Inhomogeneous-spin-momentum-induced orbital motion of birefringent particles in tight focusing of vector beams in optical tweezers

Spin-orbit interaction (SOI) due to the tight focusing of light in optical tweezers has led to exciting and exotic avenues towards inducing rotation in microscopic particles. However, instances where the back action of the particles influences and modifies SOI effects so as to induce rotational motion are rarely known. Here we tightly focus a vector beam having radial/azimuthal polarization carrying no intrinsic angular momentum into a refractive index stratified medium and observe the orbital rotation of birefringent particles around the beam propagation axis. In order to validate our experimental findings, we perform numerical simulations of the governing equations. Our simulations reveal that the interaction of light with a birefringent particle gives rise to inhomogeneous spin currents near the focus, resulting in a finite spin momentum. This spin momentum combines with the canonical momentum to finally generate an origin-dependent orbital angular momentum, which is manifested in the rotation of the birefringent particles around the beam axis. Our study describes a unique modulation of the SOI of light due to its interaction with anisotropic particles that can open up new avenues for exotic and complex particle manipulation in optical tweezers.