Surface sensing is widely used to detect liquids and solids on surfaces and to characterize their properties. Guided-wave and pulse-echo methods are commonly used for these tasks. Guided-waves enable large-area inspection but lack accurate local thickness resolution and multi-frequency sensing capabilities. In contrast, pulse-echo techniques offer precise thickness measurements but are limited in spatial coverage. Consequently, neither method alone can provide both large-area coverage and high-precision local characterization. In this paper, we proposed a linear piezoelectric micromachined ultrasonic transducer (PMUT) array with a hexagonal close-packed layout, achieving a 70% fill factor and generating acoustic pressures up to 190 kPa at a distance of 5 mm from the array. By exciting higher-order vibrational modes, the PMUT array produces a broad response from 100 kHz to 10 MHz. Using a PMUT linear array for low-frequency guided-wave sensing enables the inspection of large areas on thin media. By employing phased-array excitation and receive beamforming, the guided-wave amplitude is enhanced to 283% and 208% of its original value, respectively, while eliminating the bulky wedge structures required by conventional transducers. Moreover, the broadband response supports multi-frequency pulse-echo operation, thereby improving thickness measurement accuracy and providing multi-frequency characterization of the medium. Therefore, the proposed dual-mode surface sensing approach based on broadband, high-acoustic-pressure PMUT arrays offers significant potential for compact, high-precision sensing in industrial and consumer electronics applications.[2026-0021]

Multi-frequency ultrasound devices have shown great potential in imaging and range-finding applications for their capabilities in balancing sensing spatial resolution and penetration depth. Traditional multi-frequency ultrasound devices are typically achieved by integrating highfrequency and low-frequency transducer elements, but suffer from mutual radiation impedance coupling. Here, by utilizing the inherent harmonic response of a single nonlinear mode curved Sc -doped AlN piezoelectric micromachined ultrasonic transducer (PMUT), a novel method to generate multi-frequency ultrasound outputs with balanced magnitudes is presented. Significant enhancements of the vibration harmonics of the PMUT diaphragm were achieved through excitation signal tuning. Increasing frequency sweep continuous sine wave excitation was implemented to induce nonlinear behavior, further increasing the harmonic amplitudes. The proposed method yielded harmonic vibration outputs with intensities comparable to, or surpassing, those of the fundamental vibration mode. Consequently, with optimized modulation of the excitation frequency and the driving voltage, the second and even third-order harmonics of the PMUT can be effectively utilized for multi-frequency transmission.

A piezoelectric actuated planar lightwave circuit (PLC) for quantum key distribution (QKD) is designed and fabricated. The PLC is composed of high photoelastic coefficient polymer waveguide and high-performance piezoelectric layer. It is compatible with BB84-phase, BB84 time-bin, differential phase shift (DPS), and coherent one-way (COW) protocols. Experimental results demonstrate that this PLC has a V pi L of 4 V & sdot;cm and response time is only 12 ns. In single-photon experiment, it achieved interference visibility exceeding 92%, which can be further improved through material optimization and polarization feedback loop or real-time stabilization system.

This paper presents a piezoelectric acoustic pressure sensor designed for high sound pressure level (SPL) applications, achieving both high sensitivity and high stiffness. The sensor employs a concave hexagonal structure to amplify the local stress, resulting in superior output charge density compared to conventional circular designs. The optimization of wedge-shaped electrode and series connection effectively utilizes stress concentration within limited areas while maximizing output voltage. Lead zirconate titanate (PZT) is chosen as the piezoelectric material due to its high piezoelectric coefficient and dielectric constant, which provides sufficient capacitance to support series electrode configurations, which can be a challenge for AlN based sensors where limited device capacitance leads to significant parasitic effects. The enhanced structural stiffness contributes to a high acoustic overload point (AOP) and a broad bandwidth with flat frequency response. Experimental results demonstrate that the concave hexagonal sensor achieves a sensitivity of $22.5~\mu $ V/Pa at 1 kHz, a characteristic frequency of 270 kHz, and a total harmonic distortion (THD) of only 2.4% at 170 dB SPL. In comparison, the circular design exhibits a sensitivity of $14.5~\mu $ V/Pa at 1 kHz, a characteristic frequency of 146 kHz, and 5.3% THD at 170 dB SPL. These results indicate that the concave hexagonal sensor successfully integrates high sensitivity, high stiffness, and low harmonic distortion, validating the stress concentration strategy as an effective approach for advanced acoustic pressure sensor development in high SPL environments.

