## Monday, December 19, 2016

### Studying the Mechanochemistry of Processive Cytoskeletal Motors With an Optical Trap

V. Belyy, A. Yildiz

Cytoskeletal motors utilize the energy stored in ATP to generate linear motion along rigid filaments. Because their enzymatic cycles are tightly coupled to the production of force and forward movement, the optical-trapping technique is uniquely suited for studying their mechanochemical cycle. Here, we discuss the practical aspects of optical trapping in connection with single-motor assays and describe three distinct experimental modes (fixed-trap, force feedback, and square wave) that are typically used to investigate the enzymatic and biophysical properties of cytoskeletal motors. The principal outstanding questions in the field involve motor regulation by cargo adaptor proteins and cargo transport by teams of motors, ensuring that the optical trap's ability to apply precise forces and measure nanometer-scale displacements will remain crucial to the study of intracellular motility in the foreseeable future.

DOI

### Dynamic analysis of hyperbolic waveguide resonator driven by optical gradient force

Zuo-Yang Zhong ; Hai-Lian Zhang ; Wen-Ming Zhang ; Yan Liu

As a unique type of driving force, the transverse optical gradient force has been extensively studied and applied in the nanowaveguides resonator. Recently, it is demonstrated that the optical forces in slot waveguides of hyperbolic metamaterials can be over two orders of magnitude stronger than that in conventional dielectric slot waveguides. To investigate the nonlinear dynamic characteristic of hyperbolic waveguide resonator driven by optical gradient force, a continuum elastic model of the optoresonator is presented and analytically solved using the methods of Rayleigh–Ritz and multiple scales. The results show that the optical force is strengthened with the increase of the filling ratio of silver in the hyperbolic waveguide. The resonance frequency becomes greater with the increase of the filling ratio of silver no matter what the geometric parameters and physical property parameters are. However, the steady maximum vibration amplitude becomes smaller, and the degree of system stiffness softening also reduces.

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### Single-Bond Association Kinetics Determined by Tethered Particle Motion: Concept and Simulations

Koen E. Merkus, Menno W.J. Prins, Cornelis Storm

Tethered particle motion (TPM), the motion of a micro- or nanoparticle tethered to a substrate by a macromolecule, is a system that has proven to be extremely useful for its ability to reveal physical features of the tether, because the thermal motion of the bound particle reports sensitively on parameters like the length, the rigidity, or the folding state of its tether. In this article, we survey the applicability of TPM to probe the kinetics of single secondary bonds, bonds that form and break between the tethered particle and a substrate due, for instance, to receptor/ligand pairs on particle and substrate. Much like the tether itself affects the motion pattern, so do the presence and absence of such secondary connections. Keeping the tether properties constant, we demonstrate how raw positional TPM data may be parsed to generate detailed insights into the association and dissociation kinetics of single secondary bonds. We do this using coarse-grained molecular dynamics simulations specifically developed to treat the motion of particles close to interfaces.

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### Light-Directed Reversible Assembly of Plasmonic Nanoparticles Using Plasmon-Enhanced Thermophoresis

Linhan Lin, Xiaolei Peng, Mingsong Wang, Leonardo Scarabelli, Zhangming Mao, Luis M. Liz-Marzán, Michael F. Becker, and Yuebing Zheng

Reversible assembly of plasmonic nanoparticles can be used to modulate their structural, electrical, and optical properties. Common and versatile tools in nanoparticle manipulation and assembly are optical tweezers, but these require tightly focused and high-power (10–100 mW/μm2) laser beams with precise optical alignment, which significantly hinders their applications. Here we present light-directed reversible assembly of plasmonic nanoparticles with a power intensity below 0.1 mW/μm2. Our experiments and simulations reveal that such a low-power assembly is enabled by thermophoretic migration of nanoparticles due to the plasmon-enhanced photothermal effect and the associated enhanced local electric field over a plasmonic substrate. With software-controlled laser beams, we demonstrate parallel and dynamic manipulation of multiple nanoparticle assemblies. Interestingly, the assemblies formed over plasmonic substrates can be subsequently transported to nonplasmonic substrates. As an example application, we selected surface-enhanced Raman scattering spectroscopy, with tunable sensitivity. The advantages provided by plasmonic assembly of nanoparticles are the following: (1) low-power, reversible nanoparticle assembly, (2) applicability to nanoparticles with arbitrary morphology, and (3) use of simple optics. Our plasmon-enhanced thermophoretic technique will facilitate further development and application of dynamic nanoparticle assemblies, including biomolecular analyses in their native environment and smart drug delivery.

