Quasi-bandgap behavior in non-Hermitian photonic crystals

TL;DR

This paper explores how introducing small losses in non-Hermitian photonic crystals creates quasi bandgaps, leading to sharp reflection peaks and enabling wavelength-selective reflection with broadband absorption.

Contribution

It reveals the formation of quasi bandgaps due to loss in non-Hermitian photonic crystals and demonstrates their application in wavelength-selective reflection devices.

Findings

Small losses open a quasi bandgap at the Brillouin-zone boundary.

Quasi bandgaps cause sharp reflectivity peaks.

Application shown in wavelength-selective reflection with broadband absorption.

Abstract

We investigate non-Hermitian photonic crystals in which the lossy and lossless constituents share the same real permittivity and differ only in their imaginary part. We characterize the complex band structure and reflection response of both one-dimensional (1D) and two-dimensional (2D) systems, and show that introducing even a small amount of material loss opens a quasi bandgap at the Brillouin-zone boundary. This quasi bandgap, absent in the lossless limit of the same structure, gives rise to sharp reflectivity peaks whose origin we explain through second-order perturbation theory. As an application of this behavior, we demonstrate a selective reflector combining a conventional photonic-crystal waveguide with a non-Hermitian photonic crystal, achieving wavelength-selective reflection with broadband absorption.