Mode Surface Acoustic Wave Research Articles

Generation of GHz surface acoustic waves in (Sc,Al)N thin films grown on free-standing polycrystalline diamond wafers by plasma-assisted molecular beam epitaxy

Abstract Telecommunication of the next generation demands filters that can operate in the 10 GHz range with sufficient bandwidths. For surface-acoustic-wave (SAW) devices this prerequisite translates into high sound velocities and high piezoelectric couplings. Wurtzite AlN on diamond, which exploits the strong piezoelectricity of AlN with the very high SAW velocity of diamond, has been considered a promising platform. A significant boost (up to a factor of 4) of the piezoelectric response can be obtained by alloying AlN with Sc. Here, the main challenge lies in the synthesis of highly-oriented thin (Sc,Al)N films on diamond. In this work, we aim at establishing a platform for SAW devices using plasma-assisted molecular beam epitaxy for the deposition of Sc0.2Al0.8N on diamond. We investigate the structural properties related to SAW generation gearing towards applications at high frequencies. To this end, we prepare (Sc,Al)N thin films using plasma-assisted MBE on polished polycrystalline diamond wafers and demonstrate the efficient generation of SAW modes with frequencies up to 8 GHz. Systematic studies of the dependence of the SAW velocity and electromechanical coupling coefficient on the Sc0.2Al0.8N film thickness is presented for various SAW modes. Our result demonstrates the potential of this material combination for future application that requires large bandwidth in the ultra-high frequency range.

Dual-mode Shear Horizontal / Rayleigh-like surface-acoustic-wave configuration on lithium niobate 64° Y-cut

Technologies based on surface acoustic waves (SAWs) find widespread use in wireless communications, signal processing, and sensing applications. In this study, we present an innovative dual-mode SAW configuration using a 64° Y-cut lithium niobate (LN) substrate. The conventional setup, employing interdigital transducers (IDTs) oriented along the crystal X-axis, is known for generating the shear horizontal mode (SH-SAW) with in-plane polarization. Conversely, by utilizing IDTs oriented perpendicular to the X-axis, we introduce effective excitation of a Rayleigh-like SAW (R-SAW), a novel achievement in our research. Through finite element simulations and electro-mechanical analyses, we comprehensively characterize these modes in terms of frequencies, polarizations, propagation losses, and temperature coefficients. Furthermore, we investigate their interaction with liquids by analyzing damping characteristics and acoustic streaming effects using particle image velocimetry (PIV). SH-SAWs exhibit a leaky displacement during propagation with minimal damping in liquids, resulting in weak acoustic streaming—ideal for real-time sensing applications in wet environments. In contrast, non-leaky R-SAWs, characterized by a strong displacement component normal to the surface, exhibit enhanced acoustic streaming, facilitating microfluidic operations. Additionally, R-SAWs demonstrate high sensitivity to mass loading, making them optimal for real-time sensing in dry environments. The ability to generate both SAW modes on the same substrate offers the potential to develop fully electrical-driven, multifunctional integrated sensing platforms with SAW-driven microfluidic capabilities. This unique capability promises novel solutions in bio-sensing, leveraging the complementary strengths of the two acoustic modes in detection and sample handling.

Design and implementation of shear-horizontal mode surface acoustic wave formaldehyde gas sensor

Surface acoustic wave (SAW) sensors have garnered significant attention due to high performance in gas detection. Love wave as the special mode of SAW has greater attractiveness for molecules detection in gas or liquid. To investigate the detection performance of Love wave SAW (Love-SAW) devices towards toxic gases, the influence of interdigitated electrodes on the propagating performance of Rayleigh and shear horizontal (SH) waves along ST-X and ST-90°X quartz substrates was analyzed by finite element method (FEM). The effect of the guiding layer based on different substrates of SAW devices has been discussed by changing the thickness of the zinc oxide (ZnO) layer. The results indicate that the SH wave excited on ST-90°X quartz substrate couples into the Love wave mode through the ZnO guiding layer, and Rayleigh waves in ST-X quartz do not exhibit mode coupling. The Love-SAW sensor covering the ZnO guiding layer (0.5 µm) has shown high mass sensitivity. The vibration distribution of SH wave excited by different thicknesses of electrodes is different, indicating that the mass-loading is one of the reasons for the transformation of SH wave into Love wave. Furthermore, the 50 ppm, 95 ppm and 110 formaldehyde-load leads the short-circuit resonance frequency of the polyetherimide (PEI) covered Love-SAW devices to drop by 100.1 kHz, 200.2 kHz and 300.3 kHz, respectively. The maximum detection sensitivity reached about 2.73 kHz/ppm. This work can provide valuable insights for designing and investigating the SAW formaldehyde gas sensors.

