assemblies of nanoparticles will allow a new generation of programmable composites for tunable metamaterials, data storage devices, sensors and displays.

DOI

Nanostructured grating patterns over a large area fabricated by optically directed assembly

Xiaoping Huang, Kai Chen, Mingxi Qi, Yu Li, Yumeng Hou, Ying Wang, Qing Zhao, Xiangang Luo and Qingyu Xu

Optical trapping and manipulation of nanoparticles (NPs) have been widely used in nanotechnology and biology. Here, we demonstrate an optically directed assembly (ODA) route for bottom-up fabrication of stable nanostructured grating patterns in solution using laser standing evanescent wave (LSEW) fields. The control mechanism is the intriguing cooperative action of the periodically line-centered attractive optical gradient force and the near field dipolar coupling force induced by LSEW, which leads to assembly of the colloidal silver NPs into robust grating patterns within minutes. The anisotropic polarization nature of the grating patterns was studied further by examining the morphology correlation of the surface-enhanced Raman scattering (SERS)-based signal amplification. We show the LSEW ODA method can optimize and stabilize the strongest dipolar coupling style among the NPs during pattern assembly. These results advance the further understanding of ODA of colloid NPs and might have many potential applications in SERS, catalysis, nanophotonics and nano-fabrication.

DOI

Pharmacological chaperone reshapes the energy landscape for folding and aggregation of the prion protein

Amar Nath Gupta, Krishna Neupane, Negar Rezajooei, Leonardo M. Cortez, Valerie L. Sim & Michael T. Woodside
The development of small-molecule pharmacological chaperones as therapeutics for protein misfolding diseases has proven challenging, partly because their mechanism of action remains unclear. Here we study Fe-TMPyP, a tetrapyrrole that binds to the prion protein PrP and inhibits misfolding, examining its effects on PrP folding at the single-molecule level with force spectroscopy. Single PrP molecules are unfolded with and without Fe-TMPyP present using optical tweezers. Ligand binding to the native structure increases the unfolding force significantly and alters the transition state for unfolding, making it more brittle and raising the barrier height. Fe-TMPyP also binds the unfolded state, delaying native refolding. Furthermore, Fe-TMPyP binding blocks the formation of a stable misfolded dimer by interfering with intermolecular interactions, acting in a similar manner to some molecular chaperones. The ligand thus promotes native folding by stabilizing the native state while also suppressing interactions driving aggregation.

DOI

Flexural Rigidity and Shear Stiffness of Flagella Estimated from Induced Bends and Counterbends

Gang Xu, Kate S. Wilson, Ruth J. Okamoto, Jin-Yu Shao, Susan K. Dutcher, Philip V. Bayly

Motile cilia and flagella are whiplike cellular organelles that bend actively to propel cells or move fluid in passages such as airways, brain ventricles, and the oviduct. Efficient motile function of cilia and flagella depends on coordinated interactions between active forces from an array of motor proteins and passive mechanical resistance from the complex cytoskeletal structure (the axoneme). However, details of this coordination, including axonemal mechanics, remain unclear. We investigated two major mechanical parameters, flexural rigidity and interdoublet shear stiffness, of the flagellar axoneme in the unicellular alga Chlamydomonas reinhardtii. Combining experiment, theory, and finite element models, we demonstrate that the apparent flexural rigidity of the axoneme depends on both the intrinsic flexural rigidity (EI) and the elastic resistance to interdoublet sliding (shear stiffness, ks). We estimated the average intrinsic flexural rigidity and interdoublet shear stiffness of wild-type Chlamydomonas flagella in vivo, rendered immotile by vanadate, to be EI = 840 ± 280 pN⋅μm2 and ks = 79.6 ± 10.5 pN/rad, respectively. The corresponding values for the pf3; cnk11-6 double mutant, which lacks the nexin-dynein regulatory complex (N-DRC), were EI = 1011 ± 183 pN·μm2 and ks = 39.3 ± 6.0 pN/rad under the same conditions. Finally, in the pf13A mutant, which lacks outer dynein arms and inner dynein arm c, the estimates were EI = 777 ± 184 pN·μm2 and ks = 43.3 ± 7.7 pN/rad. In the two mutant strains, the flexural rigidity is not significantly different from wild-type (p > 0.05), but the lack of N-DRC (in pf3; cnk11-6) or dynein arms (in pf13A) significantly reduces interdoublet shear stiffness. These differences may represent the contributions of the N-DRCs (∼40 pN/rad) and residual dynein interactions (∼35 pN/rad) to interdoublet sliding resistance in these immobilized Chlamydomonas flagella.

DOI

Single-fiber tweezers applied for dye lasing in a fluid droplet

Zhihai Liu, Yunhao Chen, Li Zhao, Yu Zhang, Yong Wei, Hanyang Li, Yongjun Liu, Yaxun Zhang, Enming Zhao, Xinghua Yang, Jianzhong Zhang, and Libo Yuan

We report on the first demonstration of a single-fiber optical tweezer that is utilized to stabilize and control the liquid droplet for dye lasing. In order to trap a liquid droplet with a diameter of 15–30 μm, an annular core micro-structured optical fiber is adopted. By using wavelength division multiplexing technology, we couple a trapping light source (980 nm) and a pumping light source (532 nm) into the annular core of the fiber to realize the trapping, controlling, and pumping of the oil droplet. We show that the laser emission spectrum tunes along the same size as the oil droplet. The lasing threshold of the oil droplet with the diameter of 24 μm is 0.7 μJ. The presented fiber-based optical manipulation of liquid droplet micro-lasers can be easily combined with the micro-fluidic chip technology and also may extend the application of optical fiber tweezers for micro-droplet lasing technology in the biological field.

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Efficient Formation of Site-Specific Protein–DNA Hybrids Using Copper-Free Click Chemistry

Ann Mukhortava and Michael Schlierf

Protein–DNA hybrids have become increasingly popular molecular building blocks in bionanotechnology and single-molecule studies to synergistically combine the programmability of DNA with the chemical diversity of proteins. The growing demand for protein–DNA hybrids requires powerful strategies for their conjugation. Here, we present an efficient two-step method for protein–DNA assembly based on copper-free click chemistry. The method allows site-specificity and high coupling efficiency, while maintaining the conservation of protein activity. We compare our method to a commonly used protocol of direct linkage of maleimide-modified oligos. We demonstrate the significantly higher yield with a protein–DNA conjugate, which is analyzed using single-molecule force spectroscopy.