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## Friday, December 16, 2016

### Nanomechanical measurement of adhesion and migration of leukemia cells with phorbol 12-myristate 13-acetate treatment

Zhuo Long Zhou, Jing Ma, Ming-Hui Tong, Barbara Pui Chan, Alice Sze Tsai Wong, Alfonso Hing Wan Ngan

The adhesion and traction behavior of leukemia cells in their microenvironment is directly linked to their migration, which is a prime issue affecting the release of cancer cells from the bone marrow and hence metastasis. In assessing the effectiveness of phorbol 12-myristate 13-acetate (PMA) treatment, the conventional batch-cell transwell-migration assay may not indicate the intrinsic effect of the treatment on migration, since the treatment may also affect other cellular behavior, such as proliferation or death. In this study, the pN-level adhesion and traction forces between single leukemia cells and their microenvironment were directly measured using optical tweezers and traction-force microscopy. The effects of PMA on K562 and THP1 leukemia cells were studied, and the results showed that PMA treatment significantly increased cell adhesion with extracellular matrix proteins, bone marrow stromal cells, and human fibroblasts. PMA treatment also significantly increased the traction of THP1 cells on bovine serum albumin proteins, although the effect on K562 cells was insignificant. Western blots showed an increased expression of E-cadherin and vimentin proteins after the leukemia cells were treated with PMA. The study suggests that PMA upregulates adhesion and thus suppresses the migration of both K562 and THP1 cells in their microenvironment. The ability of optical tweezers and traction-force microscopy to measure directly pN-level cell–protein or cell–cell contact was also demonstrated.

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### Optical Trapping of Plasmonic Mesocapsules: Enhanced Optical Forces and SERS

Donatella Spadaro, Maria Antonia Iatì, Javier Perez-Pineiro, Carmen Vázquez-Vázquez, Miguel A. Correa-Duarte, Maria Grazia Donato, Pietro Giuseppe Gucciardi, Rosalba Saija, Giuseppe Strangi, and Onofrio M Marago

We demonstrate optical trapping of plasmonic silica-gold mesocapsules and their use as local SERS probes in Raman tweezers. These novel hybrid dielectric-metal particles, designed for optoplasmonic applications, are mesoscopic porous silica shells embedding gold nanospheres in their inner wall. We observe a high trapping efficiency due to plasmon-enhanced optical trapping of the gold component. Furthermore, we develop an accurate model of optical trapping of this hybrid system in the T-matrix framework studying how the plasmon-enhanced optical forces scale with gold nanoparticle number in the mesocapsule. The relevance of effective optical trapping in hollow plasmonic mesocapsules is twofold for detection and delivery technologies: Positioning and activation processes. In fact, the presented system allows for the opportunity to drag and locate cargo mesocapsules embedded with specific molecules that can be activated and released in-situ when a precise localization is required.

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### The effect of twisted light on the ring-shaped molecules: The manipulation of the photoinduced current and the magnetic moment by transferring spin and orbital angular momentum of high frequency light

Koray Köksal, Fatih Koç

In this study, we aim to investigate the possibility of the manipulation of an aromatic ring current and magnetic moment induced by spin angular momentum (SAM) and orbital angular momentum (OAM) carrying light by changing the frequency of the light beam. In the study, the possibility of transitions and the contribution of the excited states into the induced current have been investigated by analyzing the angular band structure. The induced current density has been obtained by using time-dependent perturbation theory and calculations have been performed in Cartesian coordinate. The induced magnetic field is calculated by using the well-known Biot-Savart law. The result confirms the possibility of controlling the spin direction of a magnetic atom which can be located at the center of the ring-shaped molecule.

