Optical multistability in a compact microcavity enabled by near-exceptional coupling

TL;DR

This paper demonstrates a compact photonic microcavity with engineered near-exceptional coupling that achieves optical multistability at low power, enabling potential applications in all-optical memory and computing.

Contribution

It introduces a novel design of a microcavity with near-exceptional coupling to enhance multistability at low power levels, surpassing previous limitations.

Findings

Achieved tristability and hysteresis in a 20 μm footprint microcavity.

Demonstrated optical memory with controlled switching among stable states.

Enhanced thermo-optical nonlinearity through engineered non-Hermitian coupling.

Abstract

Multistability -- the emergence of multiple stable states under identical conditions -- is a hallmark of nonlinear complexity and an enabling mechanism for multilevel optical memory and photonic computing. Its realization in a compact footprint, however, is limited by intrinsically weak optical nonlinearities and the enlarged free spectral range that raises the multistability threshold. Here, we overcome this constraint by engineering a pair of spectrally close, ultra-high-Q resonances in a photonic crystal microcavity. Leveraging structural perturbations that deliberately introduce non-Hermitian coupling through a shared radiation channel, we drive the resonances toward an exceptional point with nearly degenerate wavelengths and balanced quality factors approaching $10^6$. This configuration substantially enhances thermo-optical nonlinearity and produces pronounced tristability and hysteresis loops within a footprint of 20 {\mu}m at input powers below 240 {\mu}W. We further demonstrate proof-of-concept optical random-access memory through controlled switching among multistable states. These results establish a general strategy for nonlinear microcavities to achieve energy-efficient multistability for reconfigurable all-optical memories, logic, and neuromorphic processors.