Second-Order Topological States in Non-Hermitian Square Photonic Crystals

Non-Hermitian photonic crystals offer a versatile platform for observing exotic phenomena, including the non-Hermitian skin effect and higher-order topological phases. In this work, we construct non-Hermitian photonic crystals by embedding balanced gain and loss into a magneto-optical photonic medium. Within the associated supercell, we demonstrate the emergence of second-order topological corner states whose degeneracies are selectively lifted by non-Hermitian effects, while others remain protected. Remarkably, the bulk states exhibit strong unidirectional localization toward a single corner, providing unambiguous evidence of the non-Hermitian skin effect. The coexistence of higher-order corner states and the NHSE within the same photonic platform reveals an intricate interplay between crystalline symmetry and non-Hermitian topology. Beyond its fundamental intrigue, our approach offers a versatile means of engineering and controlling the non-Hermitian skin effect in realistic photonic architectures, paving the way for applications in topological nanolasers, robust light localization, and quantum photonic emulators.