Hypertension management requires continuous blood pressure (BP) monitoring, whereas conventional cuffbased systems provide only intermittent readings. We propose a cuffless method based on photoacoustic (PA) sensing that estimates arterial diameter from the acoustic time-of-flight between anterior and posterior wall echoes within a single axial scan line (A-line). Simulation results show that this method eliminates the need for array beamforming and tolerates millimeter-scale misalignment between the receiver and the vessel. To detect weak PA signals, a $\quad \text{Sc}_{0.3} \text{Al}_{0.7} \mathrm{N}$ piezoelectric micromachined ultrasonic transducer (PMUT) was designed with patterned electrodes and sputtered passivation layers to reduce parasitic capacitance for high receive sensitivity. Integrated with a transimpedance amplifier, the PMUT exhibits a center frequency of 6.6 MHz, a −6 dB bandwidth of 106%, a receive sensitivity of $131.07 \mu \mathrm{V} / \text{Pa}$, and a noiseequivalent pressure (NEP) of $0.053 \text{Pa} / \sqrt{ } \text{Hz}$. Based on this PMUT, a PA sensing system using a laser of only $\sim 1.5 \mu \mathrm{J} /$ pulse was developed. Such low pulse energy can be readily supplied by compact laser diodes and verticalcavity surface-emitting lasers (VCSELs). Experimental results demonstrate that the system yields a dynamic pressure measurement with a root-mean-square error (RMSE) of 3.33 mmHg in an in vitro vessel phantom. These results show great potential for future system-onchip integration and establish a pathway toward wearable and real-time cuffless BP monitoring.

Miniaturized photoacoustic (PA) sensing systems require ultrasonic receivers that combine broad bandwidth, high sensitivity, low noise, small footprint and low power consumption. To address this need, we present a compact PA receiver consisting of a 0.5-mm-diameter Sc0.3Al0.7N piezoelectric micromachined ultrasonic transducer (PMUT) with a 0.3 mm & times;0.4 mm application-specific integrated circuit (ASIC). The PMUT design is optimized based on thin-plate vibration theory and an electromechanical equivalent-circuit model. Parallel-connected PMUT cells with offset resonance frequencies exploit mode fusion to broaden the effective bandwidth. The ASIC employs a high-input-impedance capacitive positive-feedback interface together with a low-noise inverter-stack front-end for efficient weak-signal readout. Electrical, mechanical, and acoustic measurement results confirm the desirable broadband response. In water, the mode-fused PMUT exhibits a -6 dB pulse-echo fractional bandwidth of 75.1% centered at 6.17 MHz. The ASIC provides an approximately flat gain of 40 dB from 10 kHz to 8 MHz, with an input-referred noise density of 2 nV/root Hz at a power consumption of 1.8 mW. The integrated PMUT-ASIC receiver achieves a receive sensitivity of 30.1 & micro;V/Pa and a noise-equivalent pressure (NEP) of 0.0043 Pa/root Hz. In PA experiments, the receiver demonstrates an effective -6 dB system bandwidth of 104.8%, an axial resolution of 0.338 mm, and discernible signal detection at laser pulse energies down to approximately 0.4 & micro;J/pulse with 256 averages. These results demonstrate a compact and energy-efficient receiver architecture for next-generation miniaturized PA sensing and imaging systems.

