Micropascal-sensitivity ultrasound sensors based on optical microcavities

Whispering gallery mode(WGM)microcavities have been widely used for high-sensitivity ultrasound detection,owing to their optical and mechanical dual-resonance enhanced sensitivity.The ultrasound sensitivity of the cavity optomechanical system is fundamentally limited by thermal noise.In this work,we theoretically and experimentally investigate the thermal-noise-limited sensitivity of a WGM microdisk ultrasound sensor and optimize the sensitivity by varying the radius and a thickness of the microdisk,as well as using a trench structure around the disk.Utilizing a microdisk with a radius of 300μm and thickness of 2μm,we achieve a peak sensitivity of 1.18μPa Hz^(-1/2)at 82.6 k Hz.To the best of our knowledge,this represents the record sensitivity among cavity optomechanical ultrasound sensors.Such high sensitivity has the potential to improve the detection range of air-coupled ultrasound sensing technology.