Fundamental thermal noise limits for high-<i>Q/V</i> optical microcavities

Thermo-refractive noise – refractive index perturbations driven by fundamental thermal fluctuations – have recently been identified as a limiting noise source in various optical microcavities. Here, we present a theoretical and experimental characterization of this thermal noise in high quality factor (Q), small mode volume (V) photonic crystal cavities. The theory reveals a mode volume-dependent maximum “effective” Q-factor due to thermal dephasing. We quantify the implications for quantum optical devices operating in the qubit limit of cavity nonlinear optics and compare these results to calibrated frequency stability measurements of common 2D silicon photonic crystal cavities. The results highlight the importance of considering thermal noise in state-of-the-art high-Q/V optical resonators, but also inform design choices which minimize its impact on device performance.