Bioelectronic implants have enabled precise and closed-loop interfacing with biological systems to diagnose, monitor, and treat disorders. A critical challenge for these chronically implanted systems is the design of a robust, real-time wireless data uplink and a continuous, efficient wireless power delivery link within a severely constrained form factor. Ultrasound, thanks to its advantageous path loss and safety limit, has become a promising medium. However, conventional ultrasound power and communication approaches either employ two separate transducers, resulting in a larger form factor or utilize pulse-echo backscatter with a single transducer, which suffers from time-multiplexed uplink transmission, limited data rate, and inefficient power delivery. This work introduces a single-transducer architecture that enables concurrent ultrasound power delivery and backscatter-based data communication, based on a unified impedance-based operating region that allows efficient downlink power harvesting and reliable uplink backscatter modulation simultaneously. The proposed single-stage power regulator modulates its input impedance via inductor charging-time control for data transmission while remaining decoupled from load variations, ensuring uninterrupted energy harvesting. The system further implements channel-adaptive modulation (BPSK, APSK, 4-ASK) to dynamically optimize the trade-off between data rate, reliability, and power delivery. Fabricated in 180 nm CMOS with a 0.28 mm(2) core area and validated in oil at 5 cm depth with a 1 MHz acoustic wave, the system enables real-time data transmission and the highest spectral efficiency of 300 kbps/MHz with a bit error rate (BER) below 10(-6) while achieving 192 & micro;W of continuous power delivery.

This paper presents a piezoelectric MEMS (Micro Electro Mechanical Systems) microphone designed to adaptively reduce harmonic distortion across a wide range of sound pressure levels (SPL). By incorporating multiple pressure compensation structures, the nonlinear deflection of the sensing membrane is effectively suppressed, leading to a reduction in total harmonic distortion (THD) compared to conventional microphones. To further enhance performance, a ScAlN piezoelectric film with 30 % scandium doping is employed. Experimental results demonstrate that microphones implementing six compensation structures reduce the THD from 8.72 % to 1.91 % at 500 Hz and 170 dB SPL, while the sensitivity remains nearly unchanged from 1.09 to $1.04 \text{mV} / \text{Pa}$. High SPL measurement results further confirm that the microphone can acquire high-fidelity signals without significant waveform distortion. The significant reduction in THD without compromising sensitivity demonstrates that the proposed pressure compensation strategy offers a promising path for developing high-performance MEMS microphones, particularly for high SPL applications.

This paper presents an optimized piezoelectric micromachined ultrasonic transducer (PMUT) array with a multi-ring electrode configuration, designed for selective excitation of higher-order modes, addressing the critical need of high acoustic pressure in ultrasonic neuromodulation. Compared to conventional designs, simulation results predict that the proposed triple-ring architecture significantly reduces crosstalk and mitigates the adverse effects of mutual radiation impedance by precisely matching the (0,3) mode strain distribution. Experimental results obtained from a fabricated 4 mm2 triple-ring-electrode PMUT array are in good agreement with the simulation results, achieving an acoustic pressure of ~135 kPa in silicone oil at 6 mm with a 10 Vpp drive voltage and improved beam directivity. Furthermore, a quantitative acoustic radiation force (ARF) characterization method based on laser doppler velocimetry (LDV) was implemented to evaluate modulation efficiency. Experimental results demonstrated that a 50% duty cycle pulsed excitation yields superior low-frequency energy compared to sinusoidal amplitude modulation (SAM) of the excitation signal, thereby providing a high-performance hardware platform and a robust evaluation framework for low-frequency neuromodulation.

Lead-free Aluminum Nitride (AIN) piezoelectric micromachined ultrasonic transducer (PMUT) is a promising candidate for miniaturized bio-compatible ultrasonic systems and consumer electronics. However, it suffers from material-associated poor transmitting performance and a narrow mid-air bandwidth. Here, by harnessing the nonlinearity-triggered “snap-through” effect of the PMUT diaphragm, we developed a novel bistable mode 30 % Sc-doped AlN PMUT with an enhanced mid-air bandwidth and a significant vibration amplitude beyond the inherent piezoelectricity. The proposed bistable snap-through $\text{Sc}_{0.3} \text{Al}_{0.7} \mathrm{N}$ PMUT demonstrated remarkable mid-air transmission with a displacement over $6.2 \mu \mathrm{m}$, a sensitivity greater than 3 $\mu \mathrm{m} / \mathrm{V}$, a vibration velocity up to $6.74 \mathrm{m} / \mathrm{s}$, and an impressive 35 -fold velocity improvement over monostable vibration at a specific excitation frequency. Additionally, it has an exceptional bandwidth of 84.5 kHz, spanning the range from 121.5 kHz to 206 kHz in air.

