Optical Radiation Force Research Articles

Optical pulling force on a magneto-dielectric Rayleigh sphere in Bessel tractor polarized beams

The optical radiation force induced by Bessel (vortex) beams on a magneto-dielectric subwavelength sphere is investigated with particular emphasis on the beam polarization and order l (or topological charge). The analysis is focused on identifying the regions and some of the conditions to achieve retrograde motion of the sphere centered on the axis of wave propagation of the incident beam, or shifted off-axially. Exact non-paraxial analytical solutions are established, and computations for linear, circular, radial, azimuthal and mixed polarizations of the individual plane wave components forming the Bessel (vortex) beams by means of the angular spectrum decomposition method (ASDM) illustrate the theory with particular emphasis on the tractor (i.e. reversal) behavior of the force. This effect results in the pulling of the magneto-dielectric sphere against the forward linear momentum density flux associated with the incoming waves. Should some conditions related to the choice of the beam parameters as well as the permittivity and permeability of the sphere be met, the optical force vanishes and reverses sign. Moreover, the beam polarization is shown to affect differently the axial negative pulling force for either the zeroth- or the first-order Bessel beam. When the sphere is centered on the beam′s axis, the axial force component is always negative for the zeroth-order Bessel beam except for the radial and azimuthal polarization configurations. Nonetheless, for the first-order Bessel beam, the axial force is negative for the radial polarization case only. Additional tractor beam effects arise when the sphere departs from the center of the beam. It is also demonstrated that the tractor beam effect arises from the force component originating from the cross-interaction between the electric and magnetic dipoles. Potential applications are in particle manipulation, optical levitation, tractor beam tweezers, and other emergent technologies using polarized Bessel beams on a small (Rayleigh) magneto-dielectric particle.

Optical trapping by Laguerre-Gaussian beams: Far-field matching, equilibria, and dynamics

By using the method of far-field matching we obtain the far-field expressions for the optical (radiation) force exerted by Laguerre-Gaussian (LG) light beams on a spherical (Mie) particle and study the optical-force-induced dynamics of the scatterer near the trapping points represented by the equilibrium (zero-force) positions. The regimes of linearized dynamics are described in terms of the stiffness matrix spectrum and the damping constant of the ambient medium. Numerical analysis is performed for both nonvortex and optical-vortex LG beams. For the purely azimuthal LG beams, the dynamics is found to be locally nonconservative and is characterized by the presence of conditionally stable equilibria (unstable zero-force points that can be stabilized by the ambient damping). We also discuss effects related to the Mie resonances (maxima of the internal field Mie coefficients) that under certain conditions manifest themselves as the points changing the trapping properties of the particles.

Constructive spin-orbital angular momentum coupling can twist materials to create spiral structures in optical vortex illumination

It was discovered that optical vortices twist isotropic and homogenous materials, e.g., azo-polymer films to form spiral structures on a nano- or micro-scale. However, the formation mechanism has not yet been established theoretically. To understand the mechanism of the spiral surface relief formation in the azo-polymer film, we theoretically investigate the optical radiation force induced in an isotropic and homogeneous material under irradiation using a continuous-wave optical vortex with arbitrary topological charge and polarization. It is revealed that the spiral surface relief formation in azo-polymer films requires the irradiation of optical vortices with a positive (negative) spin angular momentum and a positive (negative) orbital angular momentum (constructive spin-orbital angular momentum coupling), i.e., the degeneracy among the optical vortices with the same total angular momentum is resolved.

Optical torque reversal and spin-orbit rotational Doppler shift experiments.

We report on optical rotational Doppler frequency shift experiments in the context of a counter-intuitive optomechanical phenomenon that is the angular analog of so-called negative optical radiation forces, which involves spin-orbit scattering of light. In practice, spin-orbit opto-mechanical effects arising from the interaction between polarized light and azimuthally varying birefringent optical elements are retrieved from mechano-optical experiments that involve spatial of the medium. Two kinds of experiments (single-beam and two-beam geometries) are performed and both approaches are discussed in the framework of previous dynamical geometric phase and rotational Doppler shift experiments based on spin and/or orbital angular momentum of light.