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### Numerical considerations on control of motion of nanoparticles using scattering field of laser light

Naomichi Yokoi, Yoshihisa Aizu

Most of optical manipulation techniques proposed so far depend on carefully fabricated setups and samples. Similar conditions can be fixed in laboratories; however, it is still challenging to manipulate nanoparticles when the environment is not well controlled and is unknown in advance. Nonetheless, coherent light scattered by rough object generates a speckle pattern which consists of random interference speckle grains with well-defined statistical properties. In the present study, we numerically investigate the motion of a Brownian particle suspended in water under the illumination of a speckle pattern. Particle-captured time and size of particle-captured area are quantitatively estimated in relation to an optical force and a speckle diameter to confirm the feasibility of the present method for performing optical manipulation tasks such as trapping and guiding.

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### Interactions of Colloidal Particles and Droplets with Water–Oil Interfaces Measured by Total Internal Reflection Microscopy

Laurent Helden, Kilian Dietrich, and Clemens Bechinger

Total internal reflection microscopy (TIRM) is a well-known technique to measure weak forces between colloidal particles suspended in a liquid and a solid surface by using evanescent light scattering. In contrast to typical TIRM experiments, which are carried out at liquid–solid interfaces, here we extend this method to liquid–liquid interfaces. Exemplarily, we demonstrate this concept by investigating the interactions of micrometer-sized polystyrene particles and oil droplets near a flat water–oil interface for different concentrations of added salt and ionic surfactant (SDS). We find that the interaction is well described by the superposition of van der Waals and double layer forces. Interestingly, the interaction potentials are, within the SDS concentration range studied here, rather independent of the surfactant concentration, which suggests a delicate counter play of different interactions at the oil–water interface and provides interesting insights into the mechanisms relevant for the stability of emulsions.

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## Monday, December 12, 2016

### Self-propelled round-trip motion of Janus particles in static line optical tweezers

Jing Liu, Hong-Lian Guo and Zhi-Yuan Li

Controlled propulsion of microparticles and micromachines in fluids could revolutionize many aspects of technology, such as biomedicine, microfluidics, micro-mechanics, optomechanics, and cell biology. We report the self-propelled cyclic round-trip motion of metallo-dielectric Janus particles in static line optical tweezers (LOT). The Janus particle is a 5 μm-diameter polystyrene sphere half-coated with 3 nanometer thick gold film. Both experiment and theory show that this cyclic translational and rotational motion is a consequence of the collective and fine action of the gold-face orientation dependent propulsion optical force, the gradient optical force, and the spontaneous symmetry breaking induced optical torque in different regions of the LOT. This study indicates a novel way to propel and manipulate the mechanical motion of microscopic motors and machines wirelessly in fluid, air, or vacuum environments using a static optical field with a smartly designed non-uniform intensity profile allowing fully controlled momentum and angular momentum exchange between light and the particle.

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### Rotational Dynamics and Heating of Trapped Nanovaterite Particles

Yoshihiko Arita, Joseph M. Richards, Michael Mazilu, Gabriel C. Spalding, Susan E. Skelton Spesyvtseva, Derek Craig, and Kishan Dholakia

We synthesise, optically trap and rotate individual nanovaterite crystals with a mean particle radius of $423\,\mathrm{nm}$. Rotation rates of up to $4.9\,{\rm kHz}$ in heavy water are recorded. Laser-induced heating due to residual absorption of the nanovaterite particle results in the superlinear behaviour of the rotation rate as a function of trap power. A finite element method based on the Navier-Stokes model for the system allows us to determine the residual optical absorption coefficient for a trapped nanovaterite particle. This is further confirmed by the theoretical model. Our data show that the translational Stokes drag force and rotational Stokes drag torque needs to be modified with appropriate correction factors to account for the power dissipated by the nanoparticle.

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### Manipulation and detection of single nanoparticles and biomolecules by a photonic nanojet

Yu-Chao Li, Hong-Bao Xin, Hong-Xiang Lei, Lin-Lin Liu, Yan-Ze Li, Yao Zhang and Bao-Jun Li

Optical methods to manipulate and detect nanoscale objects are highly desired in both nanomaterials and molecular biology fields. Optical tweezers have been used to manipulate objects that range in size from a few hundred nanometres to several micrometres. The emergence of near-field methods that overcome the diffraction limit has enabled the manipulation of objects below 100 nm. A highly free manipulation with signal-enhanced real-time detection, however, remains a challenge for single sub-100-nm nanoparticles or biomolecules. Here we show an approach that uses a photonic nanojet to perform the manipulation and detection of single sub-100-nm objects. With the photonic nanojet generated by a dielectric microlens bound to an optical fibre probe, three-dimensional manipulations were achieved for a single 85-nm fluorescent polystyrene nanoparticle as well as for a plasmid DNA molecule. Backscattering and fluorescent signals were detected with the enhancement factors up to ~103 and ~30, respectively. The demonstrated approach provides a potentially powerful tool for nanostructure assembly, biosensing and single-biomolecule studies.

