Torque on an Ellipsoidal Dielectric Particle in a Gaussian Beam with Circular Polarization

Simulation of torque exerted on an ellipsoidal dielectric particle in the focus of a circularly polarized spherical wave is performed. The finite difference method in time domain is used for calculation of a diffraction field, the torque was calculated based on this field using Maxwell stress tensor. It is shown that in the focus of a circularly polarized spherical wave, an ellipsoid is affected by a torque that will tend to rotate it around the optical axis. When the ellipsoid is moved from the beam center in the plane transverse to the optical axis, the torque takes a different sign if the particle rotates complete turn, therefore, there are states of balanced and unbalanced equilibria. When it is located in the beam center, the torque has only one sign, which in the absence of friction, viscosity (or others factors) will lead to continuous rotation of the particle around the optical axis.