Peisen Liu

This work proposes and experimentally demonstrates an oppositely oriented lithium niobate (LN) bilayer on SiC platform for high-frequency wideband longitudinal leaky surface acoustic wave (LLSAW) filters targeting Wi-Fi 6 bands with 5G New Radio coexistence. Through finite element method simulations and mutual-energy analysis, we clarify a bilayer-LN design paradigm for mode-selective excitation in heterostructure SAW devices, enabling destructive cancelation of the low-frequency shear horizontal surface acoustic wave (SH-SAW) mutual energy while preserving efficient LLSAW excitation. Experimentally, resonators fabricated on the optimized +X-LN/−X-LN/SiC bilayer exhibit strongly suppressed low-frequency SH-SAW spurious responses over wavelengths from 1.1 to 1.5 μm, while maintaining LLSAW electromechanical coupling (kt2) up to 20.8% and Bode-Q values above 550 near 5.5 GHz. A corresponding ladder filter centered at 5.44 GHz further achieves a minimum insertion loss of 1.91 dB and a large 3-dB fractional bandwidth of 13.9%, with the parasitic transmission near 4 GHz reduced from 19.9 to 4.3 dB compared with the single-layer LN/SiC reference. These results establish oppositely oriented LN bilayer on SiC as a viable prototype for enabling spurious-free wideband LLSAW filters, as well as a scalable interlayer coupling design paradigm for mode-selective transduction in heterogeneous SAW devices.

Surface acoustic wave (SAW) magnetic sensors offer an attractive platform for weak magnetic field detection, benefiting from compact form factor and wireless operation. However, intrinsic acoustic energy leakage into the substrate fundamentally limits the achievable sensitivity. Here, we present a compact SAW magnetic sensor based on high-order shear horizontal modes to enhance magnetoelastic coupling via acoustic energy confinement. We show that the magnetic-induced phase response is governed by harmonic-dependent acoustic energy confinement within the magnetostrictive layer. The reduced wavelength of higher-order harmonics strengthens acoustic energy confinement, increasing the fraction of acoustic energy participating in magnetoelastic coupling and thereby significantly amplifying the ΔG-induced phase modulation. As a result, the proposed device achieves a superior phase sensitivity of 3486°/mT under seventh-order harmonic excitation, corresponding to an approximately 770-fold enhancement over the fundamental mode, together with a limit of detection of 17 pT/Hz1/2 at 100 Hz. Offset frequency-dependent characterization under AC magnetic field modulation further reveals stable and high-performance detection of low-frequency weak magnetic signals. These results establish acoustic energy confinement as an effective mechanism for enhancing ΔG-based SAW magnetic sensing and provide a viable pathway toward high-density and on-chip integrated weak-field magnetic sensor arrays.

Acoustic waves in thin films always suffer from high dispersion, leading to significant sensitivity to film thickness and corresponding device instability in practical applications. The quasi-longitudinal (QL) wave, conventionally classified as a symmetric Lamb wave, is generally considered a low-dispersion plate wave. However, the mechanism responsible for its low dispersion remains underexplored, limiting the excitation and control of QL waves in complex structures beyond simple plates. Here, we show that the QL wave inherits the non-dispersive nature of the longitudinal bulk wave, whereas its dispersion is introduced by coupling with highly dispersive Lamb waves. The coupling strength is governed by their displacement similarity such that reducing their similarity suppresses the coupling-induced dispersion. Based on this mechanism, a surface-wave-like QL wave is excited in an Al/LiNbO3/SiC heterostructure. The similarity between the QL wave and Lamb waves is reduced from 0.57 to 0.16, resulting in extremely weak coupling and a nearly non-dispersive QL wave with a dispersion curve slope of only −2.6 m/s. Acoustic resonators based on this structure show less than 0.8% variation in resonant frequency corresponding to the QL wave when the LiNbO3 thickness varies from 700 to 1100 nm. Our work not only clarifies the fundamental physics of the QL wave and then isolates a nearly non-dispersive QL wave from Lamb waves but also reports a highly robust acoustic resonator that overcomes the notorious sensitivity of resonant frequency to the piezoelectric film thickness.

Efficient manipulation of surface acoustic wave (SAW) within thin-film structures is fundamental to advancing the performance of SAW devices, yet it is challenged by unavoidable energy dissipation into the substrate. To suppress acoustic wave energy leakage, this study enhances the quality factor ( ${Q}\text {)}$ of SAW devices by introducing a heterogeneous multilayer electrode structure incorporating Al/Ir and Al/Ru stacks. By strategically exploiting the drastic acoustic impedance mismatch between these heavy and light metals, a reflective boundary is engineered to strictly confine the SAW energy within the piezoelectric layer. An impedance contrast multilayer electrode structure not only minimizes acoustic wave radiation losses but also suppresses electrode damping. Experimental results confirm that acoustic impedance engineering strategy yields a transformative leap in device performance: the Al/Ir-based SAW resonator achieves a remarkable ${Q}_{\max }$ of 5846 at $3~\mu $ m wavelength—110% enhancement over the Al baseline value of 2779. Furthermore, a robust $Q$ improvement > 70% is observed across various wavelengths based on 10 nm Ru (except $2~\mu $ m) and 10 nm Ir. These findings offer deep insights into the loss mechanisms of thin-film SAW resonators and establish a generic, physically driven pathway for realizing high-performance SAW devices.