Multi-beam ultrasonic transmission (MBUT) is crucial for high-speed imaging and multi-point neural stimulation. However, conventional approaches to obtain the MBUT require complex multi-channel transducers or delay control circuits. In this work, we demonstrated a compact single-channel piezoelectric micromachined ultrasonic transducer (PMUT) capable of generating three distinct beams through cosine-apodized amplitude modulation, which was achieved by varying top electrode areas. The fabricated device with a single-crystal PZT thin film exhibits a resonant frequency of 3.5 MHz in a liquid (Fluorinert, FC-40) and possesses the ability to efficiently produce three beams at steering angles of $\pm 14^{\circ}$ and 0°, with the central beam reaching a peak acoustic pressure of $8.7 \text{kPa} / \mathrm{V}$. This electrode-geometry-based modulation methodology provides a simplified and compact solution for realizing multi-beam acoustic transmission, offering significant potential for novel applications, such as scalable neural interfaces and rapid ultrasonic flow detection.

Achieving high acoustic output from piezoelectric micromachined ultrasonic transducers (PMUTs) is critical for biomedical applications but remains challenging due to severe cross-talk effect in dense arrays. This work presents a method that exploits, rather than suppresses, such crosstalk effect by selectively exciting an array-level anti-phase resonant mode in a supercell to boost collective vibration. As a result, the proposed supercell demonstrated a measurement displacement sensitivity of 457.7 nm/V in water, which is approximately seven times that of a conventional densely arranged array. Furthermore, a 2.7 fold increase in output sound pressure compared to a conventional array of equal area is illustrated, together with a high quality-factor of 34.3. This work establishes a feasible route for boosting acoustic output via mutual radiation, offering a crucial design strategy for highperformance water-borne or in-tissue PMUTs.

Lead zirconate titanate (PZT) has been commonly used as the functional material for piezoelectric microelectromechanical system actuators, which tend to adopt multi-layer stacked structures to achieve improved actuation performance and multifunctional capabilities. However, the thermal instability of PZT has caused a great challenge in the implementation of multi-layer integrated PZT actuators using wafer-level processes. In this work, the degradation of performance of the PZT thin film was mitigated through the optimization of electrode materials and the incorporation of a protection layer. The multi-layer integration was realized through wafer-level temporary bonding and low-thermal-budget parylene bonding, both designed to preserve the functional properties of PZT. The ferroelectric and piezoelectric properties of PZT with various electrode configurations (Ti/Pt, TiW, Pt, Cr/Au), both with and without thermal sliding debonding material, were characterized after the thermal treatment to simulate the bonding process. The Cr/Au electrode was demonstrated to induce the least degradation and, therefore, was chosen as the top electrode covered by an Al2O3 protection layer. More severe degradation observed at elevated temperature highlighted the importance of the parylene bonding process. The quality of the parylene bonding was subsequently evaluated. The fabricated actuators retained 85.91% of the remanent polarization and 94.30% of the saturation polarization compared to pristine PZT films. The resonant frequency deviations of the vibrators were only 0.28% and 0.576% under separate and simultaneous driving, respectively, which can be attributed to the uniform and robust anchors formed by the wafer-level parylene bonding process.

