Topological charge of finite-size photonic crystal modes

Topological charges are the winding numbers of polarization vectors around the vortex centers of far-field radiation. In this work, the topological charge of photonic crystal modes is theoretically analyzed using an envelope function approach. A group of modes is discovered with unique polarization properties dictated by their nontrivial envelope functions. Experimentally, lasing operation on such a mode is demonstrated in an electrically pumped midinfrared photonic crystal surface-emitting laser with high slope efficiency. The topological charge is directly observed from the polarization properties of single-mode laser emission. Furthermore, it is shown that envelope functions may bring a new possibility to control the polarization profiles of photonic crystal lasers.