This paper presents the first systematic study on the collaborative high-frequency suppression of spurious modes in longitudinal leaky surface acoustic waves (LLSAW) devices on Cu-grating/X-cut LiNbO₃/SiC stack, including the first order shear-vertical (SV1), shear-horizontal (SH₁), and longitudinal spurious signals (LSS). Through detailed modal and dispersion analysis, we clarify the role of cutoff frequencies in high-order modes suppression mechanism using distance-to-cutoff as a quantitative design margin. Resonators with Al and Cu electrodes are compared through systematic simulations under matched mass-loading conditions, supported by measurements on fabricated devices with different stack configurations. The Cu-electrode LLSAW resonators achieve a maximum Bode-Q_max of 512 and a figure of merit of 107 at 6 GHz. A synchronous LLSAW filter prototype demonstrates a 3-dB fractional bandwidth of 11.9%, a minimum insertion loss of 1.6 dB, and spurious suppressed responses both in-band and out-of-band. This work provides a scalable design paradigm enabling spurious-free wideband LLSAW devices in future 5G systems.

Lithium Niobate on Insulator (LNOI) platforms are gaining attention for their potential in fabricating wideband and low-loss surface acoustic wave (SAW) filters in 5 G communication systems. However, the impact of spurious Rayleigh wave, particularly in the GHz frequency ranges, pose a significant challenge by causing in-band ripples of wideband filters. This study employes finite element analysis to design wideband SAW filter structures, aiming to maintain a high level of electromechanical coupling while seeking a window for Rayleigh mode suppression under the condition of a low LN film thickness ratio( h _LN /λ<0.1). Meanwhile, a low LN film thickness ratio can improve the performance of the wideband SAW filter, resulting in a low temperature coefficient of frequency (TCF) of −9 ppm °C. A wave-weighted interdigital-transductor design is proposed, which could effectively suppress transverse modes. As a result, a wideband filter with a center frequency of 1005 MHz, a minimum insertion loss of 0.66 dB, and a 3-dB fractional bandwidth of 12.9% was successfully fabricated. This work not only advances wideband SAW devices for high-speed data transmission but also tackles the issue of Rayleigh wave interference in its GHz applications.

This work presents an all-round optimized LiNbO₃(LN)/Quartz platform that intrinsically overcomes the long-standing spurious-mode barrier limiting wideband applications for lithium-niobate-on-insulator (LNOI), while simultaneously delivering excellent thermal stability and low-loss response in shear horizontal surface-acoustic-wave (SH-SAW) devices. Through a systematic investigation into the slowness landscape of various LN and Quartz orientation pairs, a design framework for anisotropy-matching in heterostructures was established toward the intrinsic suppression of spurious modes, and 32°YX LN/AT-Quartz and 32°YX LN/ $60^{\circ }Y 90^{\circ }X$ Quartz platforms emerged as optimal configurations due to their effective modulation of surface velocity profiles. To verify the simulation results, resonators were prepared on four LN/Quartz platforms with various Quartz orientations, including the 36°YX (AT-cut), $60^{\circ }Y 90^{\circ }X$ , $69^{\circ }Y 90^{\circ }X$ , and $Y - Z$ Quartz, along with the prevalent LN/SiO₂/Si structure for a comparison. Besides an unprecedented performance metrics in terms of electromechanical coupling factor ( $K^{2}$ ) up to 20%, Bode- $Q_{\max }$ exceeding 1000, and an ultralow temperature coefficient of frequency (TCF _a ) of −1.45 ppm/°C, the implemented resonators on LN/AT-Quartz and LN/ $60^{\circ }Y 90^{\circ }X$ Quartz demonstrated superior spectral purity with inherent suppression of transverse modes, higher order modes, and Rayleigh modes concurrently throughout a wide spectral span without additional countermeasures. Synchronously implemented surface-acoustic-wave (SAW) filters under standard interdigitated transducer (IDT) layouts further exhibited both flat in-band response and Out-of-Band (OoB) rejection beyond 30 dB, with a typical design offering a 3-dB fractional bandwidth (FBW) of 14.2% and minimum insertion loss (IL ${}_{\min }$ ) of 0.81 dB, validating the effectiveness of the proposed scheme.

LiNbO₃ thin film on insulator (LNOl) platform has emerged as a promising solution for wideband and low-loss surface acoustic wave (SAW) applications in Sub-6G era. However, Rayleigh spurious mode is a very serious problem in LNOI platform. In this work, machine learning approach random forest developed by Bayesian Optimization is employed to predict the performance of SAW resonators based on LNOI platform. Additionally, an intelligent algorithm is formulated to quantify the number of spurious modes of LNOI platform, thereby enabling the selection of parameter combinations that eliminate spurious modes and possess electromechanical coupling coefficient (K² ) higher than 20%. Moreover, this work selected parameter combinations from predicted results and designed corresponding SAW resonators based on LNOl platform to verify accuracy of the prediction and spurious modes. [2025-0081]