Topology-Enabled Switchable Unidirectional Radiative Band in a Bilayer Photonic Crystal

TL;DR

This paper introduces a topologically robust method for achieving switchable unidirectional emission in bilayer photonic crystals through non-Hermitian resonance hybridization, enabling reconfigurable and robust directional light sources.

Contribution

It presents a novel topological approach to control unidirectional emission in photonic crystals using non-Hermitian resonance hybridization and tunable parameters.

Findings

Achieves broad spectral unidirectional radiation with high quality factors.

Demonstrates robustness of emission directionality against perturbations.

Enables reversible switching of emission direction via refractive index tuning.

Abstract

Controlling how an open photonic system exchanges energy with its environment-and in particular how it radiates into the far field-is a cornerstone of non-Hermitian wave physics and a key enabler for directional photonic functionalities. Here, we propose a new route to robust unidirectional emission based on the non-Hermitian hybridization of resonances localized in spatially separated layers of a hetero-bilayer photonic crystal. By tailoring the interlayer coupling, we engineer hybrid photonc bands that exhibit strong unidirectional radiation across a broad spectral and momentum range while maintaining theoretically high quality factors. This asymmetric emission is organized by a topological vortex in a pseudo-polarization field defined from the front/back intensity imbalance, which endows the directionality with robustness against perturbations. We further show that, by tuning the surrounding refractive index, this singularity can be displaced in parameter space, enabling reversible switching of the emission direction and a reconfigurable far-field response. This framework opens perspectives for topological photonic sensing and for directional and switchable light sources, including unidirectional lasing supported by high-quality-factor modes.