DOI

Multidimensional mapping of the restoring force of an optical trap using triangular wave flow

A. Raudsepp, M. A. K. Williams & S. B. Hall

A method for mapping the spatial dependence of the restoring force of an optical trap in multiple dimensions using triangular wave flow is considered. A theoretical description of the transient motion of an optically trapped massless microsphere in triangular wave flow is introduced and verified experimentally. Guided by theory, the method is first demonstrated using one-dimensional (1D) triangular wave flow to measure the transverse and axial dependence of the restoring force of an optical trap for polystyrene microspheres with diameters from 360 to 3000 nm and is compared to more conventional measurements. While the experimental results are self-consistent, the expected size dependence predicted by theory Nieminensps2007 is not observed for the optical traps examined here. Two-dimensional (2D) triangular-wave flow is then used to the measure the spatial dependence of the restoring force in the transverse–axial plane. The resulting map shows that a spatial asymmetry, not predicted by theory, is present in the optical force field.

DOI

Different macro- and micro-rheological properties of native porcine respiratory and intestinal mucus

Harish Bokkasam, Matthias Ernst, Marco Guenther, Christian Wagner, Ulrich F. Schaefer, Claus-Michael Lehr

Aim of this study was to investigate the similarities and differences at macro- and microscale in the viscoelastic properties of mucus that covers the epithelia of the intestinal and respiratory tract. Natural mucus was collected from pulmonary and intestinal regions of healthy pigs. Macro-rheological investigations were carried out through conventional plate-plate rheometry. Microrheology was investigated using optical tweezers. Our data revealed significant differences both in macro- and micro-rheological properties between respiratory and intestinal mucus.

DOI

Evanescent field trapping of nanoparticles using nanostructured ultrathin optical fibers

Mark Daly, Viet Giang Truong, and Síle Nic Chormaic

While conventional optical trapping techniques can trap objects with submicron dimensions, the underlying limits imposed by the diffraction of light generally restrict their use to larger or higher refractive index particles. As the index and diameter decrease, the trapping difficulty rapidly increases; hence, the power requirements for stable trapping become so large as to quickly denature the trapped objects in such diffraction-limited systems. Here, we present an evanescent field-based device capable of confining low index nanoscale particles using modest optical powers as low as 1.2 mW, with additional applications in the field of cold atom trapping. Our experiment uses a nanostructured optical micro-nanofiber to trap 200 nm, low index contrast, fluorescent particles within the structured region, thereby overcoming diffraction limitations. We analyze the trapping potential of this device both experimentally and theoretically, and show how strong optical traps are achieved with low input powers.

DOI

Trochoidal trajectories of self-propelled Janus particles in a diverging laser beam

Henrique Wakil Moyses, Jeremie Palacci, Stefano Sacanna and David G Grier

We describe colloidal Janus particles with metallic and dielectric faces that swim vigorously when illuminated by defocused optical tweezers without consuming any chemical fuel. Rather than wandering randomly, these optically-activated colloidal swimmers circulate back and forth through the beam of light, tracing out sinuous rosette patterns. We propose a model for this mode of light-activated transport that accounts for the observed behavior through a combination of self-thermophoresis and optically-induced torque. In the deterministic limit, this model yields trajectories that resemble rosette curves known as hypotrochoids.

DOI

Rheological properties of cells measured by optical tweezers

Yareni A. Ayala, Bruno Pontes, Diney S. Ether, Luis B. Pires, Glauber R. Araujo, Susana Frases, Luciana F. Romão, Marcos Farina, Vivaldo Moura-Neto, Nathan B. Viana, H. Moysés Nussenzveig

The viscoelastic properties of cells have been investigated by a variety of techniques. However, the experimental data reported in literature for viscoelastic moduli differ by up to three orders of magnitude. This has been attributed to differences in techniques and models for cell response as well as to the natural variability of cells. In this work we develop and apply a new methodology based on optical tweezers to investigate the rheological behavior of fibroblasts, neurons and astrocytes in the frequency range from 1Hz to 35Hz, determining the storage and loss moduli of their membrane-cortex complex. To avoid distortions associated with cell probing techniques, we use a previously developed method that takes into account the influence of under bead cell thickness and bead immersion. These two parameters were carefully measured for the three cell types used. Employing the soft glass rheology model, we obtain the scaling exponent and the Young’s modulus for each cell type. The obtained viscoelastic moduli are in the order of Pa. Among the three cell types, astrocytes have the lowest elastic modulus, while neurons and fibroblasts exhibit a more solid-like behavior. Although some discrepancies with previous results remain and may be inevitable in view of natural variability, the methodology developed in this work allows us to explore the viscoelastic behavior of the membrane-cortex complex of different cell types as well as to compare their viscous and elastic moduli, obtained under identical and well-defined experimental conditions, relating them to the cell functions.

DOI

SERS detection of Biomolecules at Physiological pH via aggregation of Gold Nanorods mediated by Optical Forces and Plasmonic Heating

Barbara Fazio, Cristiano D’Andrea, Antonino Foti, Elena Messina, Alessia Irrera, Maria Grazia Donato, Valentina Villari, Norberto Micali, Onofrio M. Maragò & Pietro G. Gucciardi

Strategies for in-liquid molecular detection via Surface Enhanced Raman Scattering (SERS) are currently based on chemically-driven aggregation or optical trapping of metal nanoparticles in presence of the target molecules. Such strategies allow the formation of SERS-active clusters that efficiently embed the molecule at the “hot spots” of the nanoparticles and enhance its Raman scattering by orders of magnitude. Here we report on a novel scheme that exploits the radiation pressure to locally push gold nanorods and induce their aggregation in buffered solutions of biomolecules, achieving biomolecular SERS detection at almost neutral pH. The sensor is applied to detect non-resonant amino acids and proteins, namely Phenylalanine (Phe), Bovine Serum Albumin (BSA) and Lysozyme (Lys), reaching detection limits in the μg/mL range. Being a chemical free and contactless technique, our methodology is easy to implement, fast to operate, needs small sample volumes and has potential for integration in microfluidic circuits for biomarkers detection.

