Interesting manifestations of spin orbit interaction and spin Hall shift of light in an optical trap

Abstract

The spin orbit interaction (SOI) of light leading to the evolution of trajectory dependent geometric phase and associated spin Hall shift (SHS) in circularly polarized light has led to several fascinating manifestations in scattering, tight focusing, and imaging processes. However, most of these observations are at the sub-wavelength level, with somewhat limited applications of a general nature. We investigate the SOI in an optical trap for a linearly polarized trapping beam where the both the trajectory dependent geometric phase as well as the SHS are magnified significantly due to a stratified medium. The stratified medium is created using an index mismatched cover slip that modifies the radial intensity distribution near the focal plane of the trap due to diffraction effects. The modified intensity distribution causes trapping of polystyrene beads in ring-like patterns, while the tight focusing in the stratified medium also leads to a large spin redirection geometric phase that creates intensity side lobes in the azimuthal direction near the focal plane. Single trapped asymmetric particles can be trapped in the side lobes and translated along the ring by changing the polarization angle of the input beam. A 3D analysis of polarization reveals the generation of polarization vortices as well as spatially separated regions of opposite circular polarizations near the focal plane leading to controlled rotation of trapped particles, again by a linearly polarized input beam. The study can have several interesting consequences in the manipulation of mesoscopic particles in an optical trap.