Rotation Of Microparticles Research Articles

Controllable optical tweezer and spanner in evanescent fields via a single plasmonic metasurface

A dual-functional plasmonic metasurface is proposed to realize trapping and rotation of microparticles in evanescent fields by simply changing the polarization of incident light. The metasurface is constituted with subwavelength rectangular nanoslit that is perforated in an Au film on the glass substrate. Simulated near-field intensity distributions show that surface plasmon vortex with designed topological charge and focused point with enhanced intensity can be controllably generated in the center region of the designed metasurface by different circularly polarized lights. Calculated optical force and optical potential on a polystyrene sphere further demonstrate the good performances of rotating and trapping a microparticle with the generated vortex and focused surface plasmon polaritons. Moreover, two examples designed with different topological charges demonstrate the flexibility of these metasurfaces in tuning the rotation radius of microparticles. The advantages of the proposed metasurface in design flexibility, multifunctionality, and small size may provide new possibilities for applications of integrated optical manipulation devices and systems.

Acoustic rotation of multiple subwavelength cylinders for three-dimensional topography reconstruction

Accurate rotation of microparticles is of great significance in micro-rotors, multi-angle microscopic observation, microbial three-dimensional phenotyping, and microsystem assembly. However, most methods can only rotate a single object, thus limiting the throughput. In this study, we realized the simultaneous rotation of many trapped and aligned subwavelength glass cylinders inside an evanescent wave field excited by a resonant phononic crystal plate. The unique feature of the rotation lies in its periodic distribution as well as the rotation axis being perpendicular to the acoustic axis. The rotary power originates from viscous torque generated by the evanescent wave-induced near-boundary acoustic streaming's asymmetry distribution on the trapped cylinder. Furthermore, the three-dimensional topographies of rotated cylinders can be reconstructed from the microscopic images under different rotating angles. Our findings can pave the way toward developing simple, disposable, and scalable microfluidic devices for massive subwavelength acoustic rotation by carefully designing acoustic metamaterials.

Fluctuation-mediated orbital rotation of microparticles in non-coaxially counter-propagating optical tweezers

We have demonstrated in the present report that dielectric microparticles exhibited orbital rotation in the light field of non-coaxially configured two counter-propagating laser beams both in numerical simulations and experiments. A series of computational simulations indicated that when irradiated with two non-coaxially counter-propagating parallel laser beams with the same intensity distributions in the absence of thermal (Brownian) motion, a microparticle did not exhibit orbital rotation due to the symmetry of the optical field. However, the computations predicted that a microparticle exhibited one directional orbital rotation in the presence of thermal motion because of the symmetry breaking of the optical force acting on the particle. This spontaneous orbital rotation was experimentally demonstrated for 1-µm dielectric particles in water at room temperature.Graphical abstract

Orbital Angular Momentum in Nanoplasmonic Vortices

In the past decade, optical orbital angular momentum (OAM) has entered the field of plasmonics in the form of surface-confined vortices, generating vast interest. Here we give an overview of the field, starting with the pioneering analytic and experimental investigations of plasmonic OAM. We describe the advances leading to the study of suboptical cycle dynamics in time-resolved experiments and the investigation of angular momentum light–matter interactions through the mixing of circularly polarized light with plasmonic vortices. We describe how additional degrees of freedom can be controlled with metasurfaces and other design techniques leading to the complete spatial and temporal modularization of surface-confined OAM. We review maturing applications of plasmonic vortices, such as plasmonic tweezers for the selective trapping and rotation of microparticles and broadband multiplexing of angular momentum light beams for high-capacity optical communications. The major advances in the field, from the first observation to complete modularization, position the controlled surface-confined OAM at the forefront of light–matter science.

Optical Forces and Torques on Eccentric Nanoscale Core–Shell Particles

The optical trapping and manipulation of small particles is an important tool for probing fluid properties at the microscale. In particular, microrheology exploits the manipulation and rotation of micron-scale particles to probe local viscosity, especially where these properties may be perturbed as a function of their local environment, for example in the vicinity of cells. To this end, birefringent particles are useful as they can be readily controlled using optically induced forces and torques, and thereby used to probe their local environment. However the magnitude of optical torques that can be induced in birefringent particles is small, and a function of the particle diameter, meaning that rotational flow cannot readily be probed on length scales much small than the micron level. Here we show modelling that demonstrates that eccentric spherical core-shell nanoparticles can be used to generate considerable optical torques. The eccentricity is a result of the displacement of the centre of the core from the shell. Our results show that, for particles ranging from 90~nm to 180~nm in diameter, we may achieve rotation rates exceeding 800~Hz. This fills a missing size gap in the rotation of microparticles with optical forces. The diameter of particle we may rotate is almost an order of magnitude smaller than the smallest birefringent particles that have been successfully rotated to date. The rotation of eccentric core-shell nanoparticles therefore makes an important contribution to biophotonics and creates new opportunities for rheology in nanoscale environments.