Investigation of spurious mode suppression in 3.8 GHz shear-horizontal mode surface acoustic wave resonators based on LiNbO3/SiO2/Si multilayer structure

LiNbO3(LN)/SiO2/Si multilayer structures have recently attracted much attention due to their superior performance in realizing wideband radio-frequency acoustic filters. However, the spurious modes, which are commonly found in LN-on-insulator (LNOI) resonators, often cause in-band ripples and a deteriorated out-of-band suppression level. Although much research work has been done on the suppression of spurious modes in LNOI resonators operating in the <3 GHz region, little has been reported for >3 GHz devices. This work investigates the spurious mode suppression techniques for a 3.8 GHz shear-horizontal mode 36° YX-LNOI surface acoustic wave (SAW) device based on an LN/SiO2/Si multilayer structure. Specifically, we explore different techniques based on apodized electrodes, dummy electrodes and a double busbar structure. The measured results show that the effect of spurious mode suppression can be improved by combining the benefits of these techniques, which provides a promising solution for designing spurious-free LNOI SAW resonators operating above 3 GHz.

S0-like SAW mode resonator on LiNbO3/SiO2/SiC structure

Abstract This paper discusses the basic properties of a surface acoustic wave (SAW) mode like the zeroth-order symmetric (S0) Lamb mode in the LiNbO3/SiO2/SiC configuration in detail. At first, the influence of the SiO2 insertion is discussed, and it is shown that SiO2 insertion offers significant enhancement of the electromechanical coupling coefficient K 2 for the S0-like SAW mode without significant reduction of the phase velocity. Then, the discussion is extended to spurious resonance characteristics. It is shown that the dummy electrodes are not necessary in this case. Then, two kinds of piston design are applied for the transverse suppression, and their effectiveness is discussed.

Hetero Acoustic Layer Surface Acoustic Wave Resonator Composed of LiNbO3 and Quartz.

In this work, a shear-horizontal (SH) mode surface acoustic wave (SAW) resonator based on LiNbO3 (LN)/Quartz (Qz) hetero acoustic layer (HAL) structure was studied by simulation and experiment. By this HAL structure, the displacement and electric displacement are well confined in the piezoelectric layer. A lower mechanical loss of Qz than that of lossy amorphous SiO2 further enhances the quality ( Q ) factor. In addition, a negative temperature coefficient of frequency (TCF) of LN is compensated by selecting the crystalline orientation of Qz with a positive TCF. According to simulation results, the Euler angles of (0°, 101°, and 0°) and the normalized thickness of 0.2-0.3 λ (wavelength) for LN are selected to obtain a higher impedance ratio ( Z -ratio) and bandwidth (BW). The Euler angles of (0°, 160°, and 90°) for Qz are selected to obtain the positive maximum TCF. The fabricated resonator exhibits a Z -ratio of 95 dB and a BW of 15.9% in the 700 MHz range. The fit figure of merit (FoM) reaches 410, which is the best level ever reported for an LN-based resonator. The TCF of the resonator is -77 ppm/°C at anti-resonance frequency. A group of resonators composed of LN and LN/Qz with thin and thick electrodes were fabricated to further illustrate the good performance of LN/Qz. The LN/Qz HAL SAW resonator demonstrated in this work exhibits a high Z -ratio, low TCF, and wideband, which has the potential for high-performance wideband filters with steep passband and good temperature characteristics.