Underwater acoustic systems employ bulky sonar devices with inherently limited bandwidth. This work addresses the fundamental sensitivity-bandwidth trade-off in piezoelectric micromachined ultrasonic transducer (PMUT) through an innovative analog-domain multi-band fusion technique. We present a multi-channel fused transimpedance amplifier (TIA) implemented in 180-nm CMOS for high-sensitivity, broadband hydrophones. The proposed architecture combines analog-domain signal fusion with a capacitive-feedback TIA topology, achieving simultaneous area and power efficiency. When integrated with a compact 3 mm × 2 mm PMUT array, the system demonstrates 129% fractional bandwidth and 523 V/MPa sensitivity. The single-channel configuration achieves an input-referred noise (IRN) density of 10 pA/√Hz@1MHz with noise efficiency factor (NEF) of 0.093 while the triple-channel design attains a NEF of 0.156, representing an exceptional low-power solution that enables high-resolution underwater sensing.

This work presents sealed-silicon-cavity piezoelectric micromachined ultrasonic transducers (SSC-PMUTs) fabricated via high-temperature silicon surface migration, eliminating fragile release steps and complex cavity bonding while maintaining array design flexibility. Two device configurations demonstrate displacement sensitivities of $95 ~\text{nm} / \mathrm{V}$ at 3.89 MHz and $42 ~\text{nm} / \mathrm{V}$ at 7.09 MHz, with electromechanical coupling coefficients of 2.53 % and 2.0 %, respectively. Pulse-echo measurements reveal transmit sensitivities of $2.0-2.78 \text{MPa} / \mathrm{V}$ at 2 Vpp. The migration-sealed architecture exhibits fractional bandwidths of 22.6 % and 24.9 %, and beamwidths of $5-13^{\circ}$ depending on pitch, confirming excellent directivity control. These results establish SSC-PMUTs as a robust, power-efficient platform for biomedical imaging and industrial sensing, achieving superior pressure-per-volt efficiency through simplified wafer-level manufacturing and enhanced device reliability.

Conventional piezoelectric micromachined ultrasonic transducers (PMUTs) based on circular or rectangular membranes face two critical bottlenecks: inter-element coupling in array configurations, leading to crosstalk that constrains high-resolution imaging and multi-sensor integration; and poor electromechanical coupling coefficients typically below 5%, which limit sensitivity and bandwidth. In this work, we propose a Ɪ-shaped PMUT with minimized interference through optimized element arrangement while maintaining a high fill factor of 71 %. An optimized electrode distribution based on a $\text{Sc}_{0.3} \text{Al}_{0.7} ~\mathrm{N}$ film was adopted to enhance the electromechanical performance, where an impressive coupling coefficient of 8.35 % in liquid was achieved. Experimental results show that the device demonstrates a high transmitting sensitivity of $7.6 \text{kPa} / \mathrm{V}$ at 5 mm from the surface and a significant receiving sensitivity of $1.8 \mu ~\mathrm{V} / \text{Pa}$, along with a broad bandwidth of 103% and relatively good crosstalk suppression compared with circular PMUTs.

Surface sensing is widely used to detect liquids and solids on surfaces and to characterize their properties. Guided-wave and pulse-echo methods are commonly used for these tasks. However, guided waves enable large-area inspection but lack accurate local thickness resolution and frequency diversity. In contrast, pulse-echo techniques offer precise thickness measurements but are limited in spatial coverage. Consequently, neither method alone can simultaneously achieve comprehensive representation. In this paper, we proposed a linear piezoelectric micromachined ultrasonic transducer (PMUT) array with a hexagonal close-packed layout, achieving a 70 % fill factor and generating acoustic pressures up to 190 kPa at a distance of 5 mm from the array. By exciting higher-order vibrational modes, the PMUT array provides a broad response from 100 kHz to 10 MHz, supporting both guided-wave and pulse-echo detection. The low-frequency guided-wave detection using PMUT arrays eliminates the bulky wedge-shaped packaging of traditional transducers and enables large-area surface inspection of thin layers. The high-frequency pulse-echo detection complements precise thickness measurement and multi-frequency feature detection. Therefore, the proposed dual-mode surface sensing approach based on broadband, high-acoustic-pressure PMUT arrays offers significant potential for compact, high-precision sensing in industrial and consumer electronics applications.

