Ultrasensitive acoustic sensing and optical comb generation in a hybrid optomechanical system

Cavity optomechanical systems have emerged as a powerful platform for high-sensitivity acoustic sensing. However, their sensitivities at kilohertz frequencies are typically limited due to the inherent challenge in simultaneous optimization of optical and mechanical modes within a single resonator. In this work, we demonstrate a hybrid optomechanical system consisting of a tapered microfiber evanescently coupled with a microtoroid cavity. The microfiber supporting multiple kilohertz-frequency mechanical modes acts as a sensitive acoustic transducer, while the microtoroid serves as a sensitive readout cavity. Utilizing the strong acousto-optic interaction, the system enables thermal noise-limited ultrasensitive acoustic sensing in the kilohertz-frequency range, achieving an optimal noise equivalent pressure of 0.72 μPa/Hz^1/2 at 19.85 kHz. Furthermore, under strong acoustic driving, the system enables generation of optical frequency combs, with over 100 comb lines and tunable repetition rates across the kilohertz range by exciting different mechanical modes of the microfiber. This hybrid optomechanical system offers significant potential for applications in trace gas photoacoustic sensing, bioacoustic monitoring, and high-resolution spectroscopy.