Investigation of acoustoelastic surface acoustic waves in prestressed media

Dispersion of broadband surface acoustic waves (SAWs) is often used for the nondestructive evaluation of near-surface elastic properties of materials. The accurate forward model of SAW propagation is essential for inversion. In this paper, we present a combining semi-analytical finite element (SAFE) and perfectly matched layer method for accurately predicting the phase velocity dispersion, attenuation and polarization modes of SAWs in any prestressed anisotropic layered half-space. Equations of motion are formulated based on the Biot's theory of small deformations influenced by initial stress, so it does not matter whether the residual stress results from hyperelastic transformations. The simulated results are firstly validated by the solutions from the exact three-dimensional elasticity theory-based method. The SAFE method is then used to discuss the effect of material anisotropy and residual stress on the SAWs. Finally, the method is applied to two practical examples. The differences and similarities of two common types of acoustoelastic theories are also discussed.

Mode Characterization and Sensitivity Evaluation of a Surface Acoustic Wave (SAW) Resonator Biosensor: Application to the Glial-Fibrillary-Acidic-Protein (GFAP) Biomarker Detection.

Biosensors based on surface acoustic waves (SAWs) offer unique advantages due to their high sensitivity, real-time response capability, and label-free detection. The typical SAW modes are the Rayleigh mode and the shear-horizontal mode. Both present pros and cons for biosensing applications and generally need different substrates and device geometries to be efficiently generated. This study investigates and characterizes SAW resonator biosensors on lithium niobate in terms of modes generated and biosensing performance. It reveals the simultaneous presence of two typical SAW modes, the first around 1.6 GHz and the second around 1.9 GHz, differently polarized and clearly separated in frequency, which we refer to as slow and fast modes. The two modes are studied by numerical simulations and biosensing experiments with the glial-fibrillary-acidic-protein (GFAP) biomarker. The slow mode is generally more sensitive to changes in surface properties, such as temperature and mass changes, by a factor of about 1.4 with respect to the fast mode.

Observation of enhanced dynamic ΔG effect near ferromagnetic resonance frequency

The field-dependence elastic modulus of magnetostrictive films, also called ΔE or ΔG effect, is crucial for ultrasensitive magnetic field sensors based on surface acoustic waves (SAWs). In spite of a lot of demonstrations, rare attention was paid to the frequency-dependence of ΔE or ΔG effect. In this work, shear horizontal-type SAW delay lines coated with a thin FeCoSiB layer have been studied at various frequencies upon applying magnetic fields. The change of shear modulus of FeCoSiB has been extracted by measuring the field dependent phase shift of SAWs. It is found that the ΔG effect is significantly enhanced at high-order harmonic frequencies close to the ferromagnetic resonance frequency, increasing by ∼82% compared to that at the first SAW mode (128 MHz). In addition, the smaller the effective damping factor of a magnetostrictive layer, the more pronounced ΔG effect can be obtained, which is explained by our proposed dynamic magnetoelastic coupling model.

Phonon–magnon conversion using longitudinal leaky surface acoustic waves through magnetoelastic coupling

Surface acoustic waves (SAWs) can transmit magnetization oscillation inside magnetoelastic films in the form of magnetoacoustic waves. If the frequency and wavenumber of SAWs match those of spin waves, magnon excitation can be observed. In this work, we studied the phonon–magnon conversion excited by longitudinal leaky SAWs, which possess the same dominant strain component like Rayleigh-type SAWs but a much higher phase velocity. The measured transmission power absorptions of both SAW modes due to spin wave resonance follow a linear frequency dependence and exhibit a fourfold symmetry. We demonstrate that longitudinal leaky SAWs can serve as a very effective means to excite stronger phonon–magnon coupling than Rayleigh SAWs at a lower wave number.

Modulating the Performance of the SAW Strain Sensor Based on Dual-Port Resonator Using FEM Simulation.

