Harvesting the Spin-Orbit Interaction of Light to Generate Helicity-Dependent Complex Rotational Motion in Optically Trapped Mesoscopic Matter.

Spin-orbit interaction (SOI) of tightly focused light in optical tweezers underpins diverse optomechanical applications and the interconversion of spin and orbital angular momentum. Here, we demonstrate that the transfer of the spin angular momentum of a tightly focused circularly polarized beam to an on-axis birefringent particle can indirectly generate spin in an adjacent particle, leading to exotic rotational motion reminiscent of planetary trajectories. We demonstrate simultaneous rotation and revolution of birefringent liquid crystal (LC) particles by harvesting SOI, such that its two principal governing mechanisms-the momentum-dependent Pancharatnam-Berry (PB) phase and the anisotropy-induced PB phase-become coupled. In our experiments, a centrally trapped LC particle in spherically aberrated optical tweezers spins under circularly polarized illumination, generating spin-induced microfluidic flows that drive surrounding off-axis LC particles into orbital motion. Simultaneously, interaction of the input helicity with the centrally trapped particle induces spin-to-spin conversion through extrinsic SOI. The helicity thereby generated indirectly then couples to the orbiting particles, imparting an additional rotation whose direction is determined by the birefringence of the central particle. A Mueller matrix model that incorporates tight focusing and scattering quantitatively explains these observations. Thus, SOI coupled with microfluidic effects establishes exotic rotational optomechanics and microswitch applications.