Determining the Motion of an Anisotropic Particle Around an Optical Nanofiber

We theoretically explore the interaction between electromagnetic radiation and an optically small anisotropic particle by deriving the torque and force due to any electromagnetic field on such a particle. We apply the general torque and force equations derived to the case of an evanescent field outside an optical nanofiber. We analytically determine and graph the particle orientation and velocity vs. time for various polarization states of the electromagnetic  field.  We find that, for all polarization ellipticities ranging from linear up to and including a nearly circular threshold state, an anisotropic particle will assume an equilibrium orientation with respect to the polarization ellipse of the electric field, and the particle will move along a helical path at a constant speed.  For a purely circular polarized field, an anisotropic particle will spin at a constant rate, but the trajectory remains helical.  For polarization ellipticities between threshold and purely circular, an anisotropic particle will spin at a non-constant rate, and a component of the velocity will vary sinusoidally with time.