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This paper reports on the design, fabrication, and characterization of $S_{0}$ mode X-cut Lithium Niobate Laterally Vibrating Resonators (LVRs) operating between 100 MHz and 1 GHz. In the first Section of this work, analytical and numerical models are implemented to investigate the impact of topology and geometrical features on the Electromechanical Coupling ($k_{t}^{2}$) and Quality Factor ($Q_{s}$) of this class of devices. The models are used to define the design of experiments aimed at characterizing the resonator performance. The resonators are fabricated and tested in atmospheric, vacuum, and cryogenic ($T$ = 10 K) conditions to assess their performance and identify the main sources of discrepancies in $k_{t}^{2}$ and $Q_{s}$ with respect to the analytical and numerical models. Under vacuum quality factors exceeding a few thousands are recorded across the investigated spectrum, with measured $k_{t}^{2}$ as high as 28.9%. At 100 MHz, measured $Q_{s}$ and $k_{t}^{2}$ of 8,279 and 28.5%, respectively, combine into a record-breaking Figure of Merit ($FOM$) of 2,688, while cryogenic testing highlighted $Q_{s}$ as high as 26,511. In the last section, by analyzing the differences in $k_{t}^{2}$ and $Q_{s}$ between experiments and models, best design approaches to maximize the $FOM$ and minimize spurious modes responses are reported, with a focus on the 100 to 550 MHz range. [2022-0081] [ABSTRACT FROM AUTHOR]

This article presents the lamb wave contour mode resonators (CMRs) based on 22% aluminum scandium nitride (AlScN) thin film with $\lambda $ of 12–24 $\mu \text{m}$ , and operating in $\text{S}_{{0}}$ mode. We report the design, fabrication, and characterization of 500 nm-thick AlScN CMRs, which take advantage of optimized stress control of co-sputtered AlScN thin films and vertical inductively coupled plasma (ICP) etching profile. The experimental results are compared to theoretical predictions by finite element analysis (FEA). All Al $_{{0.78}}$ Sc $_{{0.22}}\text{N}$ devices show excellent agreement with simulations in piezoelectric coupling using modified AlScN film parameters. The best Al $_{{0.78}}$ Sc $_{{0.22}}\text{N}$ CMR has achieved an electromechanical coupling coefficient (${k}_{t}^{{2}}{)}$ of 5.24% and loaded quality factor (${Q}$) of 1219 with an operating frequency at approximately 300 MHz, which exhibits a high Figure-of-Merit (FoM) of 63.88 in piezoelectric microelectromechanical system (MEMS) lamb wave CMR. This article also presents the co-sputtering characteristics of the AlScN thin films under $\text{N}_{{2}}$ gas to achieve low-stress and high-quality piezoelectric materials, and the etching optimization of high concentration Sc doping aluminum nitride (AlN) thin films under Cl2/BCl3/Ar chemistry to obtain record profile angle of 77°, high selectivity of 1:1 with SiO2 hard mask. [ABSTRACT FROM AUTHOR]

This work explores micromachined heterostructured waveguides that leverage strong interactions between acoustic phonons and electrons to enable radio frequency (RF) signal amplification or attenuation. A thin-film piezoelectric-on-semiconductor stack is tailored to generate high electromechanical coupling Lamb waves that are impacted by high mobility electrons within a microacoustic waveguide. Lamb waves are generated by RF signal via interdigital electrodes on the piezoelectric layer; by applying a voltage to the semiconductor layer so that the electrons lead mechanical waves, the RF signal is amplified through the acoustoelectric (AE) effect. Conversely, the signals propagating faster or in the opposite direction of the electron flow undergo attenuation, rendering the waveguide nonreciprocal. Research on the AE effect dates to the mid-20th century and until now has been mostly focused on surface acoustic waves (SAWs). In this work, enabled by high-quality bonded thin films of lithium niobate (LN) and silicon (Si), it is shown that fundamental symmetric ($S0$) Lamb mode waveguides at 100 s of MHz can achieve more than 40 dB of AE gain and strong nonreciprocal transmission with less than 10 mW of bias power consumption. This could enable implementation of switches, delay lines, isolators, and circulators, which are critical for interference cancellation and full-duplex radio. The AE effect is observed to be stronger in some higher $S0$ harmonics, allowing for scaling to higher frequencies with optimized electrodes that have more relaxed critical dimensions. In addition, up to 5.5-dB sustained terminal gain measured in this work implies the potential for transistor-less amplifiers that could be implemented in concert with more traditional acoustic devices in a single-chip manner. [ABSTRACT FROM AUTHOR]

