Surface Acoustic Wave Propagation Research Articles

Tunable surface acoustic wave device using semiconducting MgZnO and piezoelectric NiZnO dual-layer structure on glass

Tunable surface acoustic wave (SAW) devices have attracted considerable interests due to its applications in emerging fields, such as secured wireless communication, adaptive signal processing, and smart sensing systems. We report a dual-input-voltage-controlled tunable SAW built on glass, which uses a ZnO-based dual-layer structure consisting of a piezoelectric Ni-doped ZnO (NZO) and semiconducting Mg-doped ZnO (MZO). The interdigital (IDT) electrodes are buried in the piezoelectric NZO layer to form the delay line for SAW propagation, while the semiconductor MZO layer serves as the channel of a thin film transistor (TFT) to modulate the conductivity. Results show the interface between MZO channel and SiO2 gate dielectric layer of the TFT significantly impacts on SAW tuning performances due to Zn diffusion from MZO into SiO2. The TFT-SAW device using an ultra-thin MgO layer as interface modification enables SAW frequency tuning of 0.53% under solely Vgs control with 0–12 V. The required voltage range is significantly reduced compared regular MZO-NZO TFT-SAW counterpart (Δf/fc ∼ 0.25%; Vgs −30 to −14 V) without interface modification. With additional control of Vds, the SAW frequency tunability is further expanded from 0.46% to 0.63%. This dual-input voltage-controlled frequency tuning device on glass is promising for low-voltage, low-cost portable smart sensors and voltage-controlled reconfigurable radio-frequency identification tags.

Three-dimensional finite element simulation of Love mode surface acoustic wave in layered structures including ZnO piezoelectric film and diamond substrate

Three-dimensional (3D) finite element method (FEM) was used to simulate the surface acoustic wave (SAW) propagation properties in layered structures including ZnO piezoelectric film and diamond substrate. The excitation conditions of Love mode and Rayleigh mode SAWs are discussed considering the ZnO crystal orientation and the SAW propagation direction which are specified using Euler angles (α, β, γ). The phase velocity and electromechanical coupling coefficient K2 of Love mode SAW in the ZnO/diamond structure are provided as functions of the normalized ZnO layer thickness hZnO/λ and the Euler angles (α, β, γ). The results show that the maximum K2 of 4.26% is reached at hZnO/λ = 0.3 and (α, β, γ) = (0°, 90°, 0°) for Love mode SAW; the associated phase velocity is 3436 m/s and the temperature coefficient of frequency (TCF) value is −26.5 ppm/°C. The TCF properties are greatly improved by adding a SiO2 layer between the ZnO film and the diamond substrate. A zero TCF can be obtained when the normalized SiO2 thickness hSiO2/λ is 0.25 and the hZnO/λ is 0.2, where the corresponding K2 is 3.26% and the phase velocity is 3126 m/s.

Finite element simulation of one-port surface acoustic wave resonator with thick interdigital transducer for gas sensing

The paper presents finite element simulation of a surface acoustic wave (SAW) one-port resonator with thick interdigital transducers (IDT) made of nickel metal for sensing dimethyl methylphosphonate (DMMP) gas by placing fluoroalcoholpolysiloxane (SXFA) sensing film in the space between thick IDT fingers. The absorption of DMMP gas changes density and thickness of SXFA film and affect the SAW propagation velocity, hence resonance frequency of the device. The change in resonance frequency is measured as sensor response. In this paper individual effects of change in density and thickness of SXFA film due to absorption of DMMP gas on sensor response are simulated. The results show that the change in thickness of the SXFA film placed in the space between thick IDT fingers of proposed device is dominant in sensor response and the sensitivity of the proposed device (2.1 kHz/mg/m3) is about 3.2 times greater than a conventional device using SXFA coated over the entire top surface of the device.

