Coalescence of spectral singularities and phase diagrams for one-dimensional PT symmetric photonic crystals

TL;DR

This paper investigates the spectral singularities and phase transitions in one-dimensional PT symmetric photonic crystals, revealing new types of symmetry transitions, spectral singularity coalescence, and extending topological concepts to non-Hermitian systems.

Contribution

It introduces a Hamiltonian approach to study complex band structures, identifies novel PT symmetry transitions, and demonstrates topological phase invariance in non-Hermitian photonic crystals.

Findings

Identification of two types of PT symmetry transitions.

Discovery of spectral singularity coalescence within the Brillouin zone.

Observation of band inversion with topology unaffected by non-Hermiticity.

Abstract

Non-Hermitian systems with parity-time (PT) symmetric complex potentials can exhibit a phase transition when the degree of non-Hermiticity is increased. Two eigenstates coalesce at a transition point, which is known as the exceptional point (EP) for a discrete spectrum and spectral singularity for a continuous spectrum. The existence of an EP is known to give rise to a great variety of novel behaviors in various fields of physics. In this work, we study the complex band structures of one-dimensional photonic crystals with PT symmetric complex potentials by setting up a Hamiltonian using the Bloch states of the photonic crystal without loss or gain as a basis. As a function of the degree of non-Hermiticity, two types of PT symmetry transitions are found. One is that a PT-broken phase can re-enter into a PT-exact phase at a higher degree of non-Hermiticity. The other is that two spectral singularities, one originating from the Brillouin zone center and the other from the Brillouin zone boundary, can coalesce at some k-point in the interior of the Brillouin zone and create a singularity of higher order. Furthermore, we can induce a band inversion by tuning the filling ratio of the photonic crystal, and we find that the geometric phases of the bands before and after the inversion are independent of the amount of non-Hermiticity as long as the PT-exact phase is not broken. The standard concept of topological transition can hence be extended to non-Hermitian systems.