The imaging method based on the conventional phased array (PA) can only realize angle focusing. To break through the above limitations, frequency diverse array (FDA) is first introduced from radar to Lamb wave, which opens up a possibility to realize angle–range focusing in damage localization. The energy pattern of FDA without considering the features of radar signals is re-derived, and a corresponding FDA focusing imaging method for Lamb wave is developed. In the presented method, by appending various frequency offsets to the redesigned anti-dispersive excitations, the desired FDA focusing and the dispersion elimination for Lamb wave are achieved simultaneously. Furthermore, based on full matrix capture being the data acquisition approach, a novel total focusing is realized with comprehensive consideration of positive and negative frequency offsets. Benefiting from FDA focusing, compared with the PA methods, the proposed method exhibits superior adjacent defects’ identification ability and preferable interference suppression capability with excellent location accuracy and strong parameter robustness. Its superiority is validated by the experiment on aluminum plate with multiple adjoining surface and inner defects. [ABSTRACT FROM AUTHOR]

This work presents an in-depth study of simulation and measurement results of laterally excited shear bulk acoustic resonators (XBAR) in Lithium Niobate, at 5 GHz with high electromechanical coupling factor ($k_{t}^{2}$) as high as 25%, and with impedances at resonance close to $2~\Omega $. Loaded Quality factors of up to 340 and 150 are obtained at resonance and anti-resonance, respectively. Experimental00 dispersion behaviors of main mode and spurious are presented. Several geometric parameters affecting resonator performance are studied in order to improve figure of merits (FoM) of the device for 5G filter applications. The modified fabrication process presented shows a high yield of over 90% of devices which can be scalable for mass production of high frequency, sub-6 GHz, wide band filters. [2021-0202] [ABSTRACT FROM AUTHOR]

Acoustic wave devices have great potential for integration with lab-on-chip highly efficient microfluidics. This article investigates Lamb wave-based unidirectional transducers for application in acoustic wave-driven microfluidic devices with high efficiency. The simulation of the unidirectional transducer is performed via the finite element analysis. The optimal cell design of the transducer is suggested according to the Lamb wave uneven excitation. In particular, we propose a sophisticated double-side IDT pattern to enhance Lamb wave transduction. The anti-symmetric ${A}_{{0}}$ mode implemented with double-side unidirectional transducers is determined and optimized for the microfluidic device application. The optimum Lamb wave-based devices are fabricated on a wafer of 128° YX LiNbO3 with a thickness of 300 $\mu \text{m}$ using an elaborate two-side lithography technique. The amplitude of Lamb waves excited from the unidirectional transducers are measured and confirmed the unidirectionality, accordingly. Thorough atomization and jetting experiments driven by the unidirectional transducer are presented. The results agree with the simulation and verify the efficiency of the proposed double-side patterned unidirectional transducers in microfluidic applications. [ABSTRACT FROM AUTHOR]

Advancements in the structural health monitoring (SHM) technology of composite materials are of paramount importance for early detection of critical damage. In this work, direct-write transducers (DWTs) were designed for the excitation and reception of selective ultrasonic guided waves and fabricated by spraying 25- $\mu \text{m}$ -thick piezoelectric poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TRFE)] coating with a comb-shaped electrode on carbon fiber-reinforced polymer (CFRP) plates. The characteristics and performance of the ultrasonic DWTs were benchmarked with the state-of-the-art devices, discrete lead zirconate titanate (PZT) ceramic transducers surface-mounted on the same CFRP plates. The DWTs exhibited improved Lamb wave mode excitation (A0 or S0 mode) relative to the discrete PZT transducers. Moreover, high signal-to-noise ratio was obtained by effectively canceling other modes and enhancing the directivity with the periodic comb-shaped electrode design of the DWTs, despite the smaller signal amplitudes. The enhanced directivity overcompensates for lower amplitude attenuation, making DWT a good candidate for locally monitoring critical stress hot spot regions in the CFRP structure prone to early damage initiation. It is shown that pairing a DWT sensor with a discrete PZT actuator could further achieve balanced performance in both wave mode selection and signal amplitudes, making this combination really attractive for ultrasonic SHM. [ABSTRACT FROM AUTHOR]