DOI

Annihilation dynamics of topological monopoles on a fiber in nematic liquid crystals

M. Nikkhou, M. Škarabot, and I. Muševič

We use the laser tweezers to create isolated pairs of topological point defects in a form of radial and hyperbolic hedgehogs, located close and attracted to a thin fiber with perpendicular surface orientation of nematic liquid crystal molecules in a thin planar nematic cell. We study the time evolution of the interaction between the two monopoles by monitoring their movement and reconstructing their trajectories and velocities. We find that there is a crossover in the pair interaction force between the radial and hyperbolic hedgehog. At small separation d, the elastic force between the opposite monopoles results in an increase of the attractive force with respect to the far field, and their relative velocity v scales as a v(d)∝d−2±0.2 power law. At large separations, the two oppositely charged monopoles can either attract or repel with constant interaction force. We explain this strange far-field behavior by the experimental inaccuracy in setting the fiber exactly perpendicular to the cell director.

DOI

Curl force dynamics: symmetries, chaos and constants of motion

M V Berry and Pragya Shukla

This is a theoretical study of Newtonian trajectories governed by curl forces, i.e. position-dependent but not derivable from a potential, investigating in particular the possible existence of conserved quantities. Although nonconservative and nonhamiltonian, curl forces are not dissipative because volume in the position–velocity state space is preserved. A physical example is the effective forces exerted on small particles by light. When the force has rotational symmetry, for example when generated by an isolated optical vortex, particles spiral outwards and escape, even with an attractive gradient force, however strong. Without rotational symmetry, and for dynamics in the plane, the state space is four-dimensional, and to search for possible constants of motion we introduce the Volume of section: a numerical procedure, in which orbits are plotted as dots in a three-dimensional subspace. For some curl forces, e.g. optical fields with two opposite-strength vortices, the dots lie on a surface, indicating a hidden constant of motion. For other curl forces, e.g. those from four vortices, the dots explore clouds, in an unfamiliar kind of chaos, suggesting that no constant of motion exists. The curl force dynamics generated by optical vortices could be studied experimentally.

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Edge pinning and transformation of defect lines induced by faceted colloidal rings in nematic liquid crystals

Bohdan Senyuk, Qingkun Liu, Ye Yuan, and Ivan I. Smalyukh

Nematic colloids exhibit a large diversity of topological defects and structures induced by colloidal particles in the orientationally ordered liquid crystal host fluids. These defects and field configurations define elastic interactions and medium-mediated self-assembly, as well as serve as model systems in exploiting the richness of interactions between topologies and geometries of colloidal surfaces, nematic fields, and topological singularities induced by particles in the nematic bulk and at nematic-colloidal interfaces. Here we demonstrate formation of quarter-strength surface-pinned disclinations, as well as a large variety of director field configurations with splitting and reconnections of singular defect lines, prompted by colloidal particles with sharp edges and size large enough to define strong boundary conditions. Using examples of faceted ring-shaped particles of genus g=1, we explore transformation of defect lines as they migrate between locations in the bulk of the nematic host to edge-pinned locations at the surfaces of particles and vice versa, showing that this behavior is compliant with topological constraints defined by mathematical theorems. We discuss how transformation of bulk and surface defect lines induced by faceted colloids can enrich the diversity of elasticity-mediated colloidal interactions and how these findings may impinge on prospects of their controlled reconfigurable self-assembly in nematic hosts.

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Optical Twist Induced by Plasmonic Resonance

Jun Chen, Neng Wang, Liyong Cui, Xiao Li, Zhifang Lin & Jack Ng

Harvesting light for optical torque is of significant importance, owing to its ability to rotate nano- or micro-objects. Nevertheless, applying a strong optical torque remains a challenging task: angular momentum must conserve but light is limited. A simple argument shows the tendency for two objects with strong mutual scattering or light exchange to exhibit a conspicuously enhanced optical torque without large extinction or absorption cross section. The torque on each object is almost equal but opposite, which we called optical twist. The effect is quite significant for plasmonic particle cluster, but can also be observed in structures with other morphologies. Such approach exhibits an unprecedentedly large torque to light extinction or absorption ratio, enabling limited light to exert a relatively large torque without severe heating. Our work contributes to the understanding of optical torque and introduces a novel way to manipulate the internal degrees of freedom of a structured particle cluster.

DOI

Continuous-feed optical sorting of aerosol particles

J. J. Curry and Zachary H. Levine

We consider the problem of sorting, by size, spherical particles of order 100 nm radius. The scheme we analyze consists of a heterogeneous stream of spherical particles flowing at an oblique angle across an optical Gaussian mode standing wave. Sorting is achieved by the combined spatial and size dependencies of the optical force. Particles of all sizes enter the flow at a point, but exit at different locations depending on size. Exiting particles may be detected optically or separated for further processing. The scheme has the advantages of accommodating a high throughput, producing a continuous stream of continuously dispersed particles, and exhibiting excellent size resolution. We performed detailed Monte Carlo simulations of particle trajectories through the optical field under the influence of convective air flow. We also developed a method for deriving effective velocities and diffusion constants from the Fokker-Planck equation that can generate equivalent results much more quickly. With an optical wavelength of 1064 nm, polystyrene particles with radii in the neighborhood of 275 nm, for which the optical force vanishes, may be sorted with a resolution below 1 nm.

