Measurement and simulation of atomic motion in nanoscale optical trapping potentials

Signe B. Markussen, Jürgen Appel, Christoffer Østfeldt, Jean-Baptiste S. Béguin, Eugene S. Polzik & Jörg H. Müller

Atoms trapped in the evanescent field around a nanofiber experience strong coupling to the light guided in the fiber mode. However, due to the intrinsically strong positional dependence of the coupling, thermal motion of the ensemble limits the use of nanofiber trapped atoms for some quantum tasks. We investigate the thermal dynamics of such an ensemble using short light pulses to make a spatially inhomogeneous population transfer between atomic states. As we monitor the wave packet of atoms created by this scheme, we find a damped oscillatory behavior which we attribute to sloshing and dispersion of the atoms. Oscillation frequencies range around 100 kHz, and motional dephasing between atoms happens on a timescale of 10μs. Comparison to Monte Carlo simulations of an ensemble of 1000 classical particles yields reasonable agreement for simulated ensemble temperatures between 25 and 40μK.

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Atom femto trap: experimental realization

Anton E. Afanasiev, Anna A. Meysterson, Anastasiia M. Mashko, Pavel N. Melentiev, Victor I. Balykin

In this work, we demonstrate the trapping of rubidium (Rb) atoms in a pulsed optical dipole trap formed by femtosecond laser radiation with a pulse duration as small as 70 fs. The atom localization in such trap strongly depends on the heating of the atoms caused by the momentum diffusion due to the dipole force fluctuations. The atom femto traps can be used for localization of atoms others than alkaline and alkaline earth atomic elements by conversation of pulsed laser radiation of visible or near infrared to UV spectral.

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Optical trapping and rotating of micro-particles using the circular Airy vortex beams

Musheng Chen, Sujuan Huang, Xianpeng Liu, Yi Chen, Wei Shao

The circular Airy vortex beams (CAVB) are attractive owing to their intriguing properties, such as autofocusing, and self-healing. In this paper, we experimentally study optical manipulation and rotation of the silica micro-particles by the use of the autofocusing circular Airy vortex beams, and analyze the dependence of rotational velocity on the topological charges. Experimental results prove that the circular Airy vortex beams have autofocusing property. With the increase of topological charges, the rotational velocity of the micro-particles first increases and then decreases after reaches a maximum. Optical trapping technology based on CAVB may enrich the possibility and manner for optical trapping and guiding micro-particles.

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Propagation properties of Airy hollow Gaussian vortex beams through the strongly nonlocal nonlinear media

Gengxin Chen, Qiliang Sun, Jintao Xie, Dongmei Deng

We introduce a class of Airy hollow Gaussian vortex (AiHGV) beams and derive the analytical propagating expression of the AiHGV beams in strongly nonlocal nonlinear media (SNNM) using the transfer matrix method for the first time. The nonlinear factor 𝛺 can significantly increase the intensity of the side lobe and the hollow range becomes obvious, which causes the propagation pattern to dramatically change. Moreover, the distribution factor 𝛼 can prominently affect the focusing range and the decay factor a can also flexibly change the focusing position. In addition, 3D propagation and the center of mass are fully described to enrich the propagation properties of the AiHGV beams. Interestingly, the variations of the nonlinear factor 𝛺 and the decay factor a are proportional to the Poynting vector, the angular momentum and the gradient force at the focusing position, in which the effect of the distribution factor 𝛼 is totally different.

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Trapping two types of Rayleigh particles using a focused partially coherent anomalous vortex beam

Miao Dong, Dagang Jiang, Nanhang Luo, Yuanjie Yang

The radiation forces of focused partially coherent anomalous vortex (AV) beams on Rayleigh particles of different refractive indices are studied theoretically and numerically. The influences of the topological charge, the beam order and coherence length on the radiation force are also discussed. It is shown that the focused partially coherent AV beam can be used to trap high index of refraction particles at the focus and to trap low index of refraction particles in the vicinity of the focus. It is also found that the radiation force can be modulated by the topological charge, the beam order and the coherence length.

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Reynolds number and diffusion coefficient of micro- and nano-aerosols in optical pipelines

Amin Mousavi, Fahimeh Hosseinibalam, Smaeyl Hassanzadeh

In this study, the microscopic particle motion inside an optical pipeline, such as particle motion through a mechanical tube, is investigated. The photons in an optical tube guide the particles towards the center of the light beam by inducing photophoretic and radiation pressure forces. Laguerre–Gaussian- and Bessel-like beams are examples of such optical tubes. The Reynolds number of particle motion in optical tubes is investigated. The power of the light beam and the ratio of the particle radius to the light beam ring radius influence the turbulence of the particle flow and the value of the Reynolds number. The diffusion coefficient of particle movement in such pipelines is derived, which indicates that an optical tube is a good tool for guiding and trapping particles in micron- and nanometer-scale dimensions.

