Tunable Topological Charge Coexistence in Photonic Crystal BIC Laser

Photonic crystal (PhC) lasers support lasing from optical bound states in the continuum (BICs), which are non-radiative modes stabilized by symmetry or topological protection in open systems. These BIC modes enable control over not only the frequency and emission direction but also the topological properties, including polarization and topological charge. By altering the PhC lattice geometry, such as the period and hole diameter, lasing from two non-degenerate BICs with opposite topological charges has been demonstrated. This is observed for both TE and TM modes, where modes like TE-1 and TM+1 can coexist and lase simultaneously without annihilation of their topological charge. These results suggest that BIC-based photonic platforms can provide highly tunable, topologically structured, and polarization-controllable coherent light sources, offering new opportunities for integrated photonic device development.