Topological edge states in an all-dielectric terahertz photonic crystal

Abstract

We present an analysis of the robustness of topological edge states in an all-dielectric photonic crystal slab in the terahertz (THz) frequency domain. We initially design a valley photonic crystal (VPC) exhibiting a nontrivial band topology. The excitation of the topological edge states in the structure is facilitated through a zigzag domain wall constructed by interfacing two types of VPCs with distinct band topologies. The robustness of the excited edge states is probed with respect to the magnitude and the sign of the asymmetry in terms of the hole diameters in the VPC, for different domain interfaces. Our study reveals that the topological edge states in the VPC structure are achieved only when the domain walls are formed by the larger air holes (i.e., asymmetry parameter has a positive value). In the case of the domain walls formed by relatively smaller air holes (i.e., asymmetry parameter has a negative value), the topological protection of the edge states is forbidden. For positive asymmetry, we demonstrate that the topological transport of THz becomes more robust with the increasing magnitude of asymmetry in the VPC structure. A robust propagation of topological edge states and strong confinement of electromagnetic fields within the domain wall are observed for asymmetry ranging from 28% to 42% in our structure. We have adopted a generic technique and therefore, the results of our study could be achieved at other frequency regimes by scaling the size parameters of the structure appropriately. At THz frequencies, such extensive analysis on the robustness of the topological edge states could be relevant for the realization of low-loss waveguides for 6G communication and other integrated photonic devices.