Radiation torques exerted on a sphere by focused Laguerre-Gaussian beams

Radiation torques exerted by focused Laguerre-Gaussian beams on a microsphere are investigated with the generalized Lorenz-Mie theory. It is shown that the spin and orbital torques, in particular, those exerted by circularly polarized beams, can be far larger than the net torque in magnitude, which suggests that they are not merely related to the turning effect on the sphere. It is found that they also include the contribution from the conversion between the spin and the orbital angular momentum induced by light scattering. Separations of the spin and orbital torques into mechanical and conversion components are presented. The dependences of these torque components on the beam's and sphere's parameters are also discussed.