Propagation properties of leaky surface acoustic wave on water-loaded piezoelectric substrate

To evaluate acoustical loss mainly due to Rayleigh scattering loss of a thin film material using a line-focus-beam ultrasonic material characterization system, surface acoustic waves (SAWs) were excited and detected using IDTs, the propagation properties of SAWs with and without water loading were measured, and the acoustical losses of amorphous SiO2 and Ta2O5 thin films deposited on 128°YX-LiNbO3 were evaluated by subtracting the calculated attenuation of leakage loss into water from the measured propagation loss of leaky SAWs (LSAWs) on water-loaded specimens. The results of the evaluation of acoustical losses using LSAWs with the SAW propagation mode of the small leakage loss were in good agreement with the measured propagation loss of the Rayleigh-type SAW corresponding to the acoustical loss.

Generation of multiband spoof surface acoustic waves via high-order modes

Through theoretical analysis and numerical calculation, it is found that the textured surface of a perfect rigid body whose groove height is sufficiently large can support multiple modes of propagating spoof surface acoustic waves (SSAWs). Dispersion curves and group velocities/refractive index versus height and width of the groove are depicted for infinitely thick perfect rigid body to make a thoroughly inquiry into the multiband transmission of SSAWs. We also give the dispersion relations of samples with finite thickness which are more practical to realize, and the corresponding experimental results are in accordance with simulated results. Theoretical, simulated, and measured acoustic pressure field distributions are given to further illustrate the characteristics of different modes. Our work could have implications for applications such as acoustic filter design and energy harvesting.

Nonreciprocal Surface Acoustic Waves in Multilayers with Magnetoelastic and Interfacial Dzyaloshinskii-Moriya Interactions

The design of modern circulators and isolators relies on nonreciprocal (discriminating forward and backward wave vectors, breaking time-reversal symmetry) electromagnetic waves in magnetic materials. Creating such devices that are small enough for portable electronics (think mobile phones) remains challenging. The authors show theoretically the nonreciprocal propagation of surface acoustic waves in multilayers with simultaneous magnetoelastic and Dzyaloshinskii-Moriya interactions. Despite both interactions being relatively weak, their interplay creates up to 45 dB of isolation for counterpropagating waves, making these multilayers promising for miniaturized rf isolators.

Magnetoelastic excitation of single nanomagnets for optical measurement of intrinsic Gilbert damping

We report an alternative all-optical technique to drive and probe the spin dynamics of single nanomagnets. Optically generated surface acoustic waves (SAWs) drive the magnetization precession in nanomagnets via magnetoelastic (MEL) coupling. We investigate the field-swept dynamics of isolated Ni nanomagnets at various SAW frequencies and show that this method can be used to accurately determine the intrinsic Gilbert damping of nanostructured magnetic materials. This technique opens a new avenue for studying the spin dynamics of nanoscale devices using nonthermal (``cold'') excitation, enabling direct observation of the MEL driven dynamics.

Sezawa wave acoustic humidity sensor based on graphene oxide sensitive film with enhanced sensitivity

The measurement of humidity is very important for air control in ambient, industry, cars, houses, closed apartments, museums, atomic power stations, etc. In the present work the theoretical analysis of the surface acoustic wave propagation in “graphen oxide (GO) film/ZnO film/Si substrate” layered structure has been performed. The change of GO film conductivity due to humidity has been taken into account during the calculations. Based on the obtained results an improved microwave acoustic humidity sensor has been developed. The sensor has enhanced sensitivity of about 91 kHz/% and linear response vs relative humidity in the range 20–98%RH. It is based on the mode belonging to Sezawa wave family that is shown to be more sensitive towards electric conductivity variations in GO film produced by adsorbed water molecules than the Rayleigh counterpart.

Hybridization of Guided Surface Acoustic Modes in Unconsolidated Granular Media by a Resonant Metasurface

Elastic metasurfaces can manipulate the propagation of surface acoustic waves (SAWs) for applications as wave filters, including barriers for seismic surface waves---earthquake protection, that is. Efforts so far have focused on homogenous media, in crude approximation of the real world. The authors study the dynamics of an elastic metasurface in an unconsolidated granular medium, which presents an inhomogeneous stiffness profile. The metasurface's resonance hybridizes the lowest-order SAW and down-converts all higher-order SAWs, preventing the surface-wave delocalization seen in homogeneous media. These findings could impact protective designs for inhomogeneous stratified soils.