Oscillations of nonspherical and spherical bubbles

Murray Strasberg maintained an interest in bubble oscillations throughout his career and frequently offered an incisive perspective. His early bubble research includes [Strasberg, J. Acoust. Soc. Am. 25, 536–537 (1952); Strasberg, Acustica 4, 450 (1954)]. His pertinent application of an electrostatic potential theory analogy expands interests of some of James Clerk Maxwell's students. In addition some related discussions with Murray will be recalled and experiments from the 1980s and 1990s at Washington State University pertaining to bubbles and their associated dynamics reviewed. Those experiments concern the response of bubbles to steady and modulated optical radiation forces [Unger and Marston, J. Acoust. Soc. Am. 83, 970–975 (1988); Unger and Marston, Ocean Optics IX, Proc. SPIE 925, 326–333 (1988)] and to modulated acoustical radiation forces [Asaki et al., Phys. Rev. Lett. 75, 2686–2689, (E) 4336 (1995); Asaki and Marston, J. Fluid Mech. 300, 149–167 (1995); Asaki and Marston, J. Acoust. Soc. Am. 102, 3372–3377 (1997)]. The latter papers pertain to the simultaneous measurement of the frequency and damping of bubble shape oscillations. [Work supported by ONR.]

Preface to Special Issue on Material Science Pioneered by Optical Radiation Force

Preface to Special Issue on Material Science Pioneered by Optical Radiation Force

Electrostrictive counterforce on fluid microdroplet in short laser pulse

When a micrometer-sized fluid droplet is illuminated by a laser pulse, there is a fundamental distinction between two cases. If the pulse is short in comparison with the transit time for sound across the droplet, the disruptive optical Abraham-Minkowski radiation force is countered by electrostriction, and the net stress is compressive. In contrast, if the pulse is long on this scale, electrostriction is cancelled by elastic pressure and the surviving term of the electromagnetic force, the Abraham-Minkowski force, is disruptive and deforms the droplet. Ultrashort laser pulses are routinely used in modern experiments, and impressive progress has moreover been made on laser manipulation of liquid surfaces in recent times, making a theory for combining the two pertinent. We analyze the electrostrictive contribution analytically and numerically for a spherical droplet.

Optical Funneling and Trapping of Gold Colloids in Convergent Laser Beams

The simultaneous trapping of a large number of sedimenting Au colloids by optical radiation forces has been studied in detail. The particles are collected by a convergent laser beam and compressed against gravity and osmotic pressure at the upper window of the cell, thereby forming a dense colloidal gas. A minimum critical laser power is required to transport colloids into the trap. In contrast to conventional optical tweezers, the trap cannot be described by a 3D potential. Once the trap is sufficiently filled, the laser power can be reduced below the critical value, thereby stabilizing the trap population. Some characteristic properties of the trap, like the critical laser power and the transit time, are readily understood from a simple deterministic model. A detailed description that is capable of quantitatively accounting for the time dependence of the trap population, the finite leak rate at low power levels, or hysteresis effects requires the incorporation of fluctuations by means of a proper Langevin equation. Multiple independent traps are realized by time multiplexing of the laser beam, which allows for splitting up, independent manipulation, and subsequent recombination of a trapped colloidal cloud.

Geometrical interpretation of negative radiation forces of acoustical Bessel beams on spheres

Various researchers have predicted situations where the acoustical or optical radiation force on a sphere centered on a Bessel beam is opposite the direction of beam propagation. We develop the analogy between acoustical and optical radiation forces of arbitrary-order helicoidal and ordinary Bessel beams to gain insight into negative radiation forces. The radiation force is expressed in terms of the asymmetry of the scattered field, the scattered power, the absorbed power, and the conic angle of the Bessel beam and is related to the partial-wave coefficients for the scattering. Negative forces only occur when the scattering into the backward hemisphere is suppressed relative to the scattering into the forward hemisphere. Absorbed power degrades negative radiation forces.