DOI

Optical trapping force and torque on spheroidal Rayleigh particles with arbitrary spatial orientations

Manman Li, Shaohui Yan, Baoli Yao, Yansheng Liang, Guoxia Han, and Peng Zhang

We investigate the spatial orientation dependence of optical trapping forces and intrinsic torques exerted on spheroidal Rayleigh particles under irradiation of highly focused linearly and circularly polarized beams. It is revealed that the maximal trapping forces and torques strongly depend on the orientation of the spheroid, and the spheroidal particle is driven to be stably trapped at the beam focus with its major axis perpendicular to the optical axis. For a linearly polarized trapping beam, the optical torque is always perpendicular to the plane containing the major axis and the polarization direction of the incident beam. Therefore, the spheroid tends to rotate its major axis along with the polarization direction. However, for a circularly polarized trapping beam, the optical torque is always perpendicular to the plane containing the major axis and the optical axis. What is different from the linear polarization case is that the spheroid tends to have the major axis parallel to the projection of the major axis in the transverse plane. The optical torque in the circular polarization case is half of that in the linear polarization case. These optical trapping properties may be exploited in practical optical manipulation, especially for the nonspherical particle’s trapping.

DOI

Arrested Dimer’s Diffusion by Self-Induced Back-Action Optical Forces

Jorge Luis-Hita, Juan José Sáenz, and Manuel I. Marqués

The diffusion of a dimer made out of two resonant dipolar scatters in an optical lattice is theoretically analyzed. When a small particle diffuses through an optically induced potential landscape, its Brownian motion can be strongly suppressed by gradient forces, proportional to the particle’s polarizability. For a single lossless monomer at resonance, the gradient force vanishes and the particle diffuses as in absence of external fields. However, we show that when two monomers link in a dimer, the multiple scattering among the monomers induces both a torque and a net force on the dimer’s center of mass. This “self-induced back-action” force leads to an effective potential energy landscape, entirely dominated by the mutual interaction between monomers, which strongly influences the dynamics of the dimer. Under appropriate illumination, single monomers in a colloidal suspension freely diffuse while dimers become trapped. Our theoretical predictions are tested against extensive Langevin molecular dynamics simulations.

DOI

Surface-enhanced Raman scattering via entrapment of colloidal plasmonic nanocrystals by laser generated microbubbles on random gold nano-islands

Zhiwen Kang, Jiajie Chen and Ho-Pui Ho

Surface-enhanced Raman scattering (SERS) typically requires hot-spots generated in nano-fabricated plasmonic structures. Here we report a highly versatile approach based on the use of random gold nano-island substrates (AuNIS). Hot spots are produced through the entrapment of colloidal plasmonic nano-crystals at the interface between AuNIS and a microbubble, which is generated from the localized plasmonic absorption of a focused laser beam. The entrapment strength is strongly dependent on the shape of the microbubble, which is in turn affected by the surface wetting characteristics of the AuNIS with respect to the solvent composition. The laser power intensity required to trigger microbubble-induced SERS is as low as 200 μW μm−2. Experimental results indicate that the SERS limit of detection (LOD) for molecules of 4-MBA (with –SH bonds) is 10−12 M, R6G or RhB (without –SH bonds) is 10−7 M. The proposed strategy has potential applications in low-cost lab-on-chip devices for the label-free detection of chemical and biological molecules.

DOI

Superlens induced loss-insensitive optical force

Xiaohan Cui, Shubo Wang, and C. T. Chan

A slab with relative permittivity ɛ = -1+iδ and permeability μ = ‑1+iδ has a critical distance away from the slab where a small particle will either be cloaked or imaged depending on whether it is located inside or outside that critical distance. We find that the optical force acting on a small cylinder under plane wave illumination reaches a maximum value at this critical distance. Contrary to the usual observation that superlens systems should be highly loss-sensitive, this maximum optical force remains a constant when loss is changed within a certain range. For a fixed particle-slab distance, increasing loss can even amplify the optical force acting on the small cylinder, contrary to the usual belief that loss compromises the response of supenlens.

DOI

Direct Measurement of Photon Recoil from a Levitated Nanoparticle

Vijay Jain, Jan Gieseler, Clemens Moritz, Christoph Dellago, Romain Quidant, and Lukas Novotny

The momentum transfer between a photon and an object defines a fundamental limit for the precision with which the object can be measured. If the object oscillates at a frequency Ω0, this measurement backaction adds quanta ℏΩ0 to the oscillator’s energy at a rate Γrecoil, a process called photon recoil heating, and sets bounds to coherence times in cavity optomechanical systems. Here, we use an optically levitated nanoparticle in ultrahigh vacuum to directly measure Γrecoil. By means of a phase-sensitive feedback scheme, we cool the harmonic motion of the nanoparticle from ambient to microkelvin temperatures and measure its reheating rate under the influence of the radiation field. The recoil heating rate is measured for different particle sizes and for different excitation powers, without the need for cavity optics or cryogenic environments. The measurements are in quantitative agreement with theoretical predictions and provide valuable guidance for the realization of quantum ground-state cooling protocols and the measurement of ultrasmall forces.

DOI

Imaging the position-dependent 3D force on micro beads subjected to acoustic radiation forces and streaming

Andreas Lamprecht, Stefan Lakämper, Thierry Baasch, Iwan A.T. Schaap and Jürg Dual

Acoustic particle manipulation in microfluidic channels is becoming a powerful tool in microfluidics to control micrometer sized objects in medical, chemical and biological applications. By creating a standing acoustic wave in the channel, the resulting pressure field can be employed to trap or sort particles. To design efficient and reproducible devices, it is important to characterize the pressure field throughout the volume of the microfluidic device. Here, we used an optically trapped particle as probe to measure the forces in all three dimensions. By moving the probe through the volume of the channel, we imaged spatial variations in the pressure field. In the direction of the standing wave this revealed a periodic energy landscape for 2 μm beads, resulting in an effective stiffness of 2.6 nN/m for the acoustic trap. We found that multiple fabricated devices showed consistent pressure fields. Surprisingly, forces perpendicular to the direction of the standing wave reached values of up to 20% of the main-axis-values. To separate the direct acoustic force from secondary effects, we performed experiments with different bead sizes, which attributed some of the perpendicular forces to acoustic streaming. This method to image acoustically generated forces in 3D can be used to either minimize perpendicular forces or to employ them for specific applications in novel acoustofluidic designs.

