A beam of light may possess both spin and orbital angular momentum. In non-paraxial conditions part of the spin converts into orbital angular momentum through the spin-orbit angular momentum conversion phenomenon. This effect has important consequences at the nanoscale, particularly in nano-manipulation and nano-photonics. In this work, we thoroughly analyze the rotation of microscopic beads subjected to a tightly focused Laguerre-Gaussian beam. Particularly, we observe the rotation of particles along circular trajectories that will depend strongly on the combination of topological charges and the state of polarization. Based on Richard and Wolf theory for non-paraxial beam focusing, we found a very good agreement between the experimental results and the theoretical model based on calculation of the optical forces using the generalized Lorenz-Mie theory.
We investigate motion of particle pairs optically bound in tractor beam. The tractor beam can exert a negative force on a scatterer, in contrast to the pushing force associated with radiation pressure, which can pull the scatterer towards the light source. The particle movements can be enhanced by long-range interaction between illuminated objects, called optical binding. We study optical binding of two micro-particles in various geometrical configurations and investigate their motional behaviour in tractor beam. We demonstrate that motion of two optically bound objects strongly depends on their mutual distance and spatial orientation. Such configuration-dependent optical forces add an extra flexibility to our ability to control matter with light. Understanding these interactions opens the door to new applications involving the sorting or delivery of colloidal self-organized structures.
Optical binding of polystyrene microparticle pairs in retro-reflected wide Gaussian beam, called "tractor beam", is studied experimentally and the results are compared with the numerical calculations based on the multiple-particle Mie scattering theory. To investigate the dynamics of optically bound particle pairs in three dimensions we employ holographic video microscopy technique. We show that the particle pair motion is strongly dependent on the relative distances of the particles and the switching between applying pushing and pulling force on particle pairs can be achieved only by changing their configuration even though the "tractor-beam" parameters remain unchanged.
We report on an experimental and theoretical study of optical binding of polystyrene sphere pairs illuminated by retro-reflected wide Gaussian beam, so-called "tractor beam". We show that depending on configuration of particle pairs, optically bound structure in the "tractor beam" can be pushed or pulled against the beam propagation. We employ holographic video microscopy to analyse object positions in three dimensions and their time evolution. In such a way, we investigate their dynamics in dependence on the geometrical configuration that is compared with numerical simulations. We observe strong dependence of the particle pair motion on the relative distance of the particles.
Larger golden nanoparticles grow into several preferred forms. Some of those may be easily approximated by ellipsoids. In this paper we examine the rotational dynamics of spheroidal particles in an optical trap comprising counter-propagating Gaussian beams of opposing helicity. Isolated spheroids undergo continuous rotation with frequencies determined by their size and aspect ratio. We study the rotational frequencies and stability of these golden nano-particles theoretically by the means of T-Matrix.
We study theoretically the angular momentum transfer between strongly focused laser vortex beam and a dielectric oblate spheroidal particle (OSP). We find sets of geometrical parameters of the particle and the beam for which the particle is stably trapped on the beam axis in a uniform rotating state, thereby serving as a possible test probe of the global beam angular momentum as well as its spin and orbital parts.
Following the Keplerian idea of radiative forces one would intuitively expect that an object illuminated by sunlight radiation or a laser beam is accelerated along the direction of the photon flow. Such radiation pressure forms the basis for the concept of solar sail, or laser acceleration of micro-particles. In contrast, a hypothetical optical field known from the realm of science-fiction as the "tractor" beam attracts the matter from large distances against the beam propagation. We present a geometry of such"tractor" beam in micro-scale and experimentally demonstrate how it acts upon spherical micro-particles of various sizes or optically self-arranged structures of micro-particles.
Even though a nanoparticle is much smaller than the wavelength used for their spatial confinement in an optical trap, the nanoparticle shape strongly influences force interaction between the light and the nanoparticle. The nanoparticle orientation with respect to the beam propagation and polarization strongly influences the light scattering pattern and thus the acting optical forces and torques upon the nanoparticle. We demonstrate experimental and theoretical results concerning the optical trapping of metal nanoparticles and the influence of the trapping wavelength on shaped plasmonic nanoparticles.
