Air-Coupled Whispering Gallery Mode On-Chip Microspherical Shell Resonator for High-Frequency Ultrasound Detection

In this letter, we report on a high-sensitivity whispering gallery mode (WGM) resonator-based air-coupled ultrasound sensor capable of detecting minute pressure variations across an ultrasound frequency spectrum of 0.6–3.5 MHz. The sensor comprises a microspherical glass shell of approximately 450 μm in radius and nonuniform shell thickness of 7–15 μm, which is optically coupled to a tunable laser for resonance excitation. The setup allows for the precise measurement of acoustic signals, benefiting from the high optical
Q
-factor of ∼2 million of the blown glass microspherical shells. A noise equivalent pressure as low as 40
${\rm{\mu Pa}}/\surd \text{Hz}$
was obtained at 1.72-MHz ultrasound frequency. A very good correspondence between the simulated axisymmetric resonance frequencies measured using the WGM resonator and a 3D finite-element analysis model in COMSOL was established. The sensor showed an expected linear dependence on the drive voltage of the ultrasound transducer. The distortion of the microspherical shell under acoustic pressure was also independently confirmed using a laser Doppler vibrometer. The sensor's capability to handle high-frequency ultrasonic waves with significantly better signal-to-noise ratio than conventional piezoelectric- or microphone-based systems is demonstrated, highlighting its suitability for advanced photoacoustic applications.