Direct observation of zero modes in a non-Hermitian nanocavity array

TL;DR

This paper reports the direct observation of zero modes in a non-Hermitian nanocavity array, revealing unique phase properties and robustness that differ from Hermitian systems, with implications for photonic applications.

Contribution

It provides the first direct experimental observation of non-Hermitian zero modes in a photonic crystal nanocavity array, highlighting their phase characteristics and robustness.

Findings

Zero modes observed via far-field imaging and spectral filtering.

Zero modes only appear with small sublattice detuning.

Zero modes are robust against coupling disorder.

Abstract

Zero modes are symmetry protected ones whose energy eigenvalues have zero real parts. In Hermitian arrays, they arise as a consequence of the sublattice symmetry, implying that they are dark modes. In non-Hermitian systems, that naturally emerge in gain/loss optical cavities, particle-hole symmetry prevails instead; the resulting zero modes are no longer dark but feature ${\pi}/2$ phase jumps between adjacent cavities. Here we report on the direct observation of zero modes in a non-Hermitian three coupled photonic crystal nanocavity array containing quantum wells. Unlike the Hermitian counterparts, the non-Hermitian zero modes can only be observed for small sublattice detuning, and they can be identified through far-field imaging and spectral filtering of the photoluminescence at selected pump locations. We explain the zero mode coalescence as a parity-time phase transition for small coupling. These zero modes are robust against coupling disorder, and can be used for laser mode engineering and photonic computing.