Trapping of Rayleigh spheroidal particles by highly focused radially polarized beams

The optical forces and intrinsic optical torque of a highly focused radially polarized beam on a Rayleigh spheroidal particle are calculated with the dipole approximation. Numerical results show that the maximal trapping forces depend strongly on the orientation of the particle, and the torque is always perpendicular to the plane containing the major axis of the spheroid and the optical axis. As a result of optical mechanical and torque equilibrium, the spheroidal particle will stay at the focus with its major axis of the spheroid parallel to the optical axis.