Selective activation of topological valley corner states in C-3-symmetric photonic crystals

Higher-order topological insulators have drawn great research attention in nanophotonics due to their ability to both support robust edge states and lower dimensional corner states. In this work, we present a theoretical proposal for achieving topologically switchable and valley-selective corner states based on two-dimensional C-3-symmetric photonic crystals (PCs), with breaking of inversion symmetry. Through the concatenation of two valley PCs with contrasting topological indices, we demonstrate the emergence of two types of valley-locked chiral topological edge states resulting from the valley-valley interaction. More importantly, we find that the system exhibits two distinct types of corner states, characterized by strong robustness and high localization, when the PCs are spliced at a 60 degrees angle. However, the corner states are absent when the splicing angle is set as 120 degrees. According to the theoretical analysis, the selective activation of topological valley corner states is related to the sign flip of valley Chern number at the corner. Based on this feature, we further propose a topological photonic switching device, in which the corner can be lighted up or off selectively. By combining the benefits of higher-order topology and valley degree of freedom, our work provides an efficient and flexible method for light manipulation.