Researchers propose a new way to detect very small changes near an optical microcavity by using a nonlinear effect called the Kerr nonlinearity to trigger an abrupt change in a frequency comb. A microcavity is a tiny ring or disk that traps light and makes interactions with nearby matter much stronger than in free space. A Kerr frequency comb is a set of evenly spaced colors of light generated inside such a cavity when a continuous laser and the cavity’s nonlinearity balance each other. The team shows, by theory and numerical simulation, that a minute perturbation—like a nanoparticle landing on the cavity—can push the comb from one global state into a very different one, producing a large, easy-to-see signal instead of a tiny frequency shift.

Conventional microcavity sensors look for small shifts in the cavity’s resonance frequency. Here the authors bias the microcavity so it sits close to a boundary between different dynamical states. In that condition the cavity supports localized pulses called dissipative Kerr solitons. The governing model is the Lugiato–Lefever equation, which describes how the light field evolves inside the cavity. A particle near the cavity changes the effective resonance slightly. Although that frequency change by itself would be hard to measure, the small change can move the system across a bifurcation point and cause the soliton to collapse or the comb to turn chaotic. The result is an “avalanche”: a tiny perturbation produces a macroscopic switch in the comb state.

The authors tested the idea with two independent numerical strategies. In a coupled-mode / mean-field simulation based on the Lugiato–Lefever equation they seeded a soliton with a short pulse, waited until it settled, and then introduced a step-like resonance shift meant to model particle adsorption. In one example the perturbation was applied at 400 photon lifetimes and, after about 100 photon lifetimes of cumulative nonlinear evolution, the soliton catastrophically destabilized and the field evolved into a chaotic state. They also ran full-wave finite-difference time-domain (FDTD) simulations (accelerated on GPUs and reduced to two dimensions to keep them tractable) of a microring resonator over wavelengths from 1.15 μm to 2.3 μm. Those simulations used a Kerr coefficient n2 = 2×10−20 m2/W, refractive index n = 1.5, and a model nanoparticle of radius 133 nm. The FDTD results reproduced the predicted bifurcation diagram and showed three practical scenarios: two where a particle-induced shift triggers a comb-state transition and one where a poor choice of operating point yields no observable change.