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Fluorescence-based remote irradiation sensor in liquid-filled hollow-core photonic crystal fiber

R. Zeltner, D. S. Bykov, S. Xie, T. G. Euser and P. St.J. Russell

We report an irradiation sensor based on a fluorescent “flying particle” that is optically trapped and propelled inside the core of a water-filled hollow-core photonic crystal fiber. When the moving particle passes through an irradiated region, its emitted fluorescence is captured by guided modes of the fiber core and so can be monitored using a filtered photodiode placed at the fiber end. The particle speed and position can be precisely monitored using in-fiber Doppler velocimetry, allowing the irradiation profile to be measured to a spatial resolution of ∼10 μm. The spectral response can be readily adjusted by appropriate choice of particle material. Using dye-doped polystyrene particles, we demonstrate detection of green (532 nm) and ultraviolet (340 nm) light.

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Photonic Torque Microscopy of the Nonconservative Force Field for Optically Trapped Silicon Nanowires

Alessia Irrera, Alessandro Magazzù, Pietro Artoni, Stephen H. Simpson, Simon Hanna, Philip H. Jones, Francesco Priolo, Pietro Giuseppe Gucciardi, and Onofrio M. Maragò

We measure, by photonic torque microscopy, the nonconservative rotational motion arising from the transverse components of the radiation pressure on optically trapped, ultrathin silicon nanowires. Unlike spherical particles, we find that nonconservative effects have a significant influence on the nanowire dynamics in the trap. We show that the extreme shape of the trapped nanowires yields a transverse component of the radiation pressure that results in an orbital rotation of the nanowire about the trap axis. We study the resulting motion as a function of optical power and nanowire length, discussing its size-scaling behavior. These shape-dependent nonconservative effects have implications for optical force calibration and optomechanics with levitated nonspherical particles.

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Tapping out a mechanical code for T cell triggering

Michael L. Dustin and Lance C. Kam

Mechanical forces play increasingly recognized roles in T cell receptor (TCR) signal transduction. Hu and Butte (2016. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201511053) demonstrate that actin is required for T cells to generate forces at the TCR and that exogenous application of force can emulate these cytoskeletal forces and trigger T cell activation.

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Cholesteric solid spherical micro-particles: chiral optomechanics and microphotonics

R.J. Hernández, C. Provenzano, A. Mazzulla, P. Pagliusi, M. Viola & G. Cipparrone

This manuscript reviews the main results from the investigations performed on solid chiral micro-particles based on polymerized cholesteric droplets. The procedures of particles generation, the structural characterization, optomechanics and microphotonics investigations, are shown. The aim of this work is to give a picture of the innovation introduced by exploring the combination of chirality, self-organization and solid structure.

DOI

Unravelling the structural plasticity of stretched DNA under torsional constraint

Graeme A. King, Erwin J. G. Peterman & Gijs J. L. Wuite

Regions of the genome are often held under torsional constraint. Nevertheless, the influence of such constraint on DNA–protein interactions during genome metabolism is still poorly understood. Here using a combined optical tweezers and fluorescence microscope, we quantify and explain how torsional constraint influences the structural stability of DNA under applied tension. We provide direct evidence that concomitant basepair melting and helical unwinding can occur in torsionally constrained DNA at forces >~50 pN. This striking result indicates that local changes in linking number can be absorbed by the rest of the DNA duplex. We also present compelling new evidence that an overwound DNA structure (likely P-DNA) is created (alongside underwound structures) at forces >~110 pN. These findings substantiate previous theoretical predictions and highlight a remarkable structural plasticity of torsionally constrained DNA. Such plasticity may be required in vivo to absorb local changes in linking number in DNA held under torsional constraint.

DOI

Calorimetry-Derived Composition Vectors to Resolve Component Raman Spectra in Phospholipid Phase Transitions

Jay P. Kitt, David A. Bryce, Joel M. Harris

Multidimensional least squares analysis is a well-established technique for resolving component vibrational spectra from mixed samples or systems. Component resolution of temperature-dependent vibrational spectra is challenging, however, due to the lack of a suitable model for the variation in sample composition with temperature. In this work, analysis of temperature-dependent Raman spectra of lipid membranes is accomplished by using “concentration” vectors independently derived from enthalpy changes determined by differential scanning calorimetry. Specifically, the lipid–bilayer phase transitions of DMPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) are investigated through Raman spectra acquired from individual, optically trapped vesicles in suspension as a function of temperature. Heat capacity profiles of the same vesicle suspension are measured using differential scanning calorimetry and numerically integrated to generate enthalpy change curves of each phase transition, which are in turn used to construct composition vectors. Multidimensional least squares analysis optimized for a fit to these composition vectors allows resolution of the component spectra corresponding to gel, ripple, and liquid–crystalline phases of the DMPC. The quality of fit of the calorimetry-derived results is confirmed by unstructured residual differences between the data and the model, and a composition variation predicted by the resolved spectra that matches the calorimetry results. This approach to analysis of temperature-dependent spectral data could be readily applied in other areas of materials characterization, where one is seeking to learn about structural changes that occur through temperature-dependent phase transitions.

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Observation of whispering gallery modes in elastic light scattering from microdroplets optically trapped in a microfluidic channel

S. Anand, M. Eryürek, Y. Karadag, A. Erten, A. Serpengüzel, A. Jonáš, and A. Kiraz

Optical whispering gallery modes (WGMs) were observed in elastic scattering spectra recorded from oil-in-water emulsion droplets in a microfluidic channel. Droplets with diameters ranging between 15 and 50 μm were trapped by optical tweezers near the tip of a single mode fiber that enabled the excitation of the WGMs using a tunable laser. Quality factors of the WGMs were observed to increase with droplet size. WGMs with quality factors of more than 104 were observed for droplets with diameters around 45 μm. In some cases, recorded WGMs drifted monotonically to the blue end of the spectrum due to droplet dissolution in the host liquid. Fluctuating spectral shifts to both blue and red ends of the spectrum were also observed. These were attributed to the presence of randomly diffusing particulate contaminants in the droplet liquid, indicating the potential of optically trapped droplet resonators for optical sensing applications.