The study of dispersive signals has received widespread attention in numerous fields, such as structural health monitoring, nondestructive testing, and geological exploration. The decomposition of multicomponent dispersive signals generated by real systems is challenging since the group delay (GD) of each component varies nonlinearly with frequency and may overlap with each other. Meanwhile, the amplitude mutation components also bring difficulties to the decomposition. Aiming at these problems and inspired by the dispersion compensation method (DCM) and generalized dispersive mode decomposition (GDMD), we propose the nonlinear dispersive component decomposition method (NDCD) in this article. We establish an optimization model with a hybrid regularization term for frequency domain signal decomposition. By solving the optimization problem using composite split denoising (CSD) and the split augmented Lagrange shrinkage algorithm (SALSA), we can get the signal components along with high-resolution time-frequency representation. Simulation signals, bearing fault diagnosis, and Lamb wave experiments show that NDCD has better decomposition ability for signals containing mutation amplitude and overlapped GD. Moreover, it also has better noise robustness and lowers computational complexity, better than methods such as GDMD, DCM, synchrosqueezing transform (SST), time-reassigned SST (TSST), and TSET. [ABSTRACT FROM AUTHOR]

The special form of pipe-like structure provides the helical route for ultrasonic guided wave. Considering the pipe as a flattened plate but with periodical replications, the helical wave becomes intuitional and a corresponding imaging algorithm can be constructed. This work proposes the multihelical Lamb wave imaging method by utilizing the multiple arrival wavepackets which are denoted as different orders. The helical wave signal model is presented and the constant group velocity point is illustrated. The probabilistic reconstruction algorithm is combined with the separation and fusion of different helical routes. To verify the proposed scheme, finite element simulations and corresponding experiments are conducted. The cases of single-defect simulation and two-defect simulation indicate the successful and robust implementation of the imaging algorithm. The test on actual pipe damage is also investigated to show its capability in imaging an irregular defect. The comparison with imaging results from only first arrival demonstrates the advantage of multihelical wave imaging, including the better imaging resolution and higher localization accuracy. [ABSTRACT FROM AUTHOR]

Conventional electromagnetic acoustic transducers (EMATs) are generally only used to generate and detect guided waves with a single wavelength, which increases their sensitivity at that particular wavelength but limits their application scenarios and the accuracy of defect assessment. This article proposes a design method for multiwavelength EMATs based on spatial-domain harmonic control. First, the EMAT model is analyzed, where it is then outlined that the eddy-current density distribution of the specimen is equivalent to the spatial low-pass filtering of the coil-current density distribution. This shows that the multiwavelength guided waves can be achieved as long as the spatial distribution of the coil-current density contains those multiple harmonics that are desired. It is then proposed that the structure of the EMAT coil is equivalent to the spatial sampled pulse sequences of a spatial signal. The coil parameter design based on pulse modulation technology is proposed. Taking a dual-wavelength EMAT design for Lamb waves as an example, details of the coil parameter design are presented. The simulation and experiment with the dual-wavelength EMAT proved the correctness of the proposed method. Finally, an experiment with a three-wavelength EMAT demonstrated the feasibility of the proposed method in designing multiwavelength EMATs. [ABSTRACT FROM AUTHOR]

