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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 μPa/ √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. 

We have successfully demonstrated a novel, passive layer-free, curved piezoelectric micromachined ultrasound transducer (PMUT) array, using a sacrificial curved glass template and 30% scandium-doped aluminum nitride (Sc-AlN) as the active layer. The PMUTs were fabricated using a curved, suspended borosilicate glass template created via a chip-scale glass-blowing technique, onto which electrodes and the piezoelectric layer were deposited. The glass layer was thereafter selectively removed. We characterized the performance of a 13 × 13 curved PMUT (cPMUT) array using an electrical impedance analyzer, a Laser Doppler Vibrometer (LDV), and hydrophone pressure measurements. Our results reveal a device resonance frequency of approximately 1.8 MHz in air, with LDV analysis indicating a significantly enhanced low-frequency response of 1.68 nm/V—a fivefold improvement over conventional curved PMUTs with a passive layer. Additionally, acoustic characterization in water showed that this array generates an acoustic pressure of approximately 80 kPa at a 4.4 mm focal distance, with a beam width of 5 mm, and achieves a spatial peak pulse average intensity (ISPPA) of 216 mW/cm2 when driven off-resonance. Furthermore, we demonstrate 20-degree steering capability using our data acquisition system. These advancements highlight significant potential for enhancing the precision and efficacy of medical imaging and therapeutic applications, particularly in ultrasonic diagnostics and treatments. 

Ultrasound has been extensively used and investigated in medical applications, such as medical imaging [1] and drug delivery [2], because of advantages such as noninvasiveness, good penetration, good sensitivity, and ease of use. Prior to the development of piezoelectric micromachined ultrasound transducers (pMUTs), conventional transducers were made of piezoelectric ceramics, such as lead zirconate titanate [3]. These materials when operated in thickness mode exhibit a large impedance mismatch between the transducer surface and medium resulting in lower bandwidth unless augmented with one or more matching layers. With the development of MEMS technology, improvements in MUTs have been realized in several aspects, such as wide bandwidth without the addition of matching layers [4], smaller cell size, therefore higher operating frequency and better resolution, and easier fabrication of large arrays at lower cost [5]. Despite lower electromechanical coupling coefficient, the low-power consumption feature makes pMUTs good candidates for a variety of applications, including intrabody communication [6] and fingerprint sensing [7].