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Subfemtonewton Force Spectroscopy at the Thermal Limit in Liquids

Lulu Liu, Simon Kheifets, Vincent Ginis, and Federico Capasso

We demonstrate thermally limited force spectroscopy using a probe formed by a dielectric microsphere optically trapped in water near a dielectric surface. We achieve force resolution below 1 fN in 100 s, corresponding to a 2 Å rms displacement of the probe. Our measurement combines a calibrated evanescent wave particle tracking technique and a lock-in detection method. We demonstrate the accuracy of our method by measurement of the height-dependent force exerted on the probe by an evanescent wave, the results of which are in agreement with Mie theory calculations.

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The properties of the actin-myosin interaction in the heart muscle depend on the isoforms of myosin but not of α-actin

G. Kopylova, S. Nabiev, L. Nikitina, D. Shchepkin, S. Bershitsky

In myocardium of mammals there are two isoforms of myosin heavy chains, α and β. In ventricle, together with ventricular isoforms of light chains they form two isomyosins: V1 and V3, homodimers of α- and β-heavy chains, respectively. In atria, α- and β-heavy chains together with atrial light chains form A1 (αα) and A2 (ββ) isomyosins. Besides in myocardium two isoforms of α-actin, skeletal and cardiac, are expressed. We assume that the differences in the amino acid sequence of cardiac and skeletal actin may affect its interaction with myosin. To test this hypothesis, we investigated characteristics of actin-myosin interactions of cardiac and skeletal isoforms of α-actin with the isoforms of cardiac myosin using an optical trap technique and an in vitro motility assay. It was found that the mechanical and kinetic characteristics of the interactions of the isoforms of cardiac myosin with actin depend on the isoforms of myosin not α-actin.

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Mie scattering of highly focused, scalar fields: an analytic approach

Nicole J. Moore and Miguel A. Alonso

We present a method for modeling the scattering of a focused scalar field incident on a spherical particle. This approach involves the expansion of the incident field in an orthonormal basis of closed-form solutions of the Helmholtz equation which are nonparaxial counterparts of Laguerre–Gaussian beams. This method also allows for the analytic calculation of the forces and torques exerted on a particle at any position with respect to the beam’s focus.

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Advances in the microrheology of complex fluids

Thomas Andrew Waigh

New developments in the microrheology of complex fluids are considered. Firstly the requirements for a simple modern particle tracking microrheology experiment are introduced, the error analysis methods associated with it and the mathematical techniques required to calculate the linear viscoelasticity. Progress in microrheology instrumentation is then described with respect to detectors, light sources, colloidal probes, magnetic tweezers, optical tweezers, diffusing wave spectroscopy, optical coherence tomography, fluorescence correlation spectroscopy, elastic- and quasi-elastic scattering techniques, 3D tracking, single molecule methods, modern microscopy methods and microfluidics. New theoretical techniques are also reviewed such as Bayesian analysis, oversampling, inversion techniques, alternative statistical tools for tracks (angular correlations, first passage probabilities, the kurtosis, motor protein step segmentation etc), issues in micro/macro rheological agreement and two particle methodologies. Applications where microrheology has begun to make some impact are also considered including semi-flexible polymers, gels, microorganism biofilms, intracellular methods, high frequency viscoelasticity, comb polymers, active motile fluids, blood clots, colloids, granular materials, polymers, liquid crystals and foods. Two large emergent areas of microrheology, non-linear microrheology and surface microrheology are also discussed.

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Resonant optical gradient force interaction for nano-imaging and -spectroscopy

Yang, Honghua U; Raschke, Markus B

The optical gradient force provides optomechanical interactions, for particle trapping and manipulation, as well as for near-field optical imaging in scanning probe microscopy. Based on recent spectroscopic experiments, its extension and use for a novel form of chemical scanning probe nano-imaging was proposed. Here, we provide the theoretical basis in terms of spectral behavior, resonant enhancement, and distance dependence of the optical gradient force from numerical simulations in a coupled nanoparticle model geometry. We predict an asymmetric line shape of the optical gradient force for molecular electronic or vibrational resonances, corresponding to the real part of the dielectric function of the sample materials. Yet the line shape can become symmetric and absorptive for collective polaritonic excitations. The corresponding magnitudes of the force range from fN to pN, respectively. The distance dependence scales considerably less steeply than simple point dipole model predictions due to multipole effects. The combination of these characteristics of the optical gradient force offers the chance to experimentally distinguish it from competing processes such as thermal expansion induced forces. In addition we provide a perspective for further resonant enhancement and control of optical forces.

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Arrested dimer's diffusion by self-induced back-action optical forces

Jorge Luis-Hita, Juan Jose Saenz, and Manuel I Marqués

The diffusion of a dimer made out of two resonant dipolar scatters in an optical lattice is theoretically analyzed. When a small particle diffuses through an optically induced potential landscape, its Brownian motion can be strongly suppressed by gradient forces, proportional to the particle's polarizability. For a single lossless monomer at resonance, the gradient force vanishes and the particle diffuses as in absence of external fields. However, we show that when two monomers link in a dimer, the multiple scattering among the monomers induces both a torque and a net force on the dimer's center of mass. This ``self-induced back-action'' force leads to an effective potential energy landscape, entirely dominated by the mutual interaction between monomers, which strongly influences the dynamics of the dimer. Under appropriate illumination, single monomers in a colloidal suspension freely diffuse while dimers become trapped. Our theoretical predictions are tested against extensive Langevin molecular dynamics simulations.

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Relativistic analysis of field-kinetic and canonical electromagnetic systems

Cheyenne J. Sheppard and Brandon A. Kemp

We demonstrate the relativistic electromagnetic force and power distributions of the field-kinetic and canonical electromagnetic subsystems with respect to light normally incident upon a moving, lossless magnetodielectric slab of material. Time-averaged and time-varying studies are preformed to demonstrate the continuum mathematical approach and discern between the physical characteristics of both field-kinetic and canonical subsystems. It is shown that when considering time-averaged fields, both subsystems are equivalent and thereby yield equivalent electromagnetic force and power results. The time-varying case demonstrates the differences between the field-kinetic and canonical subsystems, where the field-kinetic subsystem attempts to distort the media and the canonical subsystem satisfies global conservation principles.