Trapping and rotation of microparticles using a metasurface exciting by linearly polarized beam

We numerically demonstrate trapping and rotation of particles using a metasurface formed by arranging nanocavities as a right-handed Archimedes’ spiral. Excited by a 90° linearly polarized beam, a focused surface plasmon polariton (SPP) field is formed at the center of the spiral, and the particle can be trapped by the field. While excited by −45° linearly polarized beams, a vortex SPP field carrying orbital angular momentum is formed, and the particles can be trapped and rotated in the clockwise direction at the vortex field.

Pitch-rotational manipulation of single cells and particles using single-beam thermo-optical tweezers.

3D pitch rotation of microparticles and cells assumes importance in a wide variety of applications in biology, physics, chemistry and medicine. Applications such as cell imaging and injection benefit from pitch-rotational manipulation. Generation of such motion in single beam optical tweezers has remained elusive due to the complexities of generating high enough ellipticity perpendicular to the direction of propagation. Further, trapping a perfectly spherical object at two locations and subsequent pitch rotation hasn't yet been demonstrated to be possible. Here, we use hexagonal-shaped upconverting particles and single cells trapped close to a gold-coated glass cover slip in a sample chamber to generate complete 360 degree and continuous pitch motion even with a single optical tweezer beam. The tweezers beam passing through the gold surface is partially absorbed and generates a hot-spot to produce circulatory convective flows in the vicinity which rotates the objects. The rotation rate can be controlled by the intensity of the laser light. Thus such a simple configuration can turn the particle in the pitch sense. The circulatory flows in this technique have a diameter of about 5 μm which is smaller than those reported using acousto-fluidic techniques.

Micro-PIV measurements of flows induced by rotating microparticles near a boundary

We report on the hydrodynamics induced by single-digit micron-sized superparamagnetic particles rotating at low Reynolds number and analyze the resultant flow fields using microparticle image velocimetry (µPIV). Magnetic microparticles floating a few nanometers above a glass substrate, in an otherwise quiescent fluid, were actuated wirelessly using a rotating magnetic field controlled using two pairs of orthogonally positioned electromagnetic coils. A high-speed camera was used to sufficiently capture the motion of nanometer-sized seeding particles at 500 frames per second as well as track the rotation of microparticles. Data from µPIV are compared with the analytical solution for Stokes flow generated by a sphere in an infinite fluid and numerical simulations using finite element analysis. Two-dimensional velocity data obtained from stacks of planar flow fields at incremental depths for individual microparticles show non-symmetrical profiles that are an indication of increased viscous effects due to the boundary confining wall. Additionally, the flow fields generated by two particles, at various separation distances, are also analyzed. It is observed that as two synchronously rotating beads, of approximately equal diameter, are placed closed together, complex flows offset, superimpose, and merge into single, larger microvortices. We find that the flow fields generated by two physically bound microparticles, rotating as one unit, are well approximated by the flow generated by a single microparticle with twice the diameter.

Combined AC electroosmosis and dielectrophoresis for controlled rotation of microparticles.

Electrorotation is widely used for characterization of biological cells and materials using a rotating electric field. Generally, multiphase AC electric fields and quadrupolar electrode configuration are needed to create a rotating electric field for electrorotation. In this study, we demonstrate a simple method to rotate dielectrophoretically trapped microparticles using a stationary AC electric field. Coplanar interdigitated electrodes are used to create a linearly polarized nonuniform AC electric field. This nonuniform electric field is employed for dielectrophoretic trapping of microparticles as well as for generating electroosmotic flow in the vicinity of the electrodes resulting in rotation of microparticles in a microfluidic device. The rotation of barium titanate microparticles is observed in 2-propanol and methanol solvent at a frequency below 1 kHz. A particle rotation rate as high as 240 revolutions per minute is observed. It is demonstrated that precise manipulation (both rotation rate and equilibrium position) of the particles is possible by controlling the frequency of the applied electric field. At low frequency range, the equilibrium positions of the microparticles are observed between the electrode edge and electrode center. This method of particle manipulation is different from electrorotation as it uses induced AC electroosmosis instead of electric torque as in the case of electrorotation. Moreover, it has been shown that a microparticle can be rotated along its own axis without any translational motion.

Optical rotation of microparticles in hypergeometric beams formed by diffraction optical elements with multilevel microrelief

This paper presents an experiment on the rotation of polystyrene microparticles 5 μm in diameter in seventh-order hypergeometric beams. A multilevel diffraction optical element was used to form such a beam. It is shown that, despite the process error in fabricating the multilevel diffraction optical element, the resulting beam is fairly efficient in transferring angular momentum to a group of polystyrene microparticles.

