Tunable Localization of Higher‐Order Bound States in Non‐Hermitian Optical Waveguide Lattices

Abstract Higher‐order corner‐bound states in a 2D structure have been found to possess robust and exotic properties beyond the “ordinary” topological edge states, giving rise to a promising applicative potential. For example, the topological nanocavity designed based on the corner states exhibits much better performance than that of the conventional photonic crystal cavity. However, the corner states in the finite Hermitian system are usually coupled with each other, which results in the weakening of their localization. As such, the contradiction between the performance and footprint of topological devices is vexing. Here, it shows that introducing non‐Hermiticity in the higher‐order topological insulators is an effective strategy to enhance the localization of corner states in finite systems. By designing and analyzing the 2D finite Su‐Schriffer‐Heeger optical lattices with two types of non‐Hermitian configuration, the localization degree of higher‐order corner states is found to depend on the gain/loss strength. This is experimentally demonstrated by observing the distribution of corner states in the femtosecond‐laser‐writing loss‐controlled waveguide arrays. This scheme tuning localization of higher‐order corner‐bound states by non‐Hermiticity may offer a new avenue to design robust and compact devices, such as topological nano‐lasers with an ultra‐low threshold.