This paper presents a high-data-rate backscatter ultrasonic communication microsystem for medical Internet of Things (IoT) applications, which utilizes lead-free piezoelectric MEMS transducers. A Piezoelectric Micromachined Ultrasonic Transducer (PMUT) composed of hexagonal units was designed and fabricated, with a thickness of less than 0.4 mm and a high fill factor of 70%. The PMUT array achieved a -6 dB receiving bandwidth of 110%, which is anticipated to enhance the data rate of the ultrasonic backscatter microsystem. Furthermore, it delivered an output power in the range of $100 \mu \mathrm{W}$ under an incident acoustic power density of $14.4 \text{mW} / \text{cm}^{2}$, demonstrating significant potential as both an energy harvester and an ultrasonic backscatter node in future implants, due to its high acoustic-to-electrical energy conversion efficiency. Through co-optimization of the operating frequency and load resistances, an On-Off Keying (OOK) backscatter system was demonstrated, achieving a data rate of up to 100 kbps and a bit error rate below $1 \times 10^{-4}$ over a 4 cm distance in silicone oil. These results demonstrate a promising approach for high-speed backscatter ultrasonic communication in medical IoT.

This paper presents a piezoelectric MEMS (Micro Electro Mechanical Systems) vibration sensor designed to minimize the transverse effect while maintaining a wide dynamic range. By adopting an elliptical membrane and a series electrode configuration, the sensor significantly reduces transverse sensitivity compared to conventional circular designs. Additionally, the electrodes are positioned within the stress concentration region to enhance the axial sensitivity. Lead zirconate titanate (PZT) is selected as the piezoelectric material due to its high piezoelectric constant and large dielectric constant, which provide sufficient capacitance to mitigate parasitic effects. Moreover, the elliptical structure exhibits an increased resonant frequency, directly contributing to an enhanced stiffness and extended dynamic range. Experimental results show that the elliptical vibration sensor achieves a 29.4% reduction in transverse sensitivity along the x-axis and an 18.6% reduction along the y-axis, while slightly improving the z-axis sensitivity and the figure of merit (FOM) is increased by 184%. The sensor achieves a dynamic range of up to 50 g with a nonlinearity of 1.24%. The significant reduction in transverse sensitivity coupled with a broad dynamic range demonstrates that the proposed elliptical vibration sensor offers a promising approach for developing advanced MEMS vibration sensors, particularly for precision vibration measurement applications.

Continuous cuffless blood-pressure (BP) monitoring requires compact sensors capable of tracking arterial diameter with high repeatability. Time-resolved photoacoustic (PA) sensing is attractive because vessel diameter is directly encoded in the temporal separation between the anterior- and posterior-wall arrivals in the PA A-line signal, eliminating the need for beamforming and image reconstruction. Here, we present a compact (2.5 mm $\times\,\, 5$ mm) multi-frequency piezoelectric micromachined ultrasonic transducer (PMUT) receiver fabricated by a cavity silicon-on-insulator (CSOI) process. The high-fill-factor design provides three receive bands centered at approximately 3.5, 5.5, and 11 MHz, with PA −6 dB fractional bandwidths of 126%, 103%, and 110%, respectively. Owing to the large effective acoustic aperture and low-noise front-end readout, the low-frequency array achieves a noise-equivalent pressure (NEP) of $2.9~\boldsymbol {mPa}/\sqrt { \boldsymbol {Hz}}$ at 3.5 MHz. Using a compact 532 nm pulsed laser delivering $\sim 5~\mu $ J/pulse, the system resolved vessel phantoms with inner diameters down to 1.0 mm at a depth of 5 mm in an optically clear medium, detected PA signals up to 40 mm in clear media, and maintained vessel detection up to 6 mm in a scattering tissue-mimicking medium. These results demonstrate that complementary low-, mid-, and high-frequency PMUT arrays can jointly provide penetration depth and diameter resolution within a compact footprint, supporting the development of wearable PA sensors for cuffless vascular monitoring. [2026-0042]