Surface acoustic wave (SAW) strain sensors fabricated on piezoelectric substrates have attracted considerable attention due to their attractive features such as passive wireless sensing ability, simple signal processing, high sensitivity, compact size and robustness. To meet the needs of various functioning situations, it is desirable to identify the factors that affect the performance of the SAW devices. In this work, we perform a simulation study on Rayleigh surface acoustic wave (RSAW) based on a stacked Al/LiNbO3 system. A SAW strain sensor with a dual-port resonator was modeled using multiphysics finite element model (FEM) method. While FEM has been widely used for numerical calculations of SAW devices, most of the simulation works mainly focus on SAW modes, SAW propagation characteristics and electromechanical coupling coefficients. Herein, we propose a systematic scheme via analyzing the structural parameters of SAW resonators. Evolution of RSAW eigenfrequency, insertion loss (IL), quality factor (Q) and strain transfer rate with different structural parameters are elaborated by FEM simulations. Compared with the reported experimental results, the relative errors of RSAW eigenfrequency and IL are about 3% and 16.3%, respectively, and the absolute errors are 5.8 MHz and 1.63 dB (the corresponding Vout/Vin is only 6.6%). After structural optimization, the obtained resonator Q increases by 15%, IL decreases by 34.6% and the strain transfer rate increases by 2.4%. This work provides a systematic and reliable solution for the structural optimization of dual-port SAW resonators.

A near spurious-free 6 GHz LLSAW resonator with large electromechanical coupling on X-cut LiNbO3/SiC bilayer substrate

The fast development of the fifth-generation (5G) wireless systems and substantial growth of data usage have imposed stringent requirements for high-frequency and wideband radio frequency devices. Here, it is reported on a longitudinal leaky surface acoustic wave (LLSAW) mode acoustic resonator with a large electromechanical coupling factor (kt2), high operating frequency, and efficient spurious suppression. Through systematical finite element method simulations, available design spaces such as supporting substrate, propagation angle, and lithium niobate (LN) thickness have been fully investigated with the aim of stimulating the intended LLSAW and suppressing spurious modes concurrently. Optimization results reveal that the LLSAW mode wave propagating in X-35°Y LN/SiC piezoelectric-on-insulator (POI) bilayer structure possesses a large kt2 without significant interference from other spurious modes. To verify the theoretical analyses, LLSAW resonators were fabricated and exhibited a near spurious-free response with the operating frequency over 6 GHz, and kt2 as large as 22.7%. This work demonstrates a high-performance LLSAW resonator on the POI platform with a simple prototype as well as potentially providing a high-frequency filtering solution for 5G applications in the 6-GHz spectrum.

On-Chip Unidirectional Waveguiding for Surface Acoustic Waves along a Defect Line in a Triangular Lattice

The latest advances in topological physics have yielded a toolset for highly robust wave-propagation modalities for overcoming obstacles involving beam steering and lateral diffraction in surface acoustic waves (SAWs). However, extant proposals are limited to the exploitation of spin- or valley-polarized phases and rely on nonzero Berry curvature effects. Here, we propose and experimentally demonstrate a highly robust guiding principle, which instead employs an intrinsic chirality of phase vortices and maintains a zero Berry curvature for SAWs. Based on a line defect within a true triangular phononic lattice, the guided SAW mode spans a wide bandwidth [(\ensuremath{\Delta}\ensuremath{\omega}/\ensuremath{\omega}${}_{\mathrm{center}}$) \ensuremath{\sim} 10%] and is well confined in the lateral direction with 3-dB attenuation within half of a unit cell. SAW routing around sharp bends with negligible backscatter is demonstrated. The on-chip integrated design permits unidirectional SAW modes that can enable considerable miniaturization of SAW-based devices, with applications ranging from radio-frequency devices to quantum information transduction.

Magnetic skyrmion dynamics induced by surface acoustic waves

Magnetic skyrmions are promising information carriers for high-density memories. The dynamical motion of magnetic skyrmions have been extensively investigated in the development of magnetic racetracks. In this study, a surface acoustic wave (SAW) is theoretically investigated to drive skyrmions by using micromagnetic simulations. The in-plane type and out-of-plane particle displacement components of SAWs generate different magnetoelastic effective fields. The shear horizontal (SH) wave mode SAW drives skyrmions flow movement by the magnetoelastic coupling effect. Increasing the acoustic wave amplitude and magnetoelastic coupling constants, as well as a reduced wavelength, are beneficial for an enhanced skyrmion motion velocity. The skyrmion motion trajectory can be controlled by designing the geometry of magnetic films. Interestingly, in a circular magnetic film, the skyrmions driven by SH waves show clockwise or counterclockwise movement trajectories depending on the sign of topological charges. Our results provide an energy efficient approach to drive skyrmion dynamics including rotational motion, thus paving the way for low-power spintronics.

