  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 via micro-/nano- photonic structures, and their
momentum-space vortex topologies have been exploited in passive systems,
revealing novel spin-orbit photonic effects. However, as representative
topological properties, the spin-orbit-related phenemona of BICs in active
systems have not yet been explored. Here, we demonstrate the inherent
spin-orbit locking in topological BIC lasing. Utilizing photonic crystal (PhC)
slabs with square (C4v) and triangular (C6v) lattices, we achieve distinct
spin-orbit locking combinations in topological BIC lasing of +1 and -2
topological charges. These BIC lasing profiles manifest as vortex and
high-order anti-vortex polarization configurations, directly tied to the
topological properties of BICs. Our experimental results directly reveal the
spin-orbit locking phenomena through momentum-space spin-dependent
self-interference patterns and real-space spin separations of the lasing
emissions. This study not only highlights the inherent spin-orbit-locking
behaviours of topological BIC lasing but also opens new possibilities for
dynamically switchable orbital angular momentum (OAM) lasing by controlling
photonic spin, presenting significant potential for advancements in topological
photonic source applications.
