Conservative and nonconservative torques in optical binding.

We show that in the canonical case of two lossless spheres that are electromagnetically coupled there is interplay between conservative and nonconservative forces, which is controlled by the polarization of the bounding field. We demonstrate that this phenomenon leads to new mechanisms to induce torques on spherically symmetric, optically isotropic, and lossless objects. The electromagnetic interaction can be exploited to apply orbital torque about the mutual center of mass of the bounded spheres as well as spin around the individual axes. When the incident field is linearly polarized, the torques are mostly conservative and affect only transient behaviors while for circularly polarized fields, the torques are entirely nonconservative, resulting in steady rotations. Means to control the magnitudes of orbital and spin torques are presented and applications to nanorotator machines are discussed.