Highly Sensitive Love Mode Acoustic Wave Platform with SiO2 Wave-Guiding Layer and Gold Nanoparticles for Detection of Carcinoembryonic Antigens.

A highly sensitive and precise Love wave mode surface acoustic wave (SAW) immunosensor based on an ST-cut 90°X quartz substrate and an SiO2 wave-guiding layer was developed to detect cancer-related biomarkers of carcinoembryonic antigens (CEAs). A delay line structure of the SAW device with a resonant frequency of 196 MHz was designed/fabricated, and its surface was functionalized through CEA antibody immobilization. The CEA antibodies were bound with gold nanoparticles and CEA antibodies to form a sandwich structure, which significantly amplified the mass loading effect and enhanced the maximum responses by 30 times. The center frequency of the Love wave immunosensor showed a linear response as a function of the CEA concentration in the range of 0.2–5 ng/mL. It showed a limit of detection of 0.2 ng/mL, and its coefficient of determination was 0.983. The sensor also showed minimal interference from nonspecific adsorptions, thus demonstrating its promise for point-of-care applications for cancer biomarkers.

Highly efficient acousto-optic modulation using nonsuspended thin-film lithium niobate-chalcogenide hybrid waveguides

A highly efficient on-chip acousto-optic modulator is as a key component and occupies an exceptional position in microwave-to-optical conversion. Homogeneous thin-film lithium niobate is preferentially employed to build the suspended configuration for the acoustic resonant cavity, with the aim of improving the modulation efficiency of the device. However, the limited cavity length and complex fabrication recipe of the suspended prototype restrain further breakthroughs in modulation efficiency and impose challenges for waveguide fabrication. In this work, based on a nonsuspended thin-film lithium niobate-chalcogenide glass hybrid Mach–Zehnder interferometer waveguide platform, we propose and demonstrate a built-in push-pull acousto-optic modulator with a half-wave-voltage-length product VπL as low as 0.03 V cm that presents a modulation efficiency comparable to that of a state-of-the-art suspended counterpart. A microwave modulation link is demonstrated using our developed built-in push-pull acousto-optic modulator, which has the advantage of low power consumption. The nontrivial acousto-optic modulation performance benefits from the superior photoelastic property of the chalcogenide membrane and the completely bidirectional participation of the antisymmetric Rayleigh surface acoustic wave mode excited by the impedance-matched interdigital transducer, overcoming the issue of low modulation efficiency induced by the incoordinate energy attenuation of acoustic waves applied to the Mach–Zehnder interferometer with two arms in traditional push-pull acousto-optic modulators.

Properties of higher-order surface acoustic wave modes in Al1−xScxN/sapphire structures

In this work, surface acoustic wave (SAW) modes and their dependence on propagation directions in epitaxial Al0.68Sc0.32N(0001) films on Al2O3(0001) substrates were studied using numerical and experimental methods. In order to find optimal propagation directions for higher-order SAW modes, phase velocity dispersion branches of Al0.68Sc0.32N on Al2O3 with Pt mass loading were computed for the propagation directions &amp;lt;112¯0&amp;gt; and &amp;lt;11¯00&amp;gt; with respect to the substrate. Experimental investigations of phase velocities and electromechanical coupling were performed for comparison with the numerical results. Simulations carried out with the finite element method and a Green function approach allowed identification of each wave type, including Rayleigh, Sezawa, and shear-horizontal wave modes. For the propagation direction &amp;lt;11¯00&amp;gt;, significantly increased wave guidance of the Sezawa mode compared to other directions was observed, resulting in enhanced electromechanical coupling (keff2=1.6%) and phase velocity (vphase=6km/s). We demonstrated that selecting wave propagation in &amp;lt;11¯00&amp;gt; with high mass density electrodes results in increased electromechanical coupling without significant reduction in phase velocities for the Sezawa wave mode. An improved combination of metallization, Sc concentration x, and SAW propagation direction is suggested that exhibits both high electromechanical coupling (keff2&amp;gt;6%) and high velocity (vphase=5.5km/s) for the Sezawa mode.

