The Radiofrequency (RF) industry needs surface acoustic wave (SAW) resonators with high-quality factors ( ${Q}$ ) and spurious-free response characteristics to also have steep roll-off and a flat passband. In this work, SAW resonators based on 36°YX-LiTaO $_{{3}} /$ SiO $_{{2}} /4\text{H}$ -SiC multilayer substrates are fabricated that can effectively suppress acoustic energy leakage. Finite element method (FEM) simulations show that the effective electromechanical coupling coefficient ( ${k}^{{2}}_{\text {eff}}$ ) of the multilayer structure SAW resonators can be improved by choosing SiO2 and 36°YX-LT layers of appropriate thickness, and it is the high acoustic velocity of the slow shear bulk wave in 4H-SiC that leads to the multilayer structure SAW resonators showing excellent performance in confining acoustic energy. However, from the slowness curve, we measure the power flow angle (PFA) to be 10°, which, if uncorrected, would lead to serious transverse modes between the resonant frequency ( ${f}_{\text {r}}$ ) and the antiresonant frequency $({f}_{\text {a}})$ . To counter this, a multilayer structure SAW resonator with transverse mode suppression is fabricated using interdigital transducers (IDT) with a tilted angle of 10°. The multilayer resonator displays an extremely high Bode- ${Q}$ of over 3500 and a high impedance ratio of 85 dB. As predicted by the simulation results, ${k}^{{2}}_{\text {eff}}$ is up to 12%, and an extremely high figure of merit (FOM) of 423 is achieved. The enhanced performance of SAW resonators demonstrated in this work can open a door for constructing high-performance SAW filters with low insertion loss and high stability characteristics.

High-performance wideband acoustic filters are urgently needed for communication systems with large volumes of data. Recently, multilayered surface acoustic wave (SAW) resonators have attracted wide attention as key components of acoustic filters. In our previous work, we proposed a shear-horizontal SAW (SH-SAW) resonator based on a 30° YX -LiNbO3/SiO2/Si substrate. This resonator achieved an effective electromechanical coupling coefficient ( ${k}^{{2}}_{\text {eff}}$ ) of 24.4% and a Bode- ${Q}_{\text {max}}$ of 1092. However, the admittance/conductance showed obvious transverse and longitudinal modes spurious that degraded the resonator’s performance. To address this issue, we adopted a tilted interdigital-transducer (IDT) design in this article, which combined dispersion/slowness characteristic analysis and tilted angle optimization using the finite-element method (FEM). The simulations and experimental results have verified the successful suppression of both transverse and longitudinal modes when the tilted angle of the IDT reaches 16°. Moreover, the adoption of the tilted IDT designs for spurious modes suppression does not result in the evident sacrifice of the ${k}^{{2}}_{\text {eff}}$ . The proposed resonator has the potential to be used in the development of acoustic filters with large bandwidth, low loss, and no ripples.

With the rapid development of 5G technology, acoustic wave filters with large bandwidths are urgently required to deal with the explosive increase in data traffic. Recently, there is extensive attention paid to shear-horizontal (SH) surface acoustic wave (SAW) resonators based on lithium niobate (LiNbO₃) substrates, thanks to its large effective coupling coefficient (k²_eff). However, because of the bulk acoustic wave (BAW) energy radiation into the LiNbO₃ substrate, it is very challenging to obtain a high quality factor (Q) for SH-SAW resonators. In this study, a 30° YX-LiNbO₃/SiO₂/Si SAW resonator with the SH mode is proposed to achieve a large coupling and a high Q simultaneously. By bonding a LiNbO₃ thin film onto a thermally oxidized Si(100) substrate, the velocity mismatch between the piezoelectric layer and the SiO₂/Si substrate could significantly reduce the BAW energy leakage. Finite element method simulation is employed to optimize the cut angle of the LiNbO₃ film and the thickness of each layer. The fabricated SH-SAW resonators with a resonant frequency of 924 MHz yield a k²_eff of 24.8% and a maximum of Bode-Q (Bode-Q_max) of 1107. In comparison with the previously reported same-type SAW resonators, a higher Bode-Q_max is demonstrated in this work when their k²_eff is larger than 20%, providing a potential solution to enable wideband tunable filters in the 5G communication system.