In this work, we present a new paradigm for enabling gigahertz higher-order Lamb wave acoustic devices using complementarily oriented piezoelectric (COP) thin films. Acoustic characteristics are first theoretically explored with COP lithium niobate (LiNbO3) thin films, showing their excellent frequency scalability, low loss, and high electromechanical coupling ($k^{2}$). Acoustic resonators and delay lines are then designed and implemented, targeting efficient excitation of higher-order Lamb waves with record-breaking low loss. The fabricated resonator shows a $2^{\mathbf {nd}}$ -order symmetric (S2) resonance at 3.05 GHz with a high quality factor ($Q$) of 657, and a large $k^{2}$ of 21.5% and a $6^{\mathbf {th}}$ -order symmetric (S6) resonance at 9.05 GHz with a high $Q$ of 636 and a $k^{2}$ of 3.71%, both among the highest demonstrated for higher-order Lamb wave devices. The delay lines show an average insertion loss (IL) of 7.5 dB and the lowest reported propagation loss of 0.014 dB/ $\mu \text{m}$ at 4.4 GHz for S2. Notable acoustic passbands up to 15.1 GHz are identified. Upon further optimizations, the proposed COP platform can lead to gigahertz low-loss wideband acoustic components. [2020-0127] [ABSTRACT FROM AUTHOR]

Ultrasonic Lamb wave testing allows health monitoring for plate structures and has been extensively applied in the modern industry. To obtain the location and further form the damage imaging, the time of flight (TOF) is a pivotal parameter. However, the dispersive and multimodal nature of the Lamb wave obstructs the accurate measurement of TOF. The objective of this article is to develop efficient techniques to separate multimodal Lamb wave signals and accomplish accurate damage localization. The narrowband Lamb wave is generated by the excitation signal with a center frequency of 150 kHz. The fundamental symmetric and antisymmetric modes are exploited. The adaptive polynomial chirplet transform (CT) is proposed to decompose the Lamb wave signals and identify the modes. Then, the instantaneous frequency characteristics are utilized to extract time information. Thus, the TOF is obtained and damage location can be calculated. The reconstruction error is defined to quantitate the difference between the reconstructed signal and the received signal. The obtained reconstruction errors from the simulation and experiment are 0.1631 and 0.2875, respectively. The varied defect location is adopted to obtain the TOF estimation. The results show that the proposed method outperforms the CT and cross correlation, especially for the longer propagation distances where the dispersion effect is distinct. For experiments considering the defect and boundary reflection, the localization errors of the collinear defect are 2.51% and 3.04% for the two Lamb wave modes. In comparison with smoothed-pseudo-Wigner–Ville distribution and CT, the proposed method owns superior performance on mode identification and location accuracy. [ABSTRACT FROM AUTHOR]

Lamb wave delay lines with acoustoelectri-cally-induced non-reciprocity are demonstrated in suspended lithium niobate-on-silicon waveguides for the first time. An electric current is fed through the silicon layer in the same axis as the piezoelectrically-transduced acoustic waves which are attenuated or amplified depending on the direction of the energy exchange with drifting electrons (i.e. acoustoelectric (AE) effect). Therefore, the insertion loss (IL) and non-reciprocity of the delay line is adjustable by controlling the current bias which varies the electron drift velocity and the subsequent momentum transfer. Proof-of-concept delay lines in the range of 600 MHz to 700 MHz are demonstrated with a fractional bandwidth as high as 2.8% and AE gain as high as 5.6 dB resulting in ~20 dB of non-reciprocity. These non-reciprocal components could offer a monolithic and dynamically-tunable solution to the numerous issues that arise from the increasing congestion and interferences in the telecommunication spectrum and suggests the possibility of developing fully-switchable low-IL delay lines through design/fabrication optimizations. [ABSTRACT FROM AUTHOR]

Properties of isotropic plates in terms of material constants and thickness are characterized by making use of dispersion characteristics of propagating Lamb waves. A numerical model is inversely optimized in order to match dispersion curves measured by laser vibrometry. This kind of material characterization has gained interest in recent research. Improvements in accuracy and efficiency of the optimization process are therefore important steps toward an industrial application of this technique to nondestructive testing and online monitoring. For this purpose, the use of a fast converging numerical model based on spectral collocation has been found to be well suited. Furthermore, we improved the signal-to-noise ratio by utilizing long-time broadband excitation signals for multimodal excitation of Lamb waves. The wavenumber spectrum up to 2.5 MHz is acquired by measurements with a laser-scanning vibrometer. In order to exploit the information contained in high-order modes, we present an algorithm to match the measured data to the calculated modes during the optimization process, leading to higher accuracy of the estimated model parameters. The characterization results are verified by comparison to measurements with a conventional ultrasonic method. [ABSTRACT FROM AUTHOR]