Gas sensors based on elasticity changes of nanoparticle layers

In this paper, propagation of shear horizontal surface acoustic waves (SH-SAW) in nanoparticle layers was studied by means of dispersion acoustic theory in waveguides. These studies have allowed to obtainment of properties of nanoparticles layers, such as shear stiffness modulus. In addition, the numerical analysis of multi-guiding layers have allowed for the design of an innovative, simple and inexpensive gas sensor based on elastic properties variation of the nanoparticles layers due to their interaction with gases. Each sensor has been prepared by coating a uniform layer of nanostructured indium tin oxide (ITO) nanoparticle layer on a piezoelectric material (quartz), working as a guiding layer by confining SH-SAW energy and obtaining a Love-wave sensor. The perturbation produced in the elastic properties of the nanoparticle layer due to its interaction with gases induced a change in wave velocity that was detected by the frequency shift of an oscillator, working as a sensitive layer. Therefore, the Love-wave sensor was optimized with multi-guiding layers containing an intermediate guiding layer of SiO2. The Love-wave multi-guiding layer sensor was tested to different concentrations of toluene and benzene, measurements showed high sensitivity, short response time, and good reproducibility, and ability to detect very low concentrations of test gases, such 1 ppm of toluene and 25 ppm of benzene.

SAW propagation characteristics of TeO3/3C-SiC/LiNbO3 layered structure

Surface acoustic wave (SAW) devices based on Lithium Niobate (LiNbO3) single crystal are advantageous because of its high SAW phase velocity, electromechanical coupling coefficient and cost effectiveness. In the present work a new multi-layered TeO3/3C-SiC/128° Y-X LiNbO3 SAW device has been proposed. SAW propagation properties such as phase velocity, coupling coefficient and temperature coefficient of delay (TCD) of the TeO3/SiC/128° Y-X LiNbO3 multi layered structure is examined using theoretical calculations. It is found that the integration of 0.09λ thick 3C-SiC over layer on 128° Y-X LiNbO3 increases its electromechanical coupling coefficient from 5.3% to 9.77% and SAW velocity from 3800 ms−1 to 4394 ms−1. The SiC/128° Y-X LiNbO3 bilayer SAW structure exhibits a high positive TCD value. A temperature stable layered SAW device could be obtained with introduction of 0.007λ TeO3 over layer on SiC/128° Y-X LiNbO3 bilayer structure without sacrificing the efficiency of the device. The proposed TeO3/3C-SiC/128° Y-X LiNbO3 multi-layered SAW structure is found to be cost effective, efficient, temperature stable and suitable for high frequency application in harsh environment.

Optimization of interdigital transducers for the generation of surface acoustic waves over a large bandwidth (20–125 MHz)

In this paper, wideband interdigital transducers (IDT) to excite surface acoustic waves (SAW) have been developed and optimized. Three spatial chirp configurations were studied in order to cover a large bandwidth ranging from 20 to 125 MHz with sufficiently large displacement amplitudes to enable the characterization of surfaces (structures, thin layers, coatings, etc.). It is shown that by optimizing the configuration of the spatial distribution (width and spacing) of the electrodes, it is possible to adapt the spectral distribution of the SAW generated and to obtain a desired spectrum over the entire range of over 100 MHz. Finally, it is shown that high frequency attenuation of SAW can be overcome thanks to an additional optimization of the IDT by emitting more energy at high frequencies. It is thus possible with these transducers to measure displacements of the surface acoustic waves propagating along distances greater than 20 mm.

