  We describe the use of monolithic, buckled-dome cavities as ultrasound
sensors. Patterned delamination within a compressively stressed thin film stack
produces high-finesse plano-concave optical resonators with sealed and empty
cavity regions. The buckled mirror also functions as a flexible membrane,
highly responsive to changes in external pressure. Owing to their efficient
opto-acousto-mechanical coupling, thermal-displacement-noise limited
sensitivity is achieved at low optical interrogation powers and for modest
optical (Q ~ 10^3) and mechanical (Q ~ 10^2) quality factors. We predict and
verify broadband (up to ~ 5 MHz), air-coupled ultrasound detection with
noise-equivalent pressure (NEP) as low as ~ 30-100 $\mu$Pa/Hz^1/2. This
corresponds to an ultrasonic force sensitivity ~ 2 x 10^-13 N/Hz^1/2 and
enables the detection of MHz-range signals propagated over distances as large
as ~ 20 cm in air. In water, thermal-noise-limited sensitivity is demonstrated
over a wide frequency range (up to ~ 30 MHz), with NEP as low as ~ 100-800
$\mu$Pa/Hz^1/2. These cavities exhibit a nearly omnidirectional response, while
being ~ 3-4 orders of magnitude more sensitive than piezoelectric devices of
similar size. Easily realized as large arrays and naturally suited to direct
coupling by free-space beams or optical fibers, they offer significant
practical advantages over competing optical devices, and thus could be of
interest for several emerging applications in medical and industrial ultrasound
imaging.
