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

Zhen Liu, Xuefan Yin, Andrey Bogdanov, Yujia Nie, Yi Zuo, Hongbin Li, Feifan Wang, Chao Peng

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 μm at input powers below 240 μW. 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.

Optical multistability in a compact microcavity enabled by near-exceptional 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 . This configuration substantially enhances thermo-optical nonlinearity and produces pronounced tristability and hysteresis loops within a footprint of 20 μm at input powers below 240 μW. 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.

Paper Structure

This paper contains 15 sections, 3 equations, 5 figures.

Table of Contents

  1. Introduction
  2. Principle and design
  3. Sample fabrication and experimental setup
  4. Experimental results of multistability
  5. Discussion
  6. Conclusion
  7. Acknowledgments
  8. Author contributions
  9. Competing interests
  10. Methods
  11. Lattice stretching in the cavity
  12. Numerical simulations
  13. Sample fabrication
  14. Measurement and data processing
  15. Data availability

Figures (5)

  • Figure 1: Photonic crystal microcavity with high-$Q$ resonances at near-exceptional coupling.
  • Figure 2: Thermo-optical nonlinearity under near-exceptional coupling.
  • Figure 3: Realization and characterization of the optical cavity. (a) A top-view SEM image of the cavity. The red and yellow shades denote the core and the gradient stretching region inside the blue shaded band gap region. Inset: A zoomed-in SEM image of the core region. (b) Optical setup for sample characterization. OBJ, objective; TEC, thermoelectric cooler; LP, linear polarizer; BS, beam splitter; L, lens. (c) Measured linear radiation spectrum showing two slightly split resonances with $Q$s up to $8\times10^5$, fitted by the theoretical model.
  • Figure 4: Experimental observation of optical multistability.
  • Figure 5: Demonstration of time domain switching. Input signals and corresponding intensity responses under modulation of (a) input power and (b) wavelength, showing temporal switching behaviors across three states, marked by shaded regions as cold (blue), warm (orange), and hot (red) states. Insets show the transition pathways among the three states.