Topological materials for classical waves, e.g., electromagnetic and acoustic waves, have attracted growing interest, mainly due to the robustness, low loss, and new artificial degree of freedom conferred by their boundaries. Surface acoustic waves (SAWs), as widely used information carriers of microdevice relevance, are ubiquitous in today’s wireless communication and sensing networks. Herein, we report the implementation of a SAW topological insulator based on a monolithically integrated platform. By using a miniature acoustic resonator array working tens of megahertz on a piezoelectric half-space, we successfully endow electrically pumped Rayleigh-type SAWs with the “spin-momentum locking” feature, enabling solid-state acoustic waves on the “one-dimensional interface of the two-dimensional surface on the three-dimensional volume” detour arbitrarily and pass through defects and intersections with much smaller losses than those incurred with any other solutions. These revolutionary topological SAWs may open an avenue for monolithic electronic-(photonic)-phononic circuits with ultrahigh performance and advanced functionalities in, e.g., future mobile communicating, sensing, and quantum-information processing.

This work presents a low temperature coefficient of frequency (TCF) and high-performance S4 mode acoustic resonator based on $128^{\circ} \text{Y}$ LiNbO3/SiO2/a-Si/Si (trap-rich layer assisted LiNbO3-on-insulator, TR-LNOI) substrate. The electromechanical coupling, quality factor, and TCF are optimized for S4 mode by adjusting the thickness ratio of stack layers and the geometry design of the inter-digital transducer. The optimized resonators exhibit a TCF of resonant frequency as low as 0.24 ppm/K, an electromechanical coupling ( ${k}^{{2}}$ ) of 14.8%, a maximum quality factor ( ${Q}_{\text {max}}$ ) of 1406 at 7.43 GHz, resulting a ${k}^{{2}}\cdot {Q}_{\text {max}}$ of 208. This work greatly extends the limit of ${f}_{s}\cdot {k}^{{2}}\cdot {Q}_{\text {max}}$ for suspended thin-film resonators to ${7.43}\times {208}={1545}$ with a near-zero TCF of resonant frequency.

This work presents the laterally vibrating Lamb wave resonators (LVRs) based on a 30% aluminum scandium nitride (Al_0.7Sc_0.3N) thin film with three interdigited transducer pairs operating in the S₀ mode. In order to reduce the anchor loss, perfect matched layer-based finite element analysis simulations are utilized to design and optimize the device. Thanks to the high quality AlScN using magnetron sputtering with a single alloy target, vertical etching profile, and designed device structure, 1-μm-thick Al_0.7Sc_0.3N-based LVRs with high performance are fabricated. The resonator equivalent electric parameters are extracted utilizing the modified Butterworth–Van Dyke model. The best Al_0.7Sc_0.3N LVR achieves an electromechanical coupling coefficient (k_t²) of 9.7% and a loaded quality factor (Q_r) of 1141.5 operating at approximately 305 MHz. The same resonator shows a motional quality factor (Q_m) of 1507.2, resulting in a high figure-of-merit (FoM = k_t² · Q_m) of 146.2. A 1.8 MHz tuning range is measured for an Al_0.7Sc_0.3N LVR by applying DC voltage in the range of −40 to 40 V due to the ferroelectric property of high Sc doping in Al_0.7Sc_0.3N. With the high FoM, Q_r, Q_m, and low motional resistance (R_m), the Al_0.7Sc_0.3N-based LVRs show strong potential in applications of radio frequency communications and piezoelectric transducers.

An aluminum nitride (AlN) piezoelectric micromachined ultrasound transducer (PMUT) array was proposed and fabricated for non-invasive radial artery stiffness monitoring, which could be employed in human vascular health monitoring applications. Using surface micromachining techniques, four hexagonal PMUT arrays were fabricated within a chip area of 3 × 3 mm². The mechanical displacement sensitivity and quality factor of a single PMUT were tested and found to be 24.47 nm/V at 5.94 MHz and 278 (in air), respectively. Underwater pulse-echo tests for the array demonstrated a −3 dB bandwidth of 0.76 MHz at 3.75 MHz and distance detection limit of approximately 25 mm. Using the PMUT array as an ultrasonic probe, the depth and diameter changes over cardiac cycles of the radial artery were measured to be approximately 3.8 mm and 0.23 mm, respectively. Combined with blood pressure calibration, the biomechanical parameters of the radial artery vessel were extracted using a one-dimensional vascular model. The cross-sectional distensibility, compliance, and stiffness index were determined to be 4.03 × 10⁻³/mmHg, 1.87 × 10⁻² mm²/mmHg, and 5.25, respectively, consistent with the newest medical research. The continuous beat-to-beat blood pressure was also estimated using this model. This work demonstrated the potential of miniaturized PMUT devices for human vascular medical ultrasound applications.

This paper proposed a solid-mounted (SM) longitudinally excited shear wave resonator (i.e., YBAR). By adopting a 200 nm x-cut LiNbO₃ film, top (aluminum) and bottom (platinum) electrodes in 50 nm thickness and 500 nm width, this resonator simultaneously achieves an operating frequency over 5 GHz with an electromechanical coupling coefficient exceeding 50%. Compared with previously proposed YBAR with suspended structure, the proposed SM-YBAR can effectively suppress unwanted spurious modes with only a slight loss of the electromechanical coupling coefficient. The SM-YABR also provides better device stability, possible low-temperature drift coefficient, and a more convenient and mature device processing. It has the potential to meet the multiple requirements for the next generation signal processing devices in terms of high frequency, large bandwidth, stability, and low cost, etc.

This article presents lithium niobate (LiNbO₃) based on shear horizontal (SH0) resonators, utilizing a suspended structure, for radio frequency (RF) applications. It demonstrates the design, analysis, and fabrication of SH0 resonators based on a 36Y-cut LiNbO₃ thin film. The spurious-free SH0 resonator achieves an electromechanical coupling coefficient ($k_t^2$) of 42.67% and a quality factor (Q_r) of 254 at the wave-propagating orientation of 0° in the 36Y-cut plane.