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Chirality sorting using two-wave-interference–induced lateral optical force

Huajin Chen, Chenghua Liang, Shiyang Liu, and Zhifang Lin

We demonstrate that a lateral optical force (LOF) can be induced on chiral nanoparticles immersed in the two interfering plane waves. The LOF can push the chiral nanoparticle sideways, in the direction relying on the helicity of the particle as well as the phase difference between two waves. Analytical theory reveals that the LOF comes mostly from the direct coupling of the optical vorticity with the particle chirality. In particular, the LOF has a magnitude comparable to the gradient force and radiation pressure when the particle chirality is sufficiently large. The LOF may serve for chirality sorting due to its unusual properties and also provide an opportunity for passive chiral spectroscopy.

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Microtubule Defects Influence Kinesin-Based Transport In Vitro

Winnie H. Liang, Qiaochu Li, K.M. Rifat Faysal, Stephen J. King, Ajay Gopinathan, Jing Xu

Microtubules are protein polymers that form “molecular highways” for long-range transport within living cells. Molecular motors actively step along microtubules to shuttle cellular materials between the nucleus and the cell periphery; this transport is critical for the survival and health of all eukaryotic cells. Structural defects in microtubules exist, but whether these defects impact molecular motor-based transport remains unknown. Here, we report a new, to our knowledge, approach that allowed us to directly investigate the impact of such defects. Using a modified optical-trapping method, we examined the group function of a major molecular motor, conventional kinesin, when transporting cargos along individual microtubules. We found that microtubule defects influence kinesin-based transport in vitro. The effects depend on motor number: cargos driven by a few motors tended to unbind prematurely from the microtubule, whereas cargos driven by more motors tended to pause. To our knowledge, our study provides the first direct link between microtubule defects and kinesin function. The effects uncovered in our study may have physiological relevance in vivo.

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Spatiotemporally Resolved Tracking of Bacterial Responses to ROS-Mediated Damage at the Single-Cell Level with Quantitative Functional Microscopy

Alvaro Barroso, Malte Christian Grüner, Taylor Forbes, Cornelia Denz, and Cristian Alejandro Strassert

Herein we report on the implementation of photofunctional microparticles in combination with optical tweezers for the investigation of bacterial responses to oxidative stress by means of quantitative functional microscopy. A combination of a strongly hydrophobic axially substituted Si(IV) phthalocyanine adsorbed onto silica microparticles was developed, and the structural and photophysical characterization was carried out. The microparticles are able to produce reactive oxygen species under the fluorescence microscope upon irradiation with red light, and the behaviour of individual bacteria can be consequently investigated in situ and in real time at single cell level. For this purpose, a methodology was introduced to monitor phototriggered changes with spatiotemporal resolution. The defined distance between the photoactive particles and individual bacteria can be fixed under the microscope before the photosensitization process is started, and the photoinduced damage can be monitored by tracing the time-dependent fluorescence turn-on of a suitable marker. The results showed a distance-dependent photoinduced death time, defined as the onset of the incorporation of propidium iodide. Our methodology constitutes a new tool for the in vitro design and evaluation of photosensitizers for the treatment of cancer and infectious diseases with the aid of functional optical microscopy, as it enables a quantitative response evaluation of living systems towards oxidative stress. More generally, it provides a way to understand the response of an ensemble of living entities to reactive oxygen species by analyzing the behavior of a set of individual organisms.

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Sequence-dependent nanometer-scale conformational dynamics of individual RecBCD–DNA complexes

Ashley R. Carter, Maasa H. Seaberg, Hsiu-Fang Fan, Gang Sun, Christopher J. Wilds, Hung-Wen Li and Thomas T. Perkins

RecBCD is a multifunctional enzyme that possesses both helicase and nuclease activities. To gain insight into the mechanism of its helicase function, RecBCD unwinding at low adenosine triphosphate (ATP) (2–4 μM) was measured using an optical-trapping assay featuring 1 base-pair (bp) precision. Instead of uniformly sized steps, we observed forward motion convolved with rapid, large-scale (∼4 bp) variations in DNA length. We interpret this motion as conformational dynamics of the RecBCD–DNA complex in an unwinding-competent state, arising, in part, by an enzyme-induced, back-and-forth motion relative to the dsDNA that opens and closes the duplex. Five observations support this interpretation. First, these dynamics were present in the absence of ATP. Second, the onset of the dynamics was coupled to RecBCD entering into an unwinding-competent state that required a sufficiently long 5′ strand to engage the RecD helicase. Third, the dynamics were modulated by the GC-content of the dsDNA. Fourth, the dynamics were suppressed by an engineered interstrand cross-link in the dsDNA that prevented unwinding. Finally, these dynamics were suppressed by binding of a specific non-hydrolyzable ATP analog. Collectively, these observations show that during unwinding, RecBCD binds to DNA in a dynamic mode that is modulated by the nucleotide state of the ATP-binding pocket.

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Effect of neighboring cells on cell stiffness measured by optical tweezers indentation

Muhammad S. Yousafzai ; Giovanna Coceano ; Alberto Mariutti ; Fatou Ndoye ; Ladan Amin ; Joseph Niemela ; Serena Bonin ; Giacinto Scoles ; Dan Cojoc

We report on the modification of mechanical properties of breast cancer cells when they get in contact with other neighboring cells of the same type. Optical tweezers vertical indentation was employed to investigate cell mechanics in isolated and contact conditions, by setting up stiffness as a marker. Two human breast cancer cell lines with different aggressiveness [MCF-7 (luminal breast cancer) and MDA-MB-231 (basal-like breast cancer)] and one normal immortalized breast cell line HBL-100 (normal and myoepithelial) were selected. We found that neighboring cells significantly alter cell stiffness: MDA-MB-231 becomes stiffer when in contact, while HBL-100 and MCF-7 exhibit softer character. Cell stiffness was probed at three cellular subregions: central (above nucleus), intermediate (cytoplasm), and near the leading edge. In an isolated condition, all cells showed a significant regional variation in stiffness: higher at the center and fading toward the leading edge. However, the regional variation becomes statistically insignificant when the cells were in contact with other neighboring cells. The proposed approach will contribute to understand the intriguing temporal sequential alterations in cancer cells during interaction with their surrounding microenvironment.