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Optical trapping and controllable targeted delivery of nanoparticles by a nanofiber ring

Ying Li, Yanjun Hu

The stable manipulation and position-designated delivery of nanoparticles has shown potential application in targeted drug delivery and enhanced detection of viruses. In this paper, we demonstrate optical trapping and controllable targeted delivery of 700 nm diameter particles using a nanofiber ring. Based on 3D FDTD simulations, the bending loss for bent nanofibers was calculated at different fiber diameters, bending radio, and laser wavelengths. Therefore, according to the theoretical analysis, the 500 nm diameter nanofiber and 808 nm wavelength laser light were chosen. The experimental results indicate that, by directing a laser beam with a wavelength of 808 nm into a nanofiber ring, nanoparticles were trapped and transported along the ring, and released into the water at a designated position because of bending loss. The release position of particles was controllable by the input optical power.

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Dynamics of self-organized aggregation of resonant nanoparticles in a laser field

V. V. Slabko, A. S. Tsipotan, A. S. Aleksandrovsky, E. A. Slyusareva

Self-organized aggregation of nanoparticles in external resonant laser field is considered using Brownian dynamics model. Formation probabilities are calculated for the pair of particles in dependence on laser wavelength and mutual orientation of particles. Times required for aggregation are calculated. Possibility of efficient aggregation using pulsed laser is deduced.

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Polarization evolution characteristics of focused hybridly polarized vector fields

Bing Gu, Yang Pan, Guanghao Rui, Danfeng Xu, Qiwen Zhan, Yiping Cui

We investigate the focusing property and the polarization evolution characteristics of hybridly polarized vector fields in the focal region. Three types of hybridly polarized vector fields, namely azimuthal-variant hybridly polarized vector field, radial-variant hybridly polarized vector field, and spatial-variant hybridly polarized vector field, are experimentally generated. The intensity distributions and the polarization evolution of these hybridly polarized vector fields focused under low numerical aperture (NA) are experimentally studied and good agreements with the numerical simulations are obtained. The three-dimensional (3D) state of polarization and the field distribution within the focal volume of these hybridly polarized vector fields under high-NA focusing are studied numerically. The optical curl force on Rayleigh particles induced by tightly focused hybridly polarized vector fields, which results in the orbital motion of trapped particles, is analyzed. Simulation results demonstrate that polarization-only modulation provided by the hybridly polarized vector field allows one to control both the intensity distribution and 3D elliptical polarization in the focal region, which may find interesting applications in particle trapping, manipulation, and orientation analysis.

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Optodynamic phenomena in aggregates of polydisperse plasmonic nanoparticles

A. E. Ershov, A. P. Gavrilyuk, S. V. Karpov, P. N. Semina
We propose an optodynamical model of interaction of pulsed laser radiation with aggregates of spherical metallic nanoparticles embedded into host media. The model takes into account polydispersity of particles, pair interactions between the particles, dissipation of absorbed energy, heating and melting of the metallic core of particles and of their polymer adsorption layers, and heat exchange between electron and ion components of the particle material as well as heat exchange with the interparticle medium. Temperature dependence of the electron relaxation constant of the particle material and the effect of this dependence on interaction of nanoparticles with laser radiation are first taken into consideration. We study in detail light-induced processes in the simplest resonant domains of multiparticle aggregates consisting of two particles of an arbitrary size in aqueous medium. Optical interparticle forces are realized due to the light-induced dipole interaction. The dipole moment of each particle is calculated by the coupled dipole method (with correction for the effect of higher multipoles). We determined the role of various interrelated factors leading to photomodification of resonant domains and found an essential difference in the photomodification mechanisms between polydisperse and monodisperse nanostructures.
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Single crystal formation of amino acid with high temporal controllability by combining femtosecond and continuous wave laser trapping

Atsushi Miura, Yan-Hua Huang, Hiroshi Masuhara
We investigated laser trapping crystallization of glycine by using femtosecond (fs) laser as a trapping light source. Impulsively exerted fs laser pulses crystallized glycine more effectively than that induced by continuous wave (CW) laser trapping. Highly efficient crystallization and crystal growth behavior indicates fs laser irradiation increased the concentration not only at the focal spot, but also around the laser focus. Furthermore, we found that irradiation of fs pulses to CW laser-induced locally high supersaturation region enables immediate crystallization. Spatiotemporally controlled triggering of a single crystal formation with sub-second time resolution has achieved by integrating fs and CW laser trapping techniques.

