Precision ultrasound sensing on a chip

Ultrasound sensors have wide applications across science and technology. However, improved sensitivity is required for both miniaturisation and increased spatial resolution. Here, we introduce cavity optomechanical ultrasound sensing, where dual optical and mechanical resonances enhance the ultrasound signal. We achieve noise equivalent pressures of 8--300 $\mu$Pa/$\sqrt{\rm Hz}$ at kilohertz to megahertz frequencies in a microscale silicon-chip-based sensor with $>$120 dB dynamic range. The sensitivity far exceeds similar sensors that use optical resonance alone and, normalised to sensing area, surpasses previous air-coupled ultrasound sensors by several orders of magnitude. The noise floor is, for the first time, dominated by collisions from molecules in the gas within which the acoustic wave propagates. This new approach to acoustic sensing could find applications ranging from biomedical diagnostics, to autonomous navigation, trace gas sensing, and scientific exploration of the life-induced-vibrations of single cells.