Metal-dielectric core-shell particles represent promising tools in nanoplasmonics. In combination with optical tweezers they can be manipulated in a contactless way through fluid and their plasmonic properties can be used to probe or modify the local environment. We perform a numerical parametric study to find the particle geometry and material parameters under which such particle can be stably confined in optical tweezers. We use the theory based on Mie scattering in the focal field of an ideal water immersion objective of numerical aperture NA=1.2. For very thin metal layers we find that strong trapping on the optical axis can be achieved.
We studied experimentally self-arrangement of sub-micrometer size particles creating a one-dimensional colloidal waveguide of length several tens of micrometers. We investigated positions and behavior of particles in the chain. We also focused on 2D colloidal waveguides created in elliptical Gaussian beams. We employed geometry of counter-propagating beams enhanced with spatial light modulator to shape the intensity profile of counter-propagating beams.
We investigated the behavior of an oblate spheroidal polystyrene microparticle trapped in a focused vortex beam when the beam vorticity and polarization were modified. We demonstrated that such particles can be trapped in three dimensions, spin in a circularly polarized beam and an optical vortex beam around the axis parallel to the beam propagation. We compared the immediate frequencies and showed that contribution from the circularly polarized beam is one order of magnitude weaker comparing to the beam angular orbital momentum. Using a phase-only spatial light modulator we generated several vortex beam traps with well-defined parameters. Measuring the rotations of trapped spheroids we observed hydrodynamic phase and frequency locking for certain sets of beam parameters.
Laser manipulation with plasmonic nano-particles is a rapidly growing field with various practical applications stretching beyond physics towards biology and chemistry. For example gold nano-particles can be employed as local heat source, probes for surface enhanced Raman spectroscopy with a sensitivity going down to a single molecule or contact-less probe in scanning near-field optical microscope. A single tightly focused laser beam optical tweezers was also employed to three-dimensional trapping of gold and silver nano-particles with diameters between 20 to 250 nm. However, theoretical models assuming the spherical shape of a nano-particle predict spatial confinement only for particles with diameter lower than 100 nm. Our results indicate this discrepancy is caused by ignoring particles shape which is very important for understanding of light-matter interaction.
We demonstrate experimentally the principle of a ”tractor beam” where a microobject is pulled against the
photons flow. We show that this geometry and method can be used to delivery several microparticles over a
distance of tens of micrometers, to sorting of particles according to their sizes using rotation of beam polarization,
and to self-arrangement of microobjects to optically bound microstructures that are pulled againts the beam
We demonstrate experimentally a new method of sorting of colloidal particle suspension in wide single laser beam.
The sorting is performed in a realization of a “tractor” beam a weakly focused laser beam that is retro-reflected
under an oblique angle. In this configuration the lateral positions of particles dependent on the direction of linear
polarization of the beam. Polarization rotation by 90 degrees changes the sign of the lateral optical force acting
upon particles of certain properties and such particles are propelled in the opposite direction. This approach
provides surprisingly efficient way of passive sorting of tens of particles by pure switching the beam polarization.
In this contribution we focus on the heating and optical forces and acting upon a core-shell particles confined in
a standing-wave. The considered spheres are composed either of gold or silver layer on top of a polystyrene core.
We present the results of a computational study in which we modify the geometrical parameters of the particles
and the wavelength of the trapping beams. This study may suggest optimal particle composition that may be
utilized as an optically trapped probe for the surface enhanced Raman spectroscopy (SERS) of biomolecules.
Optical force acting upon a dielectric microparticle illuminated by a non-di racting vortex beam is expressed using the Generalized Lorenz-Mie theory (GLMT). Numerical results are presented for di erent widths and topological charges of the vortex beam. We show that such particle may be stably trapped either in the dark center of the vortex beam, in one of two stable positions placed o the optical axis, and as the third option it may circulate along almost circular trajectory having its radius smaller or equal to the radius of the smallest high intensity vortex ring.
In this contribution we focus on optical forces acting upon a metallic particle or a core-shell particle confined in a standing-wave. The considered spheres are composed either by a gold or silver or the consists of of two layers, one of them is metallic (Au, Ag). We present the results of a computational study where we modify the geometrical parameters of the particles and the wavelength of the trapping beams. Except the optical forces we also deal with heating of the particles. This study may suggest optimal particle composition that may be utilized as an optically trapped probe for Surface enhanced Raman spectroscopy of biomolecules.