Gain-induced large optical torque in optical twist settings

Inducing a significant optical torque remains a challenging task,since the law of angular momentum conservation implies that one has to harvest a lot of light.Such a problem was partially resolved by using optical twist via strong internal multiple scattering to recycle the photons,and one can induce a large torque per unit of radiation cross section.By using the Maxwell stress tensor and the generalized Lorentz-Mie scattering theory for multi-spheres,we investigate the influence of gain materials in further amplifying optical torque in the optical twist settings.It is found that,when combined with a gain layer,the optical torque of lossy(both in PT-and non-PT-symmetric structures)or lossless(low dielectric materials)clusters at resonance could be one order of magnitude larger than those of a single layer and previous studied plasmonic double layer structures.Moreover,the gain-enhanced large opposite rotations(i.e.,optical twist)of the two layers arise at resonances in these structures.In contrast,in the gain-gain double-layer cluster,optical torques on both layers have no significant increase and the two layers rotate in the same direction at resonances.This work provides an elaborate investigation on the gain media-induced optical twist,which offers more choices for optical micromanipulation.

Ray-optics model for optical force and torque on a spherical metal-coated Janus microparticle

In this paper, we develop a theoretical method based on ray optics to calculate the optical force and torque on a metallo-dielectric Janus particle in an optical trap made from a tightly focused Gaussian beam. The Janus particle is a 2.8 μm diameter polystyrene sphere half-coated with gold thin film several nanometers in thickness. The calculation result shows that the focused beam will push the Janus particle away from the center of the trap,and the equilibrium position of the Janus particle, where the optical force and torque are both zero, is located in a circular orbit surrounding the laser beam axis. The theoretical results are in good agreement qualitatively and quantitatively with our experimental observation. As the ray-optics model is simple in principle, user friendly in formalism, and cost effective in terms of computation resources and time compared with other usual rigorous electromagnetics approaches, the developed theoretical method can become an invaluable tool for understanding and designing ways to control the mechanical motion of complicated microscopic particles in various optical tweezers.