Reconfiguration of orbital angular momentum via circularly polarized multi-focal spin-to-orbit conversion.

Abstract

Circularly polarized light (CPL), when tightly focused, can generate longitudinal spin-induced orbital angular momentum (OAM) through spin-to-orbit conversion (STOC). While STOC in single-focal systems is well studied, multi-focal STOC remains underexplored. In this Letter, we proposed a multi-focal STOC strategy that leverages tightly focused CPL to generate longitudinal OAM at each focal point. Our numerical simulations demonstrate that reducing the distance between foci induces nonlinear coupling, transforming independent OAMs into a unified OAM state rather than a linear simple superposition. By increasing the number of foci in a ring-symmetric arrangement, we found that the spin angular momentum (SAM) of CPL can be converted into longitudinal OAM distributed along the entire ring. Optical tweezer experiments using non-birefringent dielectric particles confirm the feasibility of the multi-focal STOC strategy, providing direct evidence of OAM-driven rotation independent of SAM. This study reveals a unique method for STOC in multi-focal ring-symmetric structures and offers insights and methods for precision optical manipulation and optical communication.