Electrically and magnetically controlled optical spanner based on the transfer of spin angular momentum of light in an optically active medium

Abstract

An optical spanner is a light beam that can exert a torque on an object. It is demonstrated in this Rapid Communication that, with the aid of applied electric and magnetic fields, a light beam with initially linear polarization and initially zero total spin angular momentum can interact with an optically active medium, resulting in a change of the ratio of left-handed circularly polarized photons to right-handed ones. Thus the total spin angular momentum of the light is changed, which leads to a torque, creating an electrically and magnetically controlled optical spanner on the medium. For a linearly polarized $632.8\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ laser beam incident on a $100\ensuremath{-}\ensuremath{\mu}\mathrm{m}$-long $\mathrm{Ce}:{\mathrm{Bi}}_{12}\mathrm{Ti}{\mathrm{O}}_{20}$ whisker crystal with $5\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ radius, if the magnetic field is fixed at $\ensuremath{-}1.8\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, both the left- (right-)handed circularly polarized photon number and the total spin angular momentum vary with the applied electric field in a sinusoidal way, which means the torque exerted by the optical spanner on the crystal also varies sinusoidally with the electric field. It is found that at 50 $(\text{or}\ensuremath{-}50)\phantom{\rule{0.3em}{0ex}}\mathrm{kV}∕\mathrm{cm}$, 56% right- (left-)handed circularly polarized photons are translated into left- (right-)handed ones, which corresponds to a transfer of $0.56\ensuremath{\hbar}$ spin angular momentum contributed by each photon.