Analysis of Dielectric Waveguide Grating and Fabry-Perot Modes in Elastic Grating in Optical Detection of Ultrasound.

In our previous work, we have demonstrated that dielectric elastic grating can support Fabry–Perot modes and provide embedded optical interferometry to measure ultrasonic pressure. The Fabry–Perot modes inside the grating provide an enhancement in sensitivity and figure of merit compared to thin film-based Fabry–Perot structures. Here, in this paper, we propose a theoretical framework to explain that the elastic grating also supports dielectric waveguide grating mode, in which optical grating parameters control the excitation of the two modes. The optical properties of the two modes, including coupling conditions and loss mechanisms, are discussed. The proposed grating has the grating period in micron scale, which is shorter than the wavelength of the incident ultrasound leading to an ultrasonic scattering. The gap regions in the grating allow the elastic grating thickness to be compressed by the incident ultrasound and coupled to a surface acoustic wave mode. The thickness compression can be measured using an embedded interferometer through one of the optical guided modes. The dielectric waveguide grating is a narrow bandpass optical filter enabling an ultrasensitive mode to sense changes in optical displacement. This enhancement in mechanical and optical properties gives rise to a broader detectable pressure range and figure of merit in ultrasonic detection; the detectable pressure range and figure of merit can be enhanced by 2.7 times and 23 times, respectively, compared to conventional Fabry–Perot structures.

Analysis of Embedded Optical Interferometry in Transparent Elastic Grating for Optical Detection of Ultrasonic Waves.

In this paper, we propose a theoretical framework to explain how the transparent elastic grating structure can be employed to enhance the mechanical and optical properties for ultrasonic detection. Incident ultrasonic waves can compress the flexible material, where the change in thickness of the elastic film can be measured through an optical interferometer. Herein, the polydimethylsiloxane (PDMS) was employed in the design of a thin film grating pattern. The PDMS grating with the grating period shorter than the ultrasound wavelength allowed the ultrasound to be coupled into surface acoustic wave (SAW) mode. The grating gaps provided spaces for the PDMS grating to be compressed when the ultrasound illuminated on it. This grating pattern can provide an embedded thin film based optical interferometer through Fabry–Perot resonant modes. Several optical thin film-based technologies for ultrasonic detection were compared. The proposed elastic grating gave rise to higher sensitivity to ultrasonic detection than a surface plasmon resonance-based sensor, a uniform PDMS thin film, a PDMS sensor with shearing interference, and a conventional Fabry–Perot-based sensor. The PDMS grating achieved the enhancement of sensitivity up to 1.3 × 10−5 Pa−1 and figure of merit of 1.4 × 10−5 Pa−1 which were higher than those of conventional Fabry–Perot structure by 7 times and 4 times, respectively.

High quality and low loss surface acoustic wave SAW resonator based on chromium-doped AlN on sapphire

This paper aims to investigate the performances of surface acoustic wave (SAW) resonators based on chromium-doped aluminum nitride (AlCrN) piezoelectric thin film on c-cut sapphire substrate. The electromechanical properties of AlCrN (mass density, elastic, dielectric and piezoelectric constants) are obtained from density functional theory calculations for different Cr-doping concentrations. Piezoelectricity is enhanced for Al1 − xCrxN with different x concentrations. Using finite element analysis, the electroacoustic parameters such as phase velocity, electromechanical coupling factor K2 of surface acoustic modes are determined for different AlCrN thickness-to-wavelength ratios and different Cr-doping concentrations. Higher order SAW modes arise at Cr concentration x = 25%. The electrical admittance and scattering parameter S21 for the fundamental SAW modes are determined. High K2, low loss and high-quality factor are found after incorporating Cr dopant. The comparison of the obtained results with the available numerical and experimental data on AlN doped with Scandium Sc has shown a considerable improvement of SAW devices characteristics, confirming that AlCrN can be adopted as a new piezoelectric material for high performances SAW devices.