Optical torque on small chiral particles in generic optical fields

Huajin Chen, Wanli Lu, Xinning Yu, Chunhua Xue, Shiyang Liu, and Zhifang Lin

We derive an analytical expression of the optical torque (OT) on chiral particles in generic monochromatic optical fields within the dipole approximation. Besides the extinction terms owing to the interaction between the dipoles excited on the particle and the incident field, our expression includes also some long missing terms that are understood as recoil due to the radiation of the dipoles excited on the particle. The recoil terms are shown to have a significant contribution to the OT in many situations. Inspired by our expression, we further proved that the net OT vanishes for a lossless isotropic chiral spherical particle of any size illuminated by arbitrary monochromatic optical fields. Finally, we trace the origin of OT on a small chiral sphere immersed in a zeroth-order vector Bessel beam, taking advantage of our analytical expression. It is found that the azimuthal OT perpendicular to the beam’s propagation direction comes from the transfer of the spin angular momentum of the incident field to the particle, while the longitudinal OT along the illumination direction originates from the particle chirality, which generates longitudinal angular momentum on the optical beam and thus makes the particle subject to a longitudinal OT by recoil. Thus the longitudinal OT tends to rotate the chiral particle with opposite helicities in opposite directions, while the transverse OT shows little dependence on the handedness of particle chirality. Our results may help in understanding OT as an extra handle for particle manipulations in optical tweezers.

DOI