Observation of non-Hermitian point gap in photonic crystals

TL;DR

This paper reports the first direct experimental observation of a non-Hermitian point gap in photonic crystals by engineering radiation loss, enabling control over complex frequencies and demonstrating non-Hermitian skin effects without gain media.

Contribution

It introduces a radiation-loss-based method to observe non-Hermitian point gaps in photonic crystals, avoiding the need for gain media or synthetic dimensions.

Findings

Direct observation of NHPG in photonic crystals.

Reversal of non-Hermitian topology demonstrated.

Measurement system capable of mapping photonic bands in k-space.

Abstract

Non-Hermitian point gap (NHPG) is a unique phenomenon in non-Hermitian systems and induces non-Hermitian skin effect (NHSE). In photonic crystals, NHPG and the NHSE have previously been explored mainly through material loss, where the typically low $Q$ factors make direct observation of complex frequencies challenging. Here, we demonstrate the direct experimental observation of an NHPG by using a radiation-loss-based non-Hermitian photonic crystal. Radiation loss can be engineered through structural design, enabling control of the imaginary part of the complex frequency and allowing relatively high $Q$ factors. This approach is compatible with widely used absorption-free silicon-slab photonic crystals. We developed a measurement system that can measure photonic bands along arbitrary lines in $k$-space. Our measurements demonstrated direct observation of the NHPG in photonic crystals, and the reversal of non-Hermitian topology through the flip of loop rotation in a complex plane. Our platform, which requires neither gain media nor synthetic dimensions, establishes radiation-loss engineering as a simple and versatile route for photonic functionality using an NHSE in nanophotonic systems.