  Nowadays, ultrasound sensors are playing an irreplaceable role in the fields
of biomedical imaging and industrial nondestructive inspection. Currently,
piezoelectric transducers are the most widely used ultrasound sensors, but
their sensitivities drop quickly when the size becomes smaller, leading to a
typical sensor size at the millimeter to centimeter scale. In order to realize
both high sensitivity and spatial resolution, various optical ultrasound
sensors have been developed. Among them, ultrasound sensors using high-$Q$
optical microcavities have realized unprecedented sensitivities and broad
bandwidth and can be mass-produced on a silicon chip. In this review, we
introduce ultrasound sensors using three types of optical microcavities,
including Fabry-Perot cavities, $\pi$-phase-shifted Bragg gratings, and
whispering gallery mode microcavities. We introduce the sensing mechanisms
using optical microcavities and discuss several key parameters for ultrasound
sensors. We then review the recent work on ultrasound sensing using these three
types of microcavities and their applications in practical detection scenarios,
such as photoacoustic imaging, ranging, and particle detection.
