Anomalous motion of a particle levitated by Laguerre-Gaussian beams.

Abstract

Laguerre-Gaussian (LG) beams have orbital angular momentum (OAM). A particle trapped in an LG beam will rotate about the beam axis, due to the transfer of OAM. The rotation of the particle is usually in the same direction as that of the beam OAM. However, we discovered that when the LG beam is strongly focused, the rotation of the particle and the beam OAM might be in the opposite direction. This anomalous effect is caused by the negative torque on the particle exerted by the focused LG beam, which is similar to the optical pulling force in the linear case. We calculated the optical radiation force distribution of a micro-particle trapped in optical tweezers formed by a strongly focused LG beam, and showed that there exist stable trajectories of the particle that are controlled by the negative torque. We propose several necessary conditions for observing the counter-intuitive trajectories. Our work reveals that the strongly trapped micro-particle exhibits diversity of motion patterns.