## Friday, November 16, 2018

### Repulsion of polarized particles from two-dimensional materials

Francisco J. Rodríguez-Fortuño, Michela F. Picardi, and Anatoly V. Zayats

Repulsion of nanoparticles, molecules, and atoms from surfaces can have important applications in nanomechanical devices, microfluidics, optical manipulation, and atom optics. Here, through the solution of a classical scattering problem, we show that a dipole source oscillating at a frequency ω can experience a robust and strong repulsive force when its near-field interacts with a two-dimensional material. As an example, the case of graphene is considered, showing that a broad bandwidth of repulsion can be obtained at frequencies for which propagation of plasmon modes is allowed 0<ω<(5/3)μc, where μc is the chemical potential tunable electrically or by chemical doping.

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### Optical transportation of micro-particles by non-diffracting Weber beams

Weiwei Liu, Jie Gao and Xiaodong Yang

The optical transportation of solid polystyrene particles by the non-diffracting Weber beams is demonstrated with both forward and backward motion along the parabolic main lobes of Weber beams in three dimensions. The Weber beams are generated from the complex field modulation based on the spatial light modulator for realizing the optical transportation of micrometer polystyrene particles in an optical tweezers setup. The particle motion and velocity distribution along the main lobes of Weber beams in two different parabolic shapes are characterized and compared.

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### Optical manipulation of chiral nanoparticles in vector Airy beam

Wanli Lu, Xu Sun, Huajin Chen, Shiyang Liu and Zhifang Lin

The optical manipulation of chiral nanoparticles in a vector Airy beam with linear polarization is theoretically investigated, and to calculate the optical forces acting on a spherical chiral particle of an arbitrary size beyond the paraxial approximation, a rigorous numerical method based on the generalized Lorenz–Mie theory and Maxwell stress tensor method is presented. It is found that the chiral nanoparticle not only can be stably trapped within the main lobe due to the transverse optical gradient force but also can be transported faster than a conventional particle without chirality along curved trajectories because of the longitudinal scattering optical force. In addition, the particle chirality significantly enhances the longitudinal optical force while slightly affecting the transverse optical force. Our results may provide an additional handle for the optical manipulation of chiral nanoparticles.

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### Creating Multifunctional Optofluidic Potential Wells for Nanoparticle Manipulation

Fan Nan and Zijie Yan

Optical forces have enabled various nanomanipulation in microfluidics such as optical trapping, sorting, and transporting of nanoparticles (NPs), but the manipulation is usually specific with a certain optical field. Tightly focused Gaussian beams can trap NPs but not sort them; moderately focused Gaussian beams allow sorting microparticles in a flow but not NPs; quasi-Bessel beams can sort NPs in a flow but cannot control their positions due to low trapping stiffness. All these methods rely on the axial variation of laser intensity. Here we show that multifunctional and tunable optofluidic potential wells can be created for nanomanipulation by synchronizing optical phase gradient force with fluid drag force. We demonstrate controlled trapping and transporting of 150 nm Ag NPs over 10 μm and sorting of 80 and 100 nm Au NPs using optical line traps with tunable phase gradients in experiments. Our simulations further predict that simultaneous sorting and trapping of sub-50 nm Au NPs can be achieved with a sorting resolution of 1 nm using optimized optical fields. Our method provides great freedom and flexibility for nanomanipulation in optofluidics with potential applications in nanophotonics and biomedicine.

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### Kinetics coming into focus: single-molecule microscopy of riboswitch dynamics

Sujay Ray, Adrien Chauvier and Nils G. Walter

Riboswitches are dynamic RNA motifs that are mostly embedded in the 5ʹ-untranslated regions of bacterial mRNAs, where they regulate gene expression transcriptionally or translationally by undergoing conformational changes upon binding of a small metabolite or ion. Due to the small size of typical ligands, relatively little free energy is available from ligand binding to overcome the often high energetic barrier of reshaping RNA structure. Instead, most riboswitches appear to take advantage of the directional and hierarchical folding of RNA by employing the ligand as a structural ‘linchpin’ to adjust the kinetic partitioning between alternate folds. In this model, even small, local structural and kinetic effects of ligand binding can cascade into global RNA conformational changes affecting gene expression. Single-molecule (SM) microscopy tools are uniquely suited to study such kinetically controlled RNA folding since they avoid the ensemble averaging of bulk techniques that loses sight of unsynchronized, transient, and/or multi-state kinetic behavior. This review summarizes how SM methods have begun to unravel riboswitch-mediated gene regulation.

