Validity of cylindrical approximation for spherical birefringent microparticles in rotational optical tweezers

Rotational manipulation of microscopic birefringent particles has conventionally been done by manoeuvring the polarization of the trapping light in optical tweezers. The torque on the particle is a sum of contributions from the linear polarization and the circular polarization, while assuming that the difference in optical path lengths between the extraordinary and the ordinary components of polarization depends upon the wavelength of light, the thickness of the particle and the birefringence. Generally, the thickness of spherical microparticles is assumed to be the diameter which renders the particle appear cylindrical. We test this hypothesis for sizes relevant towards optical tweezers manipulation. We find that for a range of particles from the Rayleigh regime to the early Mie regime, the approximation holds good.