Trapping and Manipulation of Biological Particles Through a Plasmonic Platform

Enhanced optical radiation force can be induced through the resonant scattering field from a single plasmonic nanoparticle or a randomly distributed plasmonic nanoparticle array. In this paper, we utilized the dipole approximation for the Mie scattering field to analyze such radiation force in both far-field and near-field regime. This force can be utilized to develop noninvasive probes for trapping and manipulation of single biological particles. The trapping of single yeast cells is also demonstrated as an application of this approach.

Optical trapping and light-induced agglomeration of gold nanoparticle aggregates

This paper demonstrates the optical trapping of micron-sized gold nanoparticle aggregates (GNAs) with a ${\mathrm{TEM}}_{00}$ mode laser at $532\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and reports the observation of an unusual light-induced agglomeration phenomenon that occurs besides the trapping of the GNAs. The observed agglomerate has a $60--100\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ donut-shaped metal microstructure with the rate of formation dependent on the laser power used. Citrate capped gold nanoparticles also show light-induced agglomeration, yielding different sized microstructures from those produced with GNAs. While the observed trapping can be explained by a model including the optical radiation and radiometric forces, the light-induced agglomeration cannot be explained by these two forces alone as the size of the agglomerate is much greater than the waist of the Gaussian beam used in the optical trapping. We attribute the additional cause of the light-induced agglomeration to ion detachment from the surface of the nanoparticles (aggregates) due to light illumination or heating. This is supported by the observation of reversible electrical conductivity changes of the solution of the nanoparticles (aggregates) upon laser illumination or direct heating. Light-induced agglomeration can be useful in the design and fabrication of microstructures from nanomaterials for various device applications.

Equivalence of expressions for the acoustic radiation force on cylinders

Using an appropriate grouping of terms, a radiation force expression for cylinders in a standing wave based on far-field scattering [Wei et al., J. Acoust. Soc. Am. 116, 202–208 (2004)] is transformed to an expression given elsewhere [F. G. Mitri, Eur. Phys. J. B 44, 71–78 (2005)]. Mitri’s result is from a near-field derivation for the specific case of a circular cylinder. The far-field derivation also applies to noncircular objects having mirror symmetry about the incident wave vector. Some aspects of far-field derivations of optical and acoustical radiation forces are noted as are some implications for the radiation force on cylinders in travelling waves.

Equivalence of expressions for the radiation force on cylinders and application to elliptical cylinders

Using an appropriate grouping of terms, a radiation force expression for cylinders in a standing wave based on far‐field scattering [W. Wei, D. B. Thiessen, and P. L. Marston, J. Acoust. Soc. Am. 116, 202–208 (2004)] is transformed to an expression given elsewhere [F. G. Mitri, Eur. Phys. J. B 44, 71–78 (2005)]. Mitri’s result is from a near‐field derivation for the specific case of a circular cylinder. In the usual case, in an ideal lossless media the far‐field derivation is not an approximation. The far‐field derivation also applies to noncircular objects having mirror symmetry about the incident wave vector. Some general and historical aspects of far‐field derivations of optical and acoustical radiation force (going back to 1909) will be noted. Our formulation yields a simple low‐frequency approximation for the radiation force on elliptical cylinders by introducing approximations for the partial‐wave scattering coefficients of elliptical cylinders first derived by Rayleigh. [Work supported by NASA.]

Study on New Arrangement Technique of Fine Particles by Laser Beam. 1st Report, Arrangement and Motion Control of Fine Particles in the Water.

The fine particle manipulation using optical radiation force has already been investigated in former research. Based upon that, the manufacturing method of the micro parts made up of some fine particles has been developed. However, when the micro parts consist of a lot of fine particles, the manipulation and assembly procedures become time consuming with low work efficiency because of the one by one operation. To solve this problem, a new arrangement method with a single laser beam was realized and proposed in the study, which is able to conduct the assembly procedures efficiently and practically. In detail, the method is to arrange particles by stopping the scanning of a YAG laser beam for a short interval at some appointed positions along the scanning locus to give out a larger amount of input laser energy so that fine particles can be trapped in priority at those stop positions. Therefore it becomes realizable to trap several fine particles at the appointed locations. Furthermore, if the stop positions of the laser scanning are arranged at the same time, the plural trapped fine particles can be manipulated simultaneously. This idea has also been experimentally realized.