In this paper, we present an ultrasound nondestructive testing (NDT) method for high–temperature (HT) applications using laser-generated Lamb waves and aluminum nitride (AlN) sensors. Lamb waves were introduced to a stainless steel plate by the Nd:YAG pulsed laser at one point and detected by an AlN receiver at a distant position. The fundamental symmetric ( $S_{0}$ ) and antisymmetric ( $A_{0}$ ) Lamb waves generated by the pulsed laser were successfully detected by the AlN sensor. This detection was done on a stainless steel plate at temperatures ranging from room temperature to about 800 °C, with a signal-to-noise ratio of higher than 20 dB. Based on the time-of-flights analysis, the ability of this NDT method to localize the defect at an HT (~800 °C) has been demonstrated. [ABSTRACT FROM AUTHOR]

Characterizing the viscoelastic properties of thin-layer tissues with micro-level thickness has long remained challenging. Recently, several micro-elastography techniques have been developed to improve the spatial resolution. However, most of these techniques have not considered the medium boundary conditions when evaluating the viscoelastic properties of thin-layer tissues such as arteries and corneas; this might lead to estimation bias or errors. This paper aims to integrate the Lamb wave model with our previously developed ultrasonic micro-elastography imaging system for obtaining accurate viscoelastic properties in thin-layer tissues. A 4.5-MHz ring transducer was used to generate an acoustic radiation force for inducing tissue displacements to produce guided wave, and the wave propagation was detected using a confocally aligned 40-MHz needle transducer. The phase velocity and attenuation were obtained from k-space by both the impulse and the harmonic methods. The measured phase velocity was fit using the Lamb wave model with the Kelvin–Voigt model. Phantom experiments were conducted using 7% and 12% gelatin and 1.5% agar phantoms with different thicknesses (2, 3, and 4 mm). Biological experiments were performed on porcine cornea and rabbit carotid artery ex vivo. Thin-layer phantoms with different thicknesses were confirmed to have the same elasticity; this was consistent with the estimates of bulk phantoms from mechanical tests and the shear wave rheological model. The trend of the measured attenuations was also confirmed with the viscosity results obtained using the Lamb wave model. Through the impulse and harmonic methods, the shear viscoelasticity values were estimated to be 8.2 kPa for $0.9~\text {Pa}{\cdot} \text {s}$ and 9.6 kPa for $0.8~\text {Pa}{\cdot} \text {s}$ in the cornea and 27.9 kPa for $0.1~\text {Pa}\cdot \text {s}$ and 26.5 kPa for $0.1~\text {Pa}\cdot \text {s}$ in the artery. [ABSTRACT FROM AUTHOR]

In this paper, lamb wave acoustic (LWA) devices based on ZnO/Si plate of 30-µm thickness have been fabricated. Devices with four different designs having acoustic wavelengths of 20, 40, 60, and 80 μm have been fabricated to study its effect on the acoustic velocity for both the lowest order antisymmetric (A0) and symmetric (S0) modes. The variation in effective phase difference in all the fabricated LWA devices is studied. An optimum device design is identified with appropriate distance between centers of the input and output IDTs to obtain a phase difference of 180° in antisymmetric and symmetric modes. The obtained phase difference of 180° is very critical in fabricating an oscillator for realization of wireless sensor devices. LWA devices offer high sensitivity to mass loading and thus hold tremendous potential for chemical sensing applications. [ABSTRACT FROM AUTHOR]