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### Mechanical actuation of graphene sheets via optically induced forces

In this paper, we theoretically demonstrate the strong mechanical response of graphene sheets actuated by near-field optical forces. We study single-layer graphene and a two-layer graphene stack with large separation and show that tunable attractive and repulsive forces can be generated. A large nonlinear mechanical response can be obtained by driving the sheets through external radiation and guided modes. We report formation of graphene bubbles of several nanometers in height. Our study points towards new routes for mechanical actuation of graphene, providing new platforms for straintronics and flexible optoelectronics.

DOI

## Friday, December 9, 2016

### Control of Single-Cell Migration Using a Robot-Aided Stimulus-Induced Manipulation System

Hao Yang ; Xiangpeng Li ; Yong Wang ; Gang Feng ; Dong Sun

Cell migration is a natural movement that occurs in response to stimuli in a living environment. Analysis and control of cell-migration behavior can help elucidate various developmental and maintenance processes of multicellular organisms, thereby contributing to the development of new target therapy approaches. In this study, we successfully achieved the automated control of single-cell migration by utilizing the intrinsic migration ability of cells in an engineering control framework. Chemoattractant-loaded microsource beads trapped by robotically controlled optical tweezers were used to release the stimuli and consequently induce target cells to migrate into a desired region. Cell-movement dynamics under optical tweezer manipulation was analyzed. A new geometric model that confined microsource beads within the effective trapping area of the optical tweezers and the high-motility area of the cell under study was established. Based on this model, we developed a potential field function-based controller that handled the optical tweezers in manipulating the microsource beads, thereby stimulating cells to migrate into desired regions. Simulations and experiments were further performed to verify the effectiveness of the proposed approach.

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### 3D controlling the bead linking to DNA molecule in a single-beam nonlinear optical tweezers

Trung Thai Dinh, Khoa Doan Quoc, Kien Bui Xuan, Quy Ho Quang

The approximate expressions describing the redistribution of laser beam and optical forces in Kerr fluid, and the ratio of refractive indexes basing on the optical Kerr effect in fluid are derived. Basing on them, the dynamic of nonlinear bead in the nonlinear fluid is simulated by the finite different Langevin equation in presence of the optical Kerr and self-focused effects. The radial and axial control processes of bead linking to λ-phage WLC DNA molecule in fluid space are numerically observed by calibration of the laser power under and upper the critical value, respectively. The stable position-laser power characteristics are numerically found out. Based on the results, a sample of single-beam optical tweezers for 3D (axial and radial) control of bead is proposed and discussed.

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### Automated Transportation of Multiple Cell Types Using a Robot-Aided Cell Manipulation System with Holographic Optical Tweezers

Songyu Hu ; Shuxun Chen ; Si Chen ; Gang Xu ; Dong Sun

Transferring multiple cell types with high precision and efficiency has become increasingly important as developing cell-based assays. In this study, an enable technology is proposed for simultaneous automated transportation of multiple cell types utilizing a robot-aided cell manipulation system equipped with holographic optical tweezers. The dynamics of trapped cell is initially analyzed. A control constraint is introduced to confine the offset of cells within the optical trap to prevent cells from escaping the trap during transportation. Unlike existing methods determining the critical offset through manual calibration for only a particular cell type, this proposed approach can automatically derive and apply the control constraint to multiple cell types with different radii. A controller is then developed for automated transportation of multiple cell types with different sizes in which exact values of model parameters, such as trapping stiffness and drag coefficient, are not required. Experiments are finally performed on the transportation of yeast cells and osteoblast-like MC3T3-E1 cells to demonstrate the effectiveness of the proposed approach.

