Spatially resolved spin angular momentum mediated by spin-orbit interaction in tightly focused spinless vector beams in optical tweezers

Abstract: We demonstrate an effective and optimal strategy for generating spatially resolved longitudinal spin angular momentum (LSAM) in optical tweezers by tightly focusing first-order azimuthally radially polarized (ARP) vector beams with zero intrinsic angular momentum into a refractive index (RI) stratified medium. The stratified medium gives rise to a spherically aberrated intensity profile near the focal region of the optical tweezers, with off-axis intensity lobes in the radial direction possessing opposite LSAM (helicities corresponding to $\sigma = +1$ and -1) compared to the beam centre. We trap mesoscopic birefringent particles in an off-axis intensity lobe as well as at the beam center by modifying the trapping plane, and observe particles spinning in opposite directions depending on their location. The direction of rotation depends on particle size with large particles spinning either clockwise (CW) or anticlockwise (ACW) depending on the direction of spirality of the polarization of the ARP vector beam after tight focusing, while smaller particles spin in both directions depending on their spatial location. Numerical simulations support our experimental observations. Our results introduce new avenues in spin-orbit optomechanics to facilitate novel yet straightforward avenues for exotic and complex particle manipulation in optical tweezers.