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Separation of spin angular momentum in space-variant linearly polarized beam

Hao Chen, Zhongliang Yu, Jingjing Hao, Zhaozhong Chen, Ji Xu, Jianping Ding, Hui-Tian Wang

We show that the spin angular momentum (SAM) flux in a space-variant linearly polarized beam can be separated in the focal plane. Such a beam carries only orbital angular momentum (OAM) and develops a net SAM flux upon focusing. The radial splitting of the SAM flux density is mediated by the phase vortex (or OAM) and can be controlled by the topological charge of the phase vortex. Optical trapping experiments verify the separation of the SAM flux density. The proposed approach enriches the manipulation of the angular momentum of light fields and inspires more designs of focus engineering, which would benefit optical micromanipulation of microscopic particles.
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Photocatalytic nano-optical trapping using TiO2 nanosphere pairs mediated with Mie-scattered near field

Toshiyuki Honda, Mitsuhiro Terakawa, Minoru Obara
The localized enhanced near field on nanostructures has been attracting much attention for a template for size-selective optical trapping (tweezers) beyond the diffraction limit. The near-field optical trapping has mainly been studied using metallic substrates such as Au nanodot pairs, periodic Al nanoslits, nanoapertures on an Au film, etc. In this paper, we newly propose a Mie-scattered-near-field optical trapping scheme for size-selective photocatalytic application using pairs of poly-rutile TiO2 nanospheres. The optical intensity distribution in a 3D-nanogap space between the nanospheres was simulated by a 3D FDTD method. The simulation system consists of the two TiO2nanospheres placed on a silica substrate in water. The 400-nm excitation laser is used for both the near-field trapping and the photocatalyst excitation. The optical trapping forces were calculated based on the near-field optical intensity distribution. The trapping stiffness for 20-nm polystyrene sphere at a gap distance of 20 nm was 6.4 pN/nm/W. The optical force vector shows that the object like virus can be trapped with sufficient forces into the nanogap space and then is driven into the direct surface of the TiO2 sphere. This result suggests that this system works as a photocatalytic trapping for killing virus, protein, etc.

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Videomicroscopy calibration of optical tweezers by position autocorrelation function analysis

P. S. Alves and M. S. Rocha

We present a simple method to calibrate optical tweezers by using only videomicroscopy to measure the position autocorrelation function of the trapped bead in the potential well of the tweezers. To accomplish this task, we use a high-speed camera with ∼500 fps (frames per second), which provides a precise measurement of the relaxation time of the bead Brownian fluctuations. We also study the variation of the trap stiffness as a function of some parameters of interest such as the laser power and the distance from the bead center to the microscope coverslip, showing that the presented method returns precise results.

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Spatial light modulators for the manipulation of individual atoms

L. Brandt, C. Muldoon, T. Thiele, J. Dong, E. Brainis and A. Kuhn

We propose a versatile arrangement for the trapping and manipulation of single atoms in optical tweezers formed by the direct image of a spatial light modulator (SLM). The scheme incorporates a high numerical aperture microscope to map the intensity distribution of a SLM onto a cloud of cold atoms. The regions of high intensity act as optical dipole-force traps. With a SLM fast enough to modify the trapping potential in real time, this technique is well suited for the controlled addressing and manipulation of arbitrarily selected atoms.

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Microfluidic sorting system based on optical force switching

S.-K. Hoi, C. Udalagama, C.-H. Sow, F. Watt and A. A. Bettiol

We report a versatile, and automatic method for sorting cells and particles in a three dimensional polydimethylsiloxane (PDMS) structure consisting of two cross-microchannels. As microspheres or yeast cells are fed continuously into a lower channel, a line shaped focused laser beam is applied (perpendicular to the direction of flow) at the crossing junction of the two channels. The scattering force of the laser beam was employed to push microparticles matching specific criteria upwards from one channel to another. The force depends on the intrinsic properties of the particles such as their refractive index and size, as well as the laser power and the fluid flow speed. The combination of these parameters gives a tunable selection criterion for the effective and efficient sorting of the particles. The introduction of the cylindrical lens into the optical train allows for simultaneous manipulation of multiple particles which has significantly increased the efficiency and throughput of the sorting. A high aspect ratio microchannel (A.R.=1.6) was found to enhance the sorting performance of the device. By careful control of the microparticle flow rate, near 100% sorting efficiency was achieved.

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