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

Multistability-the presence of multiple stable states under identical conditions-is a hallmark of nonlinear complexity, and in optics, a key enabler for multilevel optical memory. Yet, realizing optical multistability in a compact footprint useful for on-chip applications remains challenging, because optical nonlinearities are intrinsically weak and large free spectral range increases the multistability threshold. Here we achieve multistability by engineering a pair of spectrally close, ultrahigh-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 towards an exceptional point with nearly degenerate wavelengths and almost-equal quality factors approaching 10⁶. This configuration produces a pronounced tristability from thermo-optical nonlinearity within a 20-μm-diameter circular footprint, evidenced by hysteresis loops with 240-μW input power. Using this concept, we then demonstrate a proof-of-concept optical random-access memory that operates via controlled switching among the multistable states.