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### Towards nano-optical tweezers with graphene plasmons: Numerical investigation of trapping 10-nm particles with mid-infrared light

Jianfa Zhang, Wenbin Liu, Zhihong Zhu, Xiaodong Yuan & Shiqiao Qin

Graphene plasmons are rapidly emerging as a versatile platform for manipulating light at the deep subwavelength scale. Here we show numerically that strong optical near-field forces can be generated under the illumination of mid-IR light when dielectric nanoparticles are located in the vicinity of a nanostructured graphene film. These near-field forces are attributed to the excitation of the graphene’s plasmonic mode. The optical forces can generate an efficient optical trapping potential for a 10-nm-diameter dielectric particle when the light intensity is only about about 4.4 mW/μm2 and provide possibilities for a new type of plasmonic nano-tweezers. Graphene plasmonic tweezers can be potentially exploited for optical manipulation of nanometric biomolecules and particles. Moreover, the optical trapping/tweezing can be combined with biosensing and provide a versatile platform for studing biology and chemistry with mid-IR light.

DOI

## Wednesday, December 7, 2016

### Stability, folding dynamics, and long-range conformational transition of the synaptic t-SNARE complex

Xinming Zhang, Aleksander A. Rebane, Lu Ma, Feng Li, Junyi Jiao, Hong Qu, Frederic Pincet, James E. Rothman, and Yongli Zhang

Synaptic soluble N-ethylmaleimide–sensitive factor attachment protein receptors (SNAREs) couple their stepwise folding to fusion of synaptic vesicles with plasma membranes. In this process, three SNAREs assemble into a stable four-helix bundle. Arguably, the first and rate-limiting step of SNARE assembly is the formation of an activated binary target (t)-SNARE complex on the target plasma membrane, which then zippers with the vesicle (v)-SNARE on the vesicle to drive membrane fusion. However, the t-SNARE complex readily misfolds, and its structure, stability, and dynamics are elusive. Using single-molecule force spectroscopy, we modeled the synaptic t-SNARE complex as a parallel three-helix bundle with a small frayed C terminus. The helical bundle sequentially folded in an N-terminal domain (NTD) and a C-terminal domain (CTD) separated by a central ionic layer, with total unfolding energy of ∼17 kBT, where kB is the Boltzmann constant and T is 300 K. Peptide binding to the CTD activated the t-SNARE complex to initiate NTD zippering with the v-SNARE, a mechanism likely shared by the mammalian uncoordinated-18-1 protein (Munc18-1). The NTD zippering then dramatically stabilized the CTD, facilitating further SNARE zippering. The subtle bidirectional t-SNARE conformational switch was mediated by the ionic layer. Thus, the t-SNARE complex acted as a switch to enable fast and controlled SNARE zippering required for synaptic vesicle fusion and neurotransmission.

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### From Surface Protrusion to Tether Extraction: A Mechanistic Model

Jin-Yu Shao, Yan Yu, and Sara J Oswald

Human leukocyte rolling on the endothelium is essential for leukocyte emigration and it is a process regulated by many factors including shear stress, receptor-ligand kinetics, and mechanical properties of cells and molecules. During this process, both leukocytes and endothelial cells (ECs) are pulled by forces due to blood flow and both may experience surface protrusion and tether extraction. In this study, we established a two-scale (cellular and molecular) model of cellular deformation due to a point pulling force and illustrated how surface protrusion makes the transition to tether extraction, either gradually or abruptly. Our simulation results matched well with what was observed in the experiments conducted with the optical trap and the atomic force microscope. We found that, although the traditional method of determining the force loading rate and the protrusional stiffness were still reasonable, the crossover force should not be simply interpreted as the rupture force of the receptor-cytoskeleton linkage. With little modification, this model can be incorporated into any leukocyte rolling model as a module for more accurate and realistic simulation.

