Photothermal mapping and free-space laser tuning of toroidal optical microcavities

Abstract

We report a tuning method for ultrahigh-quality factor toroidal optical microcavities capable of rapid modulation and resonance position control over multiple decades. A free-space laser is focused onto the resonator's silicon support pillar, rapidly heating the mode-containing silica. Microcavity photothermal response is spatially mapped. Resonance shift varies inversely with pillar diameter, reaching 1.5 × 105 fm/mW at 2 μm diameter, allowing switching with 1 μW control power. Larger pillar resonators can be modulated at high speeds (>4 kHz). Heat flow simulations accurately model observed shifts. This versatile approach fulfills an outstanding need for fast, flexible control over toroid resonances.