Ultrasound sensing at thermomechanical limits with optomechanical buckled-dome microcavities

Abstract

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 ∼ 103) and mechanical (Q ∼ 102) 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 µPa/Hz1/2. This corresponds to an ultrasonic force sensitivity ∼ 2 × 10−13 N/Hz1/2 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 µPa/Hz1/2. 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.

Ultrasonic spectroscopy of sessile droplets coupled to optomechanical sensors

“…For example, while the device is less responsive at off-resonance frequencies it also has a much lower noise floor at those same frequencies. The net result is that the noise-equivalent-pressure (NEP) of the sensor is relatively flat over the entire range for which thermomechanical noise is dominant 11,12 ( i.e. , up to ∼40 MHz in our experiment, see Fig.…”

“…This is as expected due to the additional ‘added mass’ and increased viscous damping for the ‘bulk medium’ case. 11…”

“…An extensive description of the use of our cavities for high-frequency ultrasound sensing was reported elsewhere, 11 and representative optical and mechanical properties are summarized for easy reference in the ESI† file. Our devices have sensitivities as low as ∼0.1–1 mPa Hz −1/2 in the MHz frequency range, 11 and thermal Brownian motions of the ambient medium make a substantial contribution to their noise floor. 13,16 Moreover, this corresponds to displacement sensitivity ∼10 −17 m Hz −1/2 , 16 which is approaching the best reported values for optomechanical sensors.…”

“…[12][13][14]16 This results in a direct mapping of mirror motion to the intensity of the light reflected by the cavity. An extensive description of the use of our cavities for high-frequency ultrasound sensing was reported elsewhere, 11 and representative optical and mechanical properties are summarized for easy reference in the ESI † file. Our devices have sensitivities as low as ∼0.1-1 mPa Hz −1/2 in the MHz frequency range, 11 and thermal Brownian motions of the ambient medium make a substantial contribution to their noise floor.…”

“…We use our recently reported 11 high-frequency ultrasound sensors, which have state-of-the-art sensitivity for operation in either air or water. They are all-optical (optomechanical) devices (with laser-based readout), 12,13 and offer significant advantages over conventional electrical/piezo-electrical based ultrasound sensors.…”