Parity‐Time Phase Transition of Topological Corner States in Square Lattice Photonic Crystal Structures

Abstract

Marriage of the concepts of parity‐time (PT) symmetry and topology recently gave rise to PT‐symmetric topological photonics, triggering great interest in the community of photonics. This work reveals PT‐symmetric phase transition in second‐order topological photonic systems: the photonic systems consisting of square lattice topological photonic crystals with balanced gain and loss support the corner states, two of which are in thresholdless PT broken phase and the other two exhibit PT phase transition with a threshold determined by the coupling strength between the corner modes. The main advantage of the proposed photonic system is the significantly high gain contrast between the corner states and unnecessary background modes originating from bulk and edge ones, enabling diverse photonic applications such as nanolasing with significantly high power compared with the preceding topological nanolasers made of only gain sublattices. The results presented in this work pave the way toward potential applications such as biological sensing and spectroscopy at the nanoscale.