Development of a dual temporal-spatial chirp method for the generation of broadband surface acoustic waves

This study deals with the optimization of transducers for Rayleigh-type Surface Acoustic Waves (SAW) generation. These transducers are based on Interdigital Transducers (IDT) and are specifically developed to characterize properties of thin layers, coatings, and functional surfaces. In order to characterize these coatings and structures, it is necessary to work with wide bandwidths IDT operating in the high frequencies. Therefore, in this study a spatial chirp-based on an IDT are realized for wideband SAW generation. The use of impulse temporal excitation (Dirac-type negative pulse) leads to a wide band emitter excitation but with significantly limited SAW output amplitudes due to the piezoelectric crystal breakdown voltage. This limitation can be circumvented by applying a temporal chirp excitation corresponding in terms of frequency band and duration to the spatial chirp transducer configuration. This dual temporal-spatial chirp method was studied in the 20 to 125 MHz frequency range and allowed to obtain higher SAW displacement amplitudes with an excitation voltage lower than that of the impulse excitation.

Theoretical analysis of a surface acoustic wave gas sensor mechanism using electrical conductive bi-layer nanostructures

A theoretical investigation of the surface acoustic wave (SAW) propagation on a piezoelectric substrate was performed. The elemental theory of acoustoelectrical interactions between SAWs and bi-layer sensor nanostructures is outlined. The analysis of the SAW sensing mechanism was conducted on the basis of the velocity and attenuation changes. Three varying configurations of bi-layer nanostructures: semiconductor – metal, metal – semiconductor, and dielectric – metal, have been considered. The calculations were executed for three different thicknesses of the film layered on the substrate: 50, 250 and 750 nm. Additionally, dispersion effects, i.e. the influence of various SAW wavelengths (8, 80 and 800 μm) for sensing mechanism is discussed. The experimentally achieved data of surface electrical conductivities, construction parameters and acoustoelectrical parameters for the selected single- and bi-layer sensor structures is also presented.

Wet-etched phononic crystal waveguiding on GaAs

A wet-etched phononic crystal waveguide in GaAs with approximately two micron deep inclusions is studied both numerically and experimentally for controlled surface acoustic wave propagation. Numerically, the phononic crystal was modelled using the finite element method (FEM) with COMSOL Multiphysics, and the surface displacement of the acoustic waves was measured using optical interferometry. The computed filter response of the phononic crystal confirmed that the phononic crystal was an effective stop band filter in the interval of 400 MHz and 450 MHz. An L1 linear defect waveguide with a stepped funnel entrance design is shown to perform well at a surface acoustic wave frequency of 410.344 MHz and in agreement to simulated results. The phononic crystal waveguide system shows promise for use in acoustic control of GaAs-based quantum nanostructures.

Multiharmonic Frequency-Chirped Transducers for Surface-Acoustic-Wave Optomechanics

Wide passband interdigital transducers are employed to establish a stable phase-lock between a train of laser pulses emitted by a mode-locked laser and a surface acoustic wave generated electrically by the transducer. The transducer design is based on a multi-harmonic split-finger architecture for the excitation of a fundamental surface acoustic wave and a discrete number of its overtones. Simply by introducing a variation of the transducer's periodicity $p$, a frequency chirp is added. This combination results in wide frequency bands for each harmonic. The transducer's conversion efficiency from the electrical to the acoustic domain was characterized optomechanically using single quantum dots acting as nanoscale pressure sensors. The ability to generate surface acoustic waves over a wide band of frequencies enables advanced acousto-optic spectroscopy using mode-locked lasers with fixed repetition rate. Stable phase-locking between the electrically generated acoustic wave and the train of laser pulses was confirmed by performing stroboscopic spectroscopy on a single quantum dot at a frequency of 320 MHz. Finally, the dynamic spectral modulation of the quantum dot was directly monitored in the time domain combining stable phase-locked optical excitation and time-correlated single photon counting. The demonstrated scheme will be particularly useful for the experimental implementation of surface acoustic wave-driven quantum gates of optically addressable qubits or collective quantum states or for multi-component Fourier synthesis of tailored nanomechanical waveforms.

