Tunable potential landscapes in optical microcavities using thermal expansion

Controlling the flow of light in optical microcavities enables a vast range of interesting physical phenomena. In these systems, varying the local optical path length in the microcavity is effectively equal to changing the potential landscape for the light in the resonator. Previous approaches to achieve this rely either on permanent modification of the resonator structure or reversible tuning using a fluorescent medium with a strong nonlinearity or high thermo-optic coefficient. Here, we introduce a new method based on patterned laser heating that causes a thermal expansion of the dielectric stack of the mirror. We find that our approach reaches similar potential height and width resolution compared to other reversible thermal approaches, while improving tuning speed over tenfold and removing the need for a specific medium inside of the resonator. As a proof of principle, the method is used to define and modify the potential landscape for photon Bose-Einstein condensates.