Rotation of absorbing spheres in Laguerre-Gaussian beams

It is well known that optical vortex beams carry orbital as well as spin angular momentum. This optical angular momentum can manifest itself mechanically, for example in tightly focused Laguerre-Gaussian beams, where trapped, weakly absorbing spheres rotate at a rate proportional to the total angular momentum carried by the beam. In the present paper we subject this system to a rigorous analysis involving expansions in vector spherical wave functions that culminates in a simple expression for the torque on the sphere. It is seen that, for large weakly absorbing spheres, the induced torque per unit power is independent of the detailed structure of the incident field, being a simple function of two indices that describe the helicity and polarization state of the beam, the relative refractive indices of the sphere and ambient medium, the absorption index of the sphere, and its radius. A number of relationships between the coefficients of these expansions are also developed.