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### Theory of optical-tweezers forces near a plane interface

R. S. Dutra, P. A. Maia Neto, H. M. Nussenzveig, and H. Flyvbjerg
Optical-tweezers experiments in molecular and cell biology often take place near the surface of the microscope slide that defines the bottom of the sample chamber. There, as elsewhere, force measurements require force-calibrated tweezers. In bulk, one can calculate the tweezers force from first principles, as recently demonstrated. Near the surface of the microscope slide, this absolute calibration method fails because it does not account for reverberations from the slide of the laser beam scattered by the trapped microsphere. Nor does it account for evanescent waves arising from total internal reflection of wide-angle components of the strongly focused beam. In the present work we account for both of these phenomena. We employ Weyl's angular spectrum representation of spherical waves in terms of real and complex rays and derive a fast-converging recursive series of multiple reflections that describes the reverberations, including also evanescent waves. Numerical simulations for typical setup parameters evaluate these effects on the optical force and trap stiffness, with emphasis on axial trapping. Results are in good agreement with available experimental data. Thus, absolute calibration now applies to all situations encountered in practice.

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### Out-of-equilibrium force measurements of dual-fiber optical tweezers

Jochen Fick

Optical trapping of micron-size dielectric particles in a dual-fiber tip configuration is presented. Trap oscillation and suspension flow experiments are performed to investigate the linearity of the optical forces. These measurements are completed by standard methods such as Boltzmann statistics or power spectra evaluation. Strong trapping efficiencies of 0.25 and 1.8  pN·μm−11.8  pN·μm−1 have been found in the axial and transverse directions, respectively. The values obtained by the different approaches are in good agreement. The measurements show that the optical trapping potential is harmonic over the experimentally attainable distances, i.e., 2.5 and 0.6 μm in the transverse and axial directions, respectively.

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## Friday, December 2, 2016

### Direct imaging of a digital-micromirror device for configurable microscopic optical potentials

G. Gauthier, I. Lenton, N. McKay Parry, M. Baker, M. J. Davis, H. Rubinsztein-Dunlop, and T. W. Neely

Programmable spatial light modulators have significantly advanced the configurable optical trapping of particles. Typically, these devices are utilized in the Fourier plane of an optical system, but direct imaging of an amplitude pattern can potentially result in increased simplicity and computational speed. Here we demonstrate high-resolution direct imaging of a digital micromirror device (DMD) at high numerical apertures (NAs), which we apply to the optical trapping of a Bose–Einstein condensate (BEC). We utilize a (1200×19201200×1920) pixel DMD and commercially available 0.45 NA microscope objectives, finding that atoms confined in a hybrid optical/magnetic or all-optical potential can be patterned using repulsive blue-detuned (532 nm) light with 630(10) nm full width at half-maximum resolution, within 5% of the diffraction limit. The result is near arbitrary control of the density of the BEC without the need for expensive custom optics. We also introduce the technique of time-averaged DMD potentials, demonstrating the ability to produce multiple gray-scale levels with minimal heating of the atomic cloud, by utilizing the high switching speed (20 kHz maximum) of the DMD. These techniques will enable the realization and control of diverse optical potentials for superfluid dynamics and atomtronics applications with quantum gases. The performance of this system in a direct imaging configuration has wider application for optical trapping at non-trivial NAs.

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### Tunable Fano resonant optical forces exerted on a graphene-coated dielectric particle by a Gaussian evanescent wave

Yang Yang, Xiaofu Zhang, Anping Huang and Zhisong Xiao

In this paper, we investigate the optical forces exerted on a graphene-coated dielectric particle by the Gaussian beam transmitted through the prism setup systematically. It is shown that the optical force spectra exhibit significant Fano resonance under the excitation of a Gaussian evanescent wave. The magnitude and morphology of Fano resonance can be modulated effectively by the graphene coating. Also, the modification on the threshold of the Fermi energy of graphene could help to regulate the trapping behavior efficiently. The proposed work may provide a new avenue in the specific optical tweezers and nano-optics.

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### Real-time force measurement in double wavelength optical tweezers

Sławomir Drobczyński and Kamila Duś-szachniewicz
In optical tweezers, the trap stiffness varies across the sample area. To avoid this problem, the force measurement is often performed in a fixed place where the trap stiffness is well determined. However, for some experiments, bringing the sample to the fixed position is problematic. In this paper, we describe a precise and fast procedure for mapping the trap stiffness over the whole sample area. Such a map allows development of a real-time procedure for force measurement at any point of the sample area. The presented method is particularly suitable for measuring forces, for example, in living cells samples.

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