Theory of optical trapping by an optical vortex beam.

We propose a theory to explain optical trapping by optical vortices (OVs), which are emerging as important tools to trap mesoscopic particles. The common perception is that the trapping is solely due to the gradient force and that it may be characterized by three real force constants. However, we show that the OV trap can exhibit complex force constants, implying that the trapping must be stabilized by ambient damping. At different damping levels, particles exhibit remarkably different dynamics, such as stable trapping and periodic and aperiodic orbital motions.