Inherent Spin-Orbit Locking in Topological Lasing via Bound State in the Continuum

Bound states in the continuum (BICs) are exotic optical topological singularities that defy the typical radiation within the continuum of radiative modes and carry topological polarization vortices in momentum space. Enabling ultrahigh quality factors, BICs have been applied in realizing lasing and Bose-Einstein condensation, and their momentum-space vortex topologies have been exploited in passive systems, revealing novel spin-orbit photonic effects. Here, we demonstrate the inherent spin-orbit locking in topological BIC lasing. Utilizing C_{4v} and C_{6v} photonic crystal slabs, we achieve distinct spin-orbit locking combinations in BIC lasing of +1 and -2 topological charges. Spin-orbit locking phenomena are directly observed by momentum-space spin-dependent self-interference patterns. Real-space spin separations, as a counterpart of the momentum-space spin-orbit locking, are also revealed. Our results reveal new spin-orbit locking phenomena in BIC lasing, presenting significant potential for advancements in topological photonic source applications.