Optical Vortex Harmonic Generation Enabled by Confinement-Induced Photonic Spin-Orbit Coupling.

Abstract

Strong photonic spin-orbit coupling arises under tight optical confinement in a subwavelength waveguide, where propagating fields exhibit spin-orbit entanglement. To date, while the confinement-induced spin-orbit coupling has been investigated only in linear optics, its role in nonlinear optical processes remains largely unexplored. Here, we report the first experimental demonstration and theoretical analysis of unconventional nonlinear optical frequency conversion enabled by the confinement-induced spin-orbit coupling. By pumping a silica optical nanofiber with a spin-polarized Gaussian laser beam, we generate a third-harmonic optical vortex carrying orbital angular momentum─an outcome forbidden in isotropic media under conventional conditions. This approach enables simple and cost-effective generation of optical vortices with ultrafast all-optical switching, not requiring structured pump beams, engineered media, or specialized materials. Our results open new avenues for spin-orbit-coupled nanophotonic systems, providing a platform to explore nonlinear light-matter interactions involving exotic structured light and to uncover spin-orbit-coupling-induced phenomena across various disciplines.