Abstract
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 μPa Hz−1/2 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 an optical resonance alone and, normalised to the sensing area, surpasses previous air-coupled ultrasound sensors by several orders of magnitude. The noise floor is dominated by collisions from molecules in the gas within which the acoustic wave propagates. This approach to acoustic sensing could find applications ranging from biomedical diagnostics, to autonomous navigation, trace gas sensing, and scientific exploration of the metabolism-induced-vibrations of single cells.
Highlights
Ultrasound sensors have wide applications across science and technology
In this article we extend cavity optomechanical sensing to the measurement of acoustic and ultrasonic waves, using a lithographically fabricated device suspended above a silicon chip via thin tethers
Cavity optomechanical sensors consist of a mechanically compliant element coupled to an optical cavity
Summary
Ultrasound sensors have wide applications across science and technology. improved sensitivity is required for both miniaturisation and increased spatial resolution. A characteristic feature of cavity optomechanical sensors is that they are often only limited by optical shot noise and mechanical thermal noise, allowing the intrinsic limits in sensing performance to be approached[7] This provides the ability to perform exquisitely sensitive optical measurements, with sub-attometre precision[8]. By engineering its structure for high-acoustic sensitivity, we reach the regime where gas molecule collisions dominate the noise floor This allows noise equivalent pressures of 8–300 μPa Hz−1/2 at a range of frequencies between 1 kHz and 1 MHz. Compared to acoustic sensors that use similar, but nonsuspended, optical cavities and rely on refractive index shifts and static deformations rather than nanomechanical resonances[32], the peak sensitivity represents a more than three order-of-magnitude advance. Normalised by device area, it outperforms all previous air-coupled ultrasound sensors by two orders-of-magnitude at ultrasound frequencies from 80 kHz to 1 MHz
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