High sensitivity air-coupled MHz frequency ultrasound detection using on-chip microcavities

TL;DR

This paper demonstrates high-sensitivity air-coupled ultrasound detection across kHz to MHz frequencies using whispering gallery mode microcavities, achieving sensitivities down to a few microPascals, with potential for enhanced dynamic range.

Contribution

The work introduces a novel application of microcavities for air-coupled ultrasound sensing across a broad frequency range, with detailed analysis of mechanical sidebands and noise limits.

Findings

Achieved sensitivities of 46 uPa/Hz^{1/2} in 0.25-3.2 MHz range.

Realized thermal-noise-limited sensitivity at 2.56 MHz.

Extended sensitivity to 1.83 uPa/Hz^{1/2} in 30 kHz-1.65 MHz range.

Abstract

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 (kHz) to megahertz (MHz) frequency range based on whispering gallery mode microcavities. Using a 57 um-diameter microtoroid with high optical Q factor (~10^7) and mechanical Q factor (~700), we achieve sensitivities of 46 uPa Hz^{-1/2}-10 mPa Hz^{-1/2} in a frequency range of 0.25-3.2 MHz. Thermal-noise-limited sensitivity is realized around the 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. In addition, to improve the ultrasound sensitivity in the kHz frequency range, we use a microdisk with a diameter of 200 um, and achieve sensitivities of 1.83 uPa Hz^{-1/2}-10.4 mPa Hz^{-1/2} in 30 kHz-1.65 MHz range.