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### Length dependence of viscoelasticity of entangled-DNA solution with and without external stress

Masaya Tanoguchi and Yoshihiro Murayama

We observed the diffusive motion of a micron-sized bead in an entangled-DNA solution to investigate the effect of the viscoelasticity on the bead motion. In the absence of external stress (passive microrheology), subdiffusion appears in the timescale of 0.1–10 s, and the normal diffusion recovers in longer timescales. We evaluated the apparent viscosity and elasticity, which yields a simple relaxation time for the viscoelastic medium. We found that the absence of DNA-length dependence for the time-dependent diffusion is explained by the simple relaxation of the viscoelastic media rather than the reptation dynamics, including the disentanglement. On the other hand, in the presence of a small external stress in active microrheology, the bead motion showed clear length dependence owing to the viscoelasticity. These results suggest that the viscoelasticity of the entangled DNA is highly sensitive to the external stress, even in the linear response regime.

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### An optical trapping system for particle probes in plasma diagnostics

Viktor Schneider and Holger Kersten

We present one of the first experiments for optically trapping of single microparticles as probes for low temperature plasma diagnostics. Based on the dual laser beam, counter-propagating technique, SiO2 microparticles are optically trapped at very large distances in low-temperature, low-pressure rf plasma. External forces on the particle are measured by means of the displacement of the probe particle in the trap. Measurements can be performed during plasma operation as well as without plasma. The paper focuses on the optical setup and the verification of the system and its principle. Three examples for the particle behavior in the trapping system are presented: First, we measured the neutral gas damping as a verification of the technique. Second, an experiment without a plasma studies the changing particle charge by UV light radiation, and third, by moving the probe particle in the vertical direction into the sheath or into the plasma bulk, respectively, the acting forces on the probe particle are measured.

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## Wednesday, November 14, 2018

### Experimental realization of Feynman's ratchet

Jaehoon Bang, Rui Pan, Thai M Hoang, Jonghoon Ahn, Christopher Jarzynski, H T Quan and Tongcang Li

Feynman's ratchet is a microscopic machine in contact with two heat reservoirs, at temperatures T A and T B , that was proposed by Richard Feynman to illustrate the second law of thermodynamics. In equilibrium (T A = T B ), thermal fluctuations prevent the ratchet from generating directed motion. When the ratchet is maintained away from equilibrium by a temperature difference (${T}_{A}\ne {T}_{B}$), it can operate as a heat engine, rectifying thermal fluctuations to perform work. While it has attracted much interest, the operation of Feynman's ratchet as a heat engine has not been realized experimentally, due to technical challenges. In this work, we realize Feynman's ratchet with a colloidal particle in a one-dimensional optical trap in contact with two heat reservoirs: one is the surrounding water, while the effect of the other reservoir is generated by a novel feedback mechanism, using the Metropolis algorithm to impose detailed balance. We verify that the system does not produce work when T A = T B , and that it becomes a microscopic heat engine when ${T}_{A}\ne {T}_{B}$. We analyze work, heat and entropy production as functions of the temperature difference and external load. Our experimental realization of Feynman's ratchet and the Metropolis algorithm can also be used to study the thermodynamics of feedback control and information processing, the working mechanism of molecular motors, and controllable particle transportation.

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### Directional scattering and multipolar contributions to optical forces on silicon nanoparticles in focused laser beams

Nils Odebo Länk, Peter Johansson, and Mikael Käll

Nanoparticles made of high index dielectric materials have seen a surge of interest and have been proposed for various applications, such as metalenses, light harvesting and directional scattering. With the advent of fabrication techniques enabling colloidal suspensions, the prospects of optical manipulation of such nanoparticles becomes paramount. High index nanoparticles support electric and magnetic multipolar responses in the visible regime and interference between such modes can give rise to highly directional scattering, in particular a cancellation of back-scattered radiation at the first Kerker condition. Here we present a study of the optical forces on silicon nanoparticles in the visible and near infrared calculated using the transfer matrix method. The zero-backscattering Kerker condition is investigated as an avenue to reduce radiation pressure in an optical trap. We find that while asymmetric scattering does reduce the radiation pressure, the main determining factor of trap stability is the increased particle response near the geometric resonances. The trap stability for non-spherical silicon nanoparticles is also investigated and we find that ellipsoidal deformation of spheres enables trapping of slightly larger particles.

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### Plasmonic Nanogaps: From Fabrications to Optical Applications

Panpan Gu , Wei Zhang, Gang Zhang

Metallic nanostructures with nanogap features are proved to be highly effective building blocks for plasmonic systems, as they can provide ultrastrong electromagnetic (EM) fields and controllable optical properties. A wide range of fields, including surface enhanced spectroscopy, sensing, imaging, nonlinear optics, optical trapping, and metamaterials, are benefited from these enhanced EM fields. This review outlines the latest development of the fabrication methods for nanogap structures (metal nanoparticle assembly, nanosphere lithography, electron beam lithography (EBL), focused ion beam (FIB) lithography, oblique angle shadow evaporation, edge lithography, and so on), followed by a summary of their optical applications. The present review will inspire more ingenious designs and fabrications of plasmonic nanogap structures with lithography‐free fabrication techniques, and promote their applications in optics and electronics.

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