High-Sensitivity Air-Coupled Megahertz-Frequency Ultrasound Detection Using On-Chip Microcavities

Owing to their dual-resonance enhanced sensitivity, cavity optomechanical systems provide an ideal platform for ultrasound sensing. In this work, we realize high-sensitivity air-coupled ultrasound sensing from kilohertz to megahertz frequency range based on whispering gallery mode microcavities. Using a $57$-$\ensuremath{\mu}\mathrm{m}$-diameter microtoroid with high optical Q factor (approximately ${10}^{7}$) and mechanical Q factor (approximately $700$), we achieve sensitivities of $46\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{Pa}$ ${\mathrm{Hz}}^{\ensuremath{-}1/2}$--10 mPa ${\mathrm{Hz}}^{\ensuremath{-}1/2}$ in a frequency range of 0.25--3.2 MHz. Thermal-noise-limited sensitivity is realized around a mechanical resonance at 2.56 MHz, in a frequency range of 0.6 MHz. We also observe the second- and third-order mechanical sidebands, and quantitatively study the intensities of each mechanical sideband as a function of the mechanical displacement. Measuring the combination of signal-to-noise ratios at all sidebands has the potential to extend the dynamic range of ultrasound sensing.