Lamb wave inspection is a promising structural health monitoring method for carbon fiber reinforced plastic laminates. The optimal design of the excitation waveform based on Lamb wave pulse compression is an effective way to improve the inspection resolution in elastic plates. However, due to the viscoelastic property of the composite materials, the material damping effect will influence the amplitude response of the output signals; thus, the current design strategy is no longer applicable. The purpose of this paper is to develop an excitation waveform design strategy based on pulse compression to meet the demands of high-resolution Lamb wave inspection in composite materials. Different parameters that affect the design of excitation such as material damping effect, amplitude response, relative frequency bandwidth, and the selection of excitation waveform are considered synthetically. In addition, an excitation waveform design strategy based on the corrected amplitude response is also established to eliminate the effect of material damping and thus achieve better detection resolution. Finally, an experiment was carried out on a quasi-isotropic composite plate. The experimental results validate the robustness of the amplitude response correction method and the effectiveness of the excitation waveform design strategy. [ABSTRACT FROM AUTHOR]

This paper reports on a spurious mode suppression technique for S0 mode aluminum nitride (AlN) laterally vibrating resonators. The technique utilizes a notch in the resonator body to convert spurious asymmetrical (A0) modes into the intended S0 mode vibration. Finite-element analyses were employed to theoretically investigate the mode conversion from A0 mode to S0 mode, and identify the optimal notch configuration. The technique has been experimentally validated to simultaneously eradicate the A0 spurious mode and enhance the S0 mode electromechanical coupling ( kt^{2} ) for the monolithically integrated resonators over a wide frequency range ( $100\sim 600$ MHz). It is a crucial technology development, because the spurious modes are a major bottleneck obstructing the deployment of single-chip multi-frequency AlN resonators as a commercially viable solution for radio frequency front-end filtering. [2015-0295] [ABSTRACT FROM PUBLISHER]

Commercial markets desire integrated multifrequency band-select duplexer and diplexer filters with a wide fractional bandwidth and steep roll-off to satisfy the ever-increasing demand for spectrum. In this paper, we discuss the fabrication and design of lithium niobate (LN) thin-film S0 Lamb-wave resonators on a piezoelectric-on-piezoelectric platform. Filters using these resonators have the potential to fulfill all the above requirements. In particular, we demonstrated one-port high-order S0 Lamb-wave resonators with resonant frequencies from \sim 400 MHz to \sim 1 GHz on a black rotated y-136 cut LN thin film. The effective electromechanical coupling factor ( keff^{2} ) ranges from 7% to 12%, while the measured quality factor ranges from 600 to 3300. The highest keff^{2} \times Q achieved on this chip is 194, significantly surpassing contour mode resonators manufactured in other technologies. [2014-0280] [ABSTRACT FROM AUTHOR]

In the purpose of achieving composite structure damage identification and localization of structural health monitoring, a denoising algorithm of redundant second-generation wavelet transform considering neighboring coefficients is selected as the best solution from 18 denoising schemes performed in this paper. Through introducing fractal dimension as a damage-sensitive feature and adopting the probabilistic reconstruction algorithm, the damage status on the composite panel could be identified and located as tomography maps. The practicability of the presented approach is validated in the experiment operating in the composite damage monitoring system. [ABSTRACT FROM AUTHOR]

Viscoelastic properties of the myocardium are important for normal cardiac function and may be altered by disease. Thus, quantification of these properties may aid with evaluation of the health of the heart. Lamb wave dispersion ultrasound vibrometry (LDUV) is a shear wave-based method that uses wave velocity dispersion to measure the underlying viscoelastic material properties of soft tissue with plate-like geometries. We tested this method in eight pigs in an open-chest preparation. A mechanical actuator was used to create harmonic, propagating mechanical waves in the myocardial wall. The motion was tracked using a high frame rate acquisition sequence, typically 2500 Hz. The velocities of wave propagation were measured over the 50–400 Hz frequency range in 50 Hz increments. Data were acquired over several cardiac cycles. Dispersion curves were fit with a viscoelastic, anti-symmetric Lamb wave model to obtain estimates of the shear elasticity, \mu 1, and viscosity, \mu 2 as defined by the Kelvin–Voigt rheological model. The sensitivity of the Lamb wave model was also studied using simulated data. We demonstrated that wave velocity measurements and Lamb wave theory allow one to estimate the variation of viscoelastic moduli of the myocardial walls in vivo throughout the course of the cardiac cycle. [ABSTRACT FROM PUBLISHER]