Trapping and rotating microparticles and bacteria with moiré-based optical propelling beams

We propose and demonstrate trapping and rotation of microparticles and biological samples with a moiré-based rotating optical tweezers. We show that polystyrene beads, as well as Escherichia coli cells, can be rotated with ease, while the speed and direction of rotation are fully controllable by a computer, obviating mechanical movement or phase-sensitive interference. Furthermore, we demonstrate experimentally the generation of white-light propelling beams and arrays, and discuss the possibility of optical tweezing and particle micro-manipulation based on incoherent white-light rotating patterns.

Principle of the rotation of small particles around a nodal point of strip in flexural vibration

In this report, we investigate the principle of rotation of micro particles around a nodal point of metal strip in flexural vibration, by experimental measurement and theoretical analysis. It is experimentally found that local travelling waves with the same travelling direction may exist along the circles centered at the nodal point of flexural vibration, and these local travelling waves can drive a particle cluster agglomerated at the nodal point to rotate. The acoustic radiation force, acoustic viscous force and acoustic streaming are excluded from the possible driving force by measuring and comparing the revolution speed of particle cluster in the normal environment of 1atm and 25°C and in a glass vessel of 0.04atm and 25°C. The generation of local travelling waves is related to the non-uniformity of vibration distribution along the width direction of vibration excitation part of metal strip. The physical principle obtained can well explain the measured relationships between the revolution speed and rotating particle number, and between the revolution speed and vibration displacement amplitude. There is slip between the rotating particles and vibrating surface, and air drag has little contribution to the particle rotation.

Simple optical vortices formed by a spiral phase plate

This paper discusses paraxial diffraction at a spiral phase plate of a limited light field whose phase is constant (a plane wave) while its amplitude varies as a power function with integer exponent n (positive or negative). It is shown that the Fraunhofer diffraction in this case is described by a Bessel function of the first kind of (n+1)st or (n−1)st order. Light fields that form diffraction patterns described by Bessel functions in the far zone are called simple optical vortices. Expressions are obtained for the amplitude of the Fraunhofer diffraction pattern of pure optical vortices and hypergeometric modes. The experimental part of the paper describes the formation of a vortex beam with an intensity distribution in the form of a double ring of arbitrary radius. Diffraction optical elements were created in this case by three methods: by electron lithography, by photolithography, and by means of a liquid-crystal display. Experiments are presented on the rotation of microparticles in a double ring.

Orientation of optically trapped nonspherical birefringent particles

While the alignment and rotation of microparticles in optical traps have received increased attention recently, one of the earliest examples has been almost totally neglected--the alignment of particles relative to the beam axis, as opposed to about the beam axis. However, since the alignment torques determine how particles align in a trap, they are directly relevant to practical applications. Lysozyme crystals are an ideal model system to study factors determining the orientation of nonspherical birefringent particles in a trap. Both their size and their aspect ratio can be controlled by the growth parameters, and their regular shape makes computational modeling feasible. We show that both external (shape) and internal (birefringence) anisotropy contribute to the alignment torque. Three-dimensionally trapped elongated objects either align with their long axis parallel or perpendicular to the beam axis depending on their size. The shape-dependent torque can exceed the torque due to birefringence, and can align negative uniaxial particles with their optic axis parallel to the electric field, allowing an application of optical torque about the beam axis.

Residue orbital angular momentum in interferenced double vortex beams with unequal topological charges

When two vortex beams with unequal topological charges superpose coherently, orbital angular momentum (OAM) in the two beams would not be cancelled out completely in the interference. The residual OAMs contained by the superposed beam are located at different concentric rings and may have opposite orientations owing to the difference of the charges. The residual OAM can be confirmed by the rotation of microparticles when difference between the charges of two interfering beams is large.

Rotation of microparticles with Bessel beams generated by diffractive elements

Abstract We show that imaging a non-diverging Bessel beam by a spherical lens leads to the generation of a diverging Bessel beam. Expressions for the projections of the Umov-Poynting vector for a two-dimensional TE-polarized Bessel beam and a three-dimensional paraxial linearly polarized Bessel beam are derived. A fifth-order Bessel beam is produced using a single optical element-a 16-level phase-only diffractive helical axicon fabricated using electron beam lithography. This beam was successfully used to trap and rotate 5-10 μm diameter yeast particles and polystyrene beads of diameter 5 μm.

Rotation of microparticles with Bessel beams generated by diffractive elements

We show that imaging a non-diverging Bessel beam by a spherical lens leads to the generation of a diverging Bessel beam. Expressions for the projections of the Umov-Poynting vector for a two-dimensional TE-polarized Bessel beam and a three-dimensional paraxial linearly polarized Bessel beam are derived. A fifth-order Bessel beam is produced using a single optical element-a 16-level phase-only diffractive helical axicon fabricated using electron beam lithography. This beam was successfully used to trap and rotate 5-10 μm diameter yeast particles and polystyrene beads of diameter 5 μm.