Coupled high-finesse optical Fabry-Pérot microcavities

Abstract

Dissipation is unavoidable in any realistic physical system and can strongly reshape its dynamics and spectra. Coupled cavity systems offer a minimal and highly controllable setting in which coherent photon exchange competes with radiative loss, enabling the study of interference-based control of the system's optical response. <br/>

In this thesis, we report the experimental realisation and characterisation of such an open photonic system, consisting of two coherently coupled Fabry–Pérot microcavities. The system is built from a single fibre-based resonator that is split by a high-reflectivity membrane mirror inserted at the cavity centre. Transmission through the membrane enables coherent photon exchange between the sub-cavities. This coherent coupling competes with radiative losses set by the finite mirror reflectivity, resulting in hybridised modes that extend across both resonators. In this way, the system forms a photonic molecule that provides an all-optical analogue of linear light–matter interaction, offering a controlled setting to study how coherent coupling and dissipation shape the optical response of an open system. <br/>

A central contribution of this work is the development of a dielectric-coated silicon-nitride membrane mirror that combines high reflectivity (transmittance <em>T</em> = 452(47) ppm) with sufficient surface quality to sustain high-finesse resonances on both sides. Near degeneracy of these resonances, we resolve a pronounced avoided crossing, demonstrating strong coherent coupling <em>g</em> = 2<em>&pi;</em> × 2.859(2) GHz. Beyond this regime, we achieve wide tunability of the effective coupling by exploiting higher-order transverse cavity modes, reducing the coupling strength by nearly two orders of magnitude, which allows us to examine the transition from strongly hybridised supermodes to weakly perturbed single-cavity resonances. Using dynamic cavity-length modulation to emulate an imbalance of loss rates, we additionally observe strong interference effects in the steady-state optical response, manifesting as EIT-like narrow transparency features. <br/>

We further identify absorption in the silicon-nitride membrane that induces a nonlinear response in one sub-cavity and gives rise to optical bistability. Based on these measurements, we discuss how the photonic molecule picture extends into the nonlinear regime, where interference features become photon-number dependent. We outline conditions under which nonlinear interference could suppress multiphoton excitation via the unconventional photon blockade mechanism, and discuss a realistic path toward this effect by leveraging transverse access to one cavity site for integrating additional nonlinear media.