All-optical programming of polarization singularities in a photonic-crystal laser

Singular optics has emerged as an important research area with diverse applications, yet controlling optical singularities in nanophotonic emitters remains largely constrained by the fixed subwavelength geometry of optical resonators. Here, we circumvent this limitation and demonstrate all-optical programming of real-space polarization singularities in a photonic-crystal laser, while preserving a momentum-space vortex inherited from a symmetry-protected bound state in the continuum. The principle is to use a shaped optical pump to create a smooth mesoscopic potential, whose spatial variations are slow compared with the lattice period. This potential localizes a negative-mass Bloch band into trapped lasing states whose envelope functions, and therefore far-field singularity textures, are defined by the pump geometry. Using a honeycomb photonic crystal supporting a symmetry-protected bound state in the continuum, we achieve room-temperature telecom-band lasing with real-space polarization singularities pinned to the critical points of the envelope function, where its gradient vanishes, and reconfigurable in number and position by pump shaping, while the intrinsic momentum-space singularity at the $\Gamma$ point remains fixed. The experimental observations agree quantitatively with an analytical framework combining the Bloch mode of the photonic crystal with envelope-function theory, establishing optical envelope engineering as a route to programmable structured emission from active photonic lattices.