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Modifying vibrational properties of a fused silica cantilever with optical tweezers

Rijuparna Chakraborty

This research effort is dedicated to examine the effects of an optical tweezer on the vibration of a small cantilever fiber made up of fused silica and to manipulate certain aspects of its vibrational properties with this tool. Optical tweezers has been used here to change the resonance frequency of the cantilever fiber by a small amount and also to damp its vibration.

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Direct printing of microstructures by femtosecond laser excitation of nanocrystals in solution

Wan Shou and Heng Pan

We report direct printing of micro/sub-micron structures by femtosecond laser excitation of semiconductor nanocrystals (NCs) in solution. Laser excitation with moderate intensity (1011–1012 W/cm2) induces 2D and 3D deposition of CdTe nanocrystals in aqueous solution, which can be applied for direct printing of microstructures. It is believed that laser irradiation induces charge formation on nanocrystals leading to deposition. Furthermore, it is demonstrated that the charged nanocrystals can respond to external electrical bias, enabling a printing approach based on selective laser induced electrophoretic deposition. Finally, energy dispersive X-ray analysis of deposited structures shows oxidation occurs and deposited structure mainly consists of CdxO.

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Microphotonic Forces from Superfluid Flow

D. L. McAuslan, G. I. Harris, C. Baker, Y. Sachkou, X. He, E. Sheridan, and W. P. Bowen

In cavity optomechanics, radiation pressure and photothermal forces are widely utilized to cool and control micromechanical motion, with applications ranging from precision sensing and quantum information to fundamental science. Here, we realize an alternative approach to optical forcing based on superfluid flow and evaporation in response to optical heating. We demonstrate optical forcing of the motion of a cryogenic microtoroidal resonator at a level of 1.46 nN, roughly 1 order of magnitude larger than the radiation pressure force. We use this force to feedback cool the motion of a microtoroid mechanical mode to 137 mK. The photoconvective forces we demonstrate here provide a new tool for high bandwidth control of mechanical motion in cryogenic conditions, while the ability to apply forces remotely, combined with the persistence of flow in superfluids, offers the prospect for new applications.

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The role of coupling on the statistical properties of the energy fluxes between stochastic systems at different temperatures

A Bérut, A Imparato, A Petrosyan and S Ciliberto

We experimentally study the statistical properties of the energy fluxes in two systems whose components are kept at different temperatures. The first system under consideration is an electric circuit which is composed by two resistances, kept at different temperatures and connected by a capacitance (conservative coupling) which couples the thermal noise of the two resistances. The other system is composed by two Brownian particles, trapped with optical tweezers, interacting through a dissipative hydrodynamic coupling. The particles are subjected to an effective temperature difference obtained by random forcing the position of one trap. In these two systems we measure the properties of the energy flowing between the two reservoirs. The role on these properties of the coupling and of the method used to produce the highest temperature is analyzed.

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Diffusion of a nanowire rod through an obstacle field

Dror Kasimov, Tamir Admon, and Yael Roichman

We report the experimental realization of a rod diffusing in a two-dimensional obstacle field following the single rod dynamics. We use a silver nanowire as our rod and two types of obstacles: repelling light beams and polymer pillars. We study the effect of hydrodynamic interactions on the transport of the rod, comparing both experimental realizations and recent simulations. We propose a framework for analyzing the transport through such systems, and we predict a new superdiffusive regime of rod transport at high obstacle concentration and short times.

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Kinetic modeling of molecular motors: pause model and parameter determination from single-molecule experiments

José A Morin, Borja Ibarra and Francisco J Cao

Single-molecule manipulation experiments of molecular motors provide essential information about the rate and conformational changes of the steps of the reaction located along the manipulation coordinate. This information is not always sufficient to define a particular kinetic cycle. Recent single-molecule experiments with optical tweezers showed that the DNA unwinding activity of a Phi29 DNA polymerase mutant presents a complex pause behavior, which includes short and long pauses. Here we show that different kinetic models, considering different connections between the active and the pause states, can explain the experimental pause behavior. Both the two independent pause model and the two connected pause model are able to describe the pause behavior of a mutated Phi29 DNA polymerase observed in an optical tweezers single-molecule experiment. For the two independent pause model all parameters are fixed by the observed data, while for the more general two connected pause model there is a range of values of the parameters compatible with the observed data (which can be expressed in terms of two of the rates and their force dependencies). This general model includes models with indirect entry and exit to the long-pause state, and also models with cycling in both directions. Additionally, assuming that detailed balance is verified, which forbids cycling, this reduces the ranges of the values of the parameters (which can then be expressed in terms of one rate and its force dependency). The resulting model interpolates between the independent pause model and the indirect entry and exit to the long-pause state model.

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Plasmonic trapping of sub-micro objects with metallic antenas

Sugawara, Eishi; Kato, Jun-ichi; Yamagata, Yutaka; Ozaki, Miyu; Furutani, Ryoshu

Since optical trapping was first reported, its methods and targets have been broadened. In this paper, we propose ‘plasmonic clipping’, which traps objects on the plasmonic dot array. Localized surface plasmon polaritons (LSPPs), which localize optical energy in the nanometer-scale size and enhances the optical field, are excited in gaps between the dots. The objects are trapped by electric-field-gradient forces of LSPPs along the dot array. The dot arrays are arranged radially so that LSPPs are selectively excited in dot array corresponding to polarization direction of excitation light. The selective excitation results in directionally-selective ‘plasmonic clipping’. The radial dot arrays made of silver are numerically designed and fabricated by means of a focused ion beam (FIB). The arrays are illuminated with laser beam through the half wavelength plate to rotate polarization direction. As a result, the plasmonic clipping is observed along the array corresponding to polarization of the excitation light. It is expected to be utilized to align functional components for manufacturing, measurement, and material technologies.

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