Comparative study of Hermitian and non-Hermitian topological dielectric photonic crystals

TL;DR

This paper compares Hermitian and non-Hermitian topological dielectric photonic crystals, analyzing how gain and loss affect their band structures, phase transitions, and edge states, highlighting the robustness of topology under PT symmetry.

Contribution

It provides a first-principles comparison of Hermitian and non-Hermitian topological photonic crystals, emphasizing the role of PT symmetry in topological phase stability.

Findings

PT-symmetric systems exhibit a phase transition with increasing gain/loss.

Topology remains unchanged in PT exact phase compared to Hermitian systems.

Nontrivial edge states develop complex eigenfrequencies, while trivial states stay real.

Abstract

The effects of gain and loss on the band structures of a bulk topological dielectric photonic crystal (PC) with $C_{6v}$ symmetry and the PC-air-PC interface are studied based on first-principle calculation. To illustrate the importance of parity-time (PT) symmetry, three systems are considered, namely the PT-symmetric, PT-asymmetric, and lossy systems. We find that the system with gain and loss distributed in a PT symmetric manner exhibits a phase transition from a PT exact phase to a PT broken phase as the strength of the gain and loss increases, while for the PT-asymmetric and lossy systems, no such phase transition occurs. Furthermore, based on the Wilson loop calculation, the topology of the PT-symmetric system in the PT exact phase is demonstrated to keep unchanged as the Hermitian system. At last, different kinds of edge states in Hermitian systems under the influences of gain and loss are studied and we find that while the eigenfrequencies of nontrivial edge states become complex conjugate pairs, they keep real for the trivial defect states.