  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.

Instrumentation and Detectors;Optics