For real-time millimeter-wave (MMW) imaging applications, spatially 2-D sparse sampling is one of the most promising techniques to reduce the heavy cost of signal acquisition. Therefore, in this article, we develop a new MMW synthetic aperture radar (SAR) sparse imaging method by establishing a 2-D pseudorandom spiral-sampling pattern. The method consists of two phases: sparse sampling-pattern design and sparse-imaging processing. The sparse sampling-pattern design is modeled as an optimization programming problem, in which the initial sampling pattern is a classical Archimedes spiral, and then, the number of samples per cycle is adjusted by coming up with a modified sigmoid function. This function is employed to increase the angular aperiodicity and randomness over the whole planar aperture. For sparse-imaging processing, a singular value thresholding (SVT) algorithm is applied to reconstruct the complete echo from partial samples observed by sparse sampling, and then, the inversion can be performed via the traditional imaging algorithms. In particular, a performance-guarantee condition of SVT is introduced to control the minimum number of samples, which bridges the relationship between pattern optimization and reconstruction performance. Our motivation is to answer what the undersampling pattern is and to estimate the extent to which the number of samples can be reduced by the current sparse imaging method. Finally, simulations and experiments validate the feasibility and effectiveness of the proposed sampling pattern, and the imaging results further demonstrate that our method with partially observed samples can offer the performance comparable to dense sampling.

To relieve the system cost in densely sampling when applying the millimeter-wave (MMW) synthetic aperture radar (SAR) technique for imaging applications, an MMW sparse imaging method is presented by the combination of the under-sampling scheme in space and the following sparse imaging algorithm. Different from the requirement on sub-wavelength sampling interval by Nyquist law, the system only needs to collect a random subset of spatial samples. To transform the spatial samples into wavenumber domain, a sparse range migration algorithm (SRMA) is proposed to realize MMW sparse imaging by embedding the matrix completion (MC) technique into classical RMA. In light of the relationship of Fourier transform between the scattering coefficients and the wavenumber-domain echo, the proposed algorithm utilizes the atomic norm minimization to fulfill the MC from the small set of wavenumber-domain entries to their full data matrix. The experiments are conducted by a wideband MMW transceiver mounted on a linear trajectory for imaging two specimens with different kinds of cover. Their clear imaging contrast of azimuth-range profiles is obtained through SRMA under different under-sampling conditions and RMA with fully sampled data, and verify the effectiveness of the proposed method.

With the rapid development of 5G communication technology, high-performance acoustic filters are urgently required to cope with the increasing number of frequency bands. Recently, an extensive attention has been paid to the advancement of lithium niobate (LiNbO $_{\text{3}}$ )-based thin-film acoustic resonators due to their large electromechanical coupling factor. In this article, a fundamental (0th) shear-horizontal plate acoustic wave (SH $_{\text{0}}$ -PAW) resonator based on a 30 $^{\circ}$ Y -cut LiNbO $_{\text{3}}$ /SiO $_{\text{2}}$ thin film is proposed. Using the finite element method (FEM), the effective electromechanical coupling coefficient ( $\textit{k}^{\text{2}}_{\text{eff}}$ ) and phase velocity as the function of the rotation angle $^{^{^{}}}$ of the LiNbO $_{\text{3}}$ film and the layer thicknesses are extracted. Meanwhile, the dispersion characteristic of the SH $_{\text{0}}$ mode is analyzed. The fabricated resonators yield a $\textit{k}^{\text{2}}_{\text{eff}}$ of 24.8%, a Bode- $\textit{Q}_{\text{max}}$ of 688, and a figure of merit (FoM $=$ $\textit{k}^{\text{2}}_{\text{eff}} \cdot \text{Bode-}\textit{Q}_{\text{max}}$ ) of 170. Furthermore, a tilted interdigital-transducer (IDT) method is adopted to suppress the spurious modes of the proposed resonators. It has been experimentally verified that most of the transverse modes are suppressed, and the longitudinal modes are suppressed effectively. The proposed resonator provides an effective solution to design a wideband, low loss, and unrippled acoustic filter for communication systems.