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

Hao Yang, Zhi-Gang Hu, Yuechen Lei, Xuening Cao, Min Wang, Jialve Sun, Zhanchun Zuo, Changhui Li, Xiulai Xu, and Bei-Bei Li

Phys. Rev. Applied 18, 034035 – Published 14 September 2022

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 to megahertz frequency range based on whispering gallery mode microcavities. Using a $57$ - $μ m$ -diameter microtoroid with high optical Q factor (approximately $10^{7}$ ) and mechanical Q factor (approximately $700$ ), we achieve sensitivities of $46 μ Pa$ ${Hz}^{- 1 / 2}$ –10 mPa ${Hz}^{- 1 / 2}$ in a frequency range of 0.25–3.2 MHz. Thermal-noise-limited sensitivity is realized around 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.

Received 11 March 2022
Revised 21 May 2022
Accepted 4 August 2022
Corrected 16 November 2022

DOI:https://doi.org/10.1103/PhysRevApplied.18.034035

Physics Subject Headings (PhySH)

Integrated optics Optomechanics Photoacoustics Photonics Sound detection

Optical microcavities

Cavity resonators Ultrasound techniques

Atomic, Molecular & OpticalCondensed Matter, Materials & Applied Physics

Corrections

16 November 2022

Correction: The third and fifth affiliations were inadvertently repeated, so the fifth affiliation was removed and the affiliation of the eight author was changed from 5 to 3. The postal code of the first affiliation contained an error and has been set right.

Authors & Affiliations

Hao Yang^1,2, Zhi-Gang Hu^1,2, Yuechen Lei^1,2, Xuening Cao^1,2, Min Wang^1,2, Jialve Sun³, Zhanchun Zuo^1,4, Changhui Li³, Xiulai Xu ¹, and Bei-Bei Li ^1,4,*

¹Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
²University of Chinese Academy of Sciences, Beijing 100049, China
³College of Future Technology, Peking University, Beijing 100871, China
⁴Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, China

^*libeibei@iphy.ac.cn

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Vol. 18, Iss. 3 — September 2022

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Images

Figure 1
(a) Simulated mechanical resonance frequency and (b) calculated sensitivity of the sensor, with different principal diameters. (c) Simulated mechanical resonance frequency and (d) calculated sensitivity of the sensor, with different disk thicknesses.
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Figure 2
(a) Top-view optical microscope image of the microtoroid. $D_{principal}$ , $D_{major}$ , and $D_{minor}$ denote the principal, major, and minor diameters of the microtoroid, with $D_{principal} = D_{major} + D_{minor}$ . The scale bar corresponds to $10 μ m$ . (b) Simulated optical field intensity distribution of the fundamental WGM of the microtoroid, normalized to its maximum value. (c) Measured transmission spectrum (black solid curve) of the microtoroid around 1550 nm, with the red solid curve showing the double Lorentzian fitting result, from which we can obtain $Q_{int} \approx 10^{7}$ . (d) Schematic diagram of the experimental setup for ultrasound sensing, with the inset showing the principle of ultrasound detection. PD, photodetector; VNA, vector network analyzer; OSC, oscilloscope; ESA, electronic spectrum analyzer. A SEM image of the microtoroid is shown in the setup.
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Figure 3
(a) Noise power spectrum (black solid curve) and the response of the microtoroid (green solid curve) to ultrasound at 2.56 MHz, with a SNR of 41.39 dB. The orange dashed, blue dotted, and red dash-dotted curves are the calculated thermal noise, shot noise, and total noise, respectively, in the frequency range of 2–3.2 MHz. The inset shows the simulated displacement distribution of the first-order flapping mode, normalized to its maximum value. (b) System response of the microtoroid versus the ultrasound frequency. (c) Derived pressure (left axis) and force (right axis) sensitivity spectra of the microtoroid.
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Figure 4
(a) Noise power spectrum with (black curve) and without (red curve) applying a single-frequency ultrasound at 2.56 MHz. (b) $\sqrt{SNR} s$ versus ultrasound pressure at $Ω$ (black squares), $2 Ω$ (red circles), and $3 Ω$ (blue triangles) frequencies, with the black dashed line, red dash-dotted line, and blue dotted line representing the corresponding fitting results.
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Figure 5
Enlargement of system response of Fig. 3 in the main text, normalized by the ultrasound pressure of the transducer at different frequencies.
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Figure 6
(a) Noise power spectrum of the taper-microdisk coupled system, showing multiple mechanical resonances of the tapered fiber and the microdisk in the range of tens to hundreds of kilohertz. (b) The simulated normalized displacement distributions of the three taper modes at 90, 180, and 282 kHz.
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