Optical Multistability in Microcavities Achieves Near-exceptional Coupling with Balanced Quality Factors Approaching, Enabling Photonic Memory

Multistability, the ability of a system to exist in multiple stable states simultaneously, holds immense promise for advanced optical technologies, including high-density memory and complex photonic processing. Zhen Liu, Xuefan Yin, and Andrey Bogdanov, along with colleagues at their respective institutions, now demonstrate a significant advance in achieving this phenomenon within a remarkably small device. The team overcomes limitations imposed by weak optical effects and energy requirements by carefully designing a microcavity that manipulates light in a unique way. By bringing two light-trapping resonances incredibly close together and linking them through a shared channel, they amplify the system’s nonlinear response, achieving stable multistability within a footprint of just 20 micrometres and at exceptionally low power levels. This breakthrough establishes a new pathway for creating energy-efficient, reconfigurable optical components for future data storage and processing applications.

Photonic Crystal Cavities and Exceptional Points

Scientists are developing a new approach to optical computing and information processing by harnessing non-Hermitian exceptional points within photonic crystal cavities. These cavities, created within a silicon-on-insulator wafer, are specifically designed with a unique heterostructure consisting of a central core, a surrounding photonic bandgap region, and a gradient stretching area to ensure smooth transitions for light. The team utilizes a technique called lattice stretching, carefully adjusting the positions of air holes within the structure to fine-tune the cavity’s properties, allowing them to create and manipulate non-Hermitian exceptional points critical for achieving multistability and fast switching capabilities. The fabrication process begins with a commercial silicon-on-insulator wafer and employs electron beam lithography to define the intricate photonic crystal structure.

Inductively coupled plasma etching then sculpts the silicon layer, creating the desired pattern, while under-etching with hydrofluoric acid forms a membrane cavity measuring approximately 20μm in diameter. Researchers use a tunable laser to direct light onto the sample, observed through a high-powered objective lens, and employ cross-polarized detection to minimize background noise. Numerical simulations, using the finite element method, model the cavity resonances and behaviour, validating experimental results and guiding design optimization. The team intentionally introduces structural perturbations to create the non-Hermitian exceptional points, carefully scanning and adjusting the positions of air holes within the lattice.

Non-Hermitian Coupling in Photonic Crystal Microcavities

Researchers have engineered a silicon photonic crystal microcavity to achieve multistability, a phenomenon where multiple stable states can coexist under identical conditions. The cavity confines light within a hexagonal core, creating two closely spaced resonant modes, enabling efficient light emission and absorption, which forms the basis for nonlinear dynamics. By deliberately introducing structural imperfections, scientists induce non-Hermitian coupling, breaking the system’s symmetry and bringing the resonant wavelengths of the two modes closer together, balancing the rates at which each mode loses energy for enhanced interaction and control. The team controls the frequency difference between the two resonant modes, starting with nearly identical modes and then applying a slight perturbation.

They then couple each mode to a radiation bath, describing the interaction with a mathematical model. By exploiting the symmetry of the hexagonal lattice, they lift the degeneracy while preserving non-radiative characteristics. Selective structural perturbations, achieved by shifting air holes within the lattice, independently couple each resonance to polarized radiation, providing precise control over the radiative coupling strength. Researchers quantify this strength, demonstrating that increasing it narrows the frequency splitting, enhancing optical nonlinearity and maximizing energy within the cavity.

Low-Power Tristability in Silicon Photonics

Scientists have achieved a significant breakthrough in optical multistability, demonstrating a compact and energy-efficient system for reconfigurable photonics. They engineered a silicon photonic crystal microcavity to support multiple stable states within a footprint of just 20μm, accomplished by carefully designing a pair of ultra-high-Q resonances, bringing them into near-exceptional coupling, and leveraging structural perturbations to introduce non-Hermitian coupling through a shared radiation channel. This configuration substantially enhances thermo-optical nonlinearity, enabling pronounced tristability and hysteresis loops at remarkably low input powers below 240 μW. The team achieved quality factors approaching 10 6 for the cavity resonances, ensuring strong intracavity fields and efficient excitation.

By engineering the cavity modes to be closely spaced yet spectrally resolvable with balanced radiative decay rates, they maximized the nonlinear response and facilitated multi-level state transitions. Data demonstrates a proof-of-concept optical random-access memory, achieved through controlled switching among the multiple stable states. This breakthrough delivers a compact and energy-efficient building block for reconfigurable photonic logic, all-optical signal processing, and advanced memory devices.

Compact Multistability and Optical Random Access Memory

This research demonstrates a compact and robust realisation of optical multistability through precise control of radiative coupling between degenerate cavity modes operating in a near-exceptional coupling regime. By engineering a photonic crystal microcavity with ultra-high-Q resonances, the team achieved clear hysteresis loops and transition dynamics governed by thermo-optical nonlinearity. Efficient excitation of these resonances, with quality factors reaching up to 8x 10 5 , was crucial to observing these effects. The work extends beyond observation, successfully demonstrating proof-of-concept optical random-access memory behaviour based on the observed multistable states, validating the feasibility and reliability of the approach and paving the way for scalable, reconfigurable photonic memories and potentially, neuromorphic information processors.