Grating-patterned FeCo coated surface acoustic wave device for sensing magnetic field

This study addresses the theoretical and experimental investigations of grating-patterned magnetostrictive FeCo coated surface acoustic wave (SAW) device for sensing magnetic field. The proposed sensor is composed of a configuration of differential dual-delay-line oscillators, and a magnetostrictive FeCo grating array deposited along the SAW propagation path of the sensing device, which suppresses effectively the hysteresis effect by releasing the internal binding force in FeCo. The magnetostrictive strain and ΔE effect from the FeCo coating modulates the SAW propagation characteristic, and the corresponding shift in differential oscillation frequency was utilized to evaluate the measurant. A theoretical model is performed to investigate the wave propagation in layered structure of FeCo/LiNbO3 in the effect of magnetostrictive, and allowing determining the optimal structure. The experimental results indicate that higher sensitivity, excellent linearity, and lower hysteresis error over the typical FeCo thin-film coated sensor were achieved from the grating-patterned FeCo coated sensor successfully.

Time-domain imaging of gigahertz surface waves on an acoustic metamaterial

We extend time-domain imaging in acoustic metamaterials to gigahertz frequencies. Using a sample consisting of a regular array of ∼1 μm diameter silica microspheres forming a two-dimensional triangular lattice on a substrate, we implement an ultrafast technique to probe surface acoustic wave propagation inside the metamaterial area and incident on the metamaterial from a region containing no microspheres, which reveals the acoustic metamaterial dispersion, the presence of band gaps and the acoustic transmission properties of the interface. A theoretical model of this locally resonant metamaterial based on normal and shear-rotational resonances of the spheres fits the data well. Using this model, we show analytically how the sphere elastic coupling parameters influence the gap widths.

Residual stress effect on coupling electromechanical factor of epitaxial Barium Strontium Titanate (BST) thin films

Acoustoelastic (AE) effect of a mechanically pre-stressed layered piezoelectric structure of BST is investigated. Two samples of BST (Ba0.8Sr0.2TiO3) thin films were grown by rf-magnetron sputtering deposition techniques on a Pt(100)/MgO(100) and Pt(110)/MgO(110) substrates. The crystallographic orientation of BST thin films is analyzed by X-ray diffraction (XRD). A laser acoustic waves (LA-waves) technique is used to generate surface acoustic waves (SAW) propagating in both samples for epitaxial relationship examination. We perform an extended model to determine the residual stress and strain in BST films using the (XRD) X-ray diffraction measurement and the piezoelectric constitutive equations. Theoretical analysis of (SAW) propagation in a pre-stressed layered piezoelectric structure is achieved to discuss the effect of the measured residual stresses of BST films on the propagation behavior of Rayleigh waves and on the coupled electromechanical factor.

Non-leaky modes and bandgaps of surface acoustic waves in wrinkled stiff-film/compliant-substrate bilayers

Surface acoustic wave (SAW) devices have found a wide variety of technical applications, including SAW filters, SAW resonators, microfluidic actuators, biosensors, flow measurement devices, and seismic wave shields. Stretchable/flexible electronic devices, such as sensory skins for robotics, structural health monitors, and wearable communication devices, have received considerable attention across different disciplines. Flexible SAW devices are essential building blocks for these applications, wherein piezoelectric films may need to be integrated with the compliant substrates. When piezoelectric films are much stiffer than soft substrates, SAWs are usually leaky and the devices incorporating them suffer from acoustic losses. In this study, the propagation of SAWs in a wrinkled bilayer system is investigated, and our analysis shows that non-leaky modes can be achieved by engineering stress patterns through surface wrinkles in the system. Our analysis also uncovers intriguing bandgaps (BGs) related to the SAWs in a wrinkled bilayer system; these are caused by periodic deformation patterns, which indicate that diverse wrinkling patterns could be used as metasurfaces for controlling the propagation of SAWs.