C 2 -Symmetric Photonic Crystal Vortex Microlaser Operating in the C-Band Based on Bound States in the Continuum

With the rapid progress of integrated photonics, miniaturized vortex lasers are crucial for chip-scale communication and computing. Bound states in the continuum with topological protection provide a promising route to realize microscale vortex lasers. Although the evolution of topological singularities in momentum space under reduced structural symmetry has been explored, systematic experimental investigations on defect-induced symmetry breaking within the unit cell and its impact on the performance of vortex lasers have yet to be conducted. Here, we propose a C2-symmetric photonic crystal vortex microlaser in the telecom C-band based on bound states in the continuum and investigate how symmetry-breaking defects affect vortex emission in C4v structures. Embedding two rectangular defects inside a circular hole of a square unit cell breaks the C4 symmetry, resulting in a cavity that emits vortex beams with reduced symmetry in the spatial intensity distribution. The proposed microlaser enables stable single-mode vortex emission at room temperature, with an experimentally measured quality factor of approximately 10500. This work reveals a new possibility of controlling vortex beam profiles through defect introduction, providing a practical route for advancing on-chip vortex lasers with customized beams.