Coupling Of Surface Acoustic Waves Research Articles

Interaction of acoustic waves with spin waves using a GHz operating GaN/Si SAW device with a Ni/NiFeSi layer between its IDTs.

A two port surface acoustic wave (SAW) device was developed to be used for the control and excitation via spin waves (SW). The structure was manufactured using advanced nanolithography techniques, on GaN/Si, enabling fundamental Rayleigh interdigitated transducer (IDT) resonances in GHz frequency range. The ferromagnetic resonance of the magnetostrictive Ni/NiFeSi layer placed between the IDTs of the SAW device can be tuned to the SAW resonance frequency by magnetic fields. Using structures with finger and interdigit spacing of 170 nm and 100 nm, fundamental Rayleigh IDT resonance frequencies of 6.4 and 10.4 GHz have been obtained. Coupling of SAW to SW was demonstrated through transmission measurements at the fundamental Rayleigh frequencies in a magnetic field, μ0H from -280 to +280 mT, at different angles (θ) between the SAW propagation direction and the magnetic field direction. For the 6.4 GHz resonator a maximum decrease of about 1.2 dB occurred in |S21|, at μ0H = 30 mT and at θ = 45. Time-gated processing of the frequency domain raw data was used to remove the direct electromagnetic cross talk and triple transit effects. Nonreciprocity associated to the coupling was analyzed for the two SAW structures. The quantitative influence of the magnetic field strength on the phase of the transmission parameters is also presented.

Swirl-like Acoustofluidic Stirring Facilitates Microscale Reactions in Sessile Droplets.

Sessile droplets play a crucial role in the microreactors of biochemical samples. Acoustofluidics provide a non-contact and label-free method for manipulating particles, cells, and chemical analytes in droplets. In the present study, we propose a micro-stirring application based on acoustic swirls in sessile droplets. The acoustic swirls are formed inside the droplets by asymmetric coupling of surface acoustic waves (SAWs). With the merits of the slanted design of the interdigital electrode, the excitation position of SAWs is selective by sweeping in wide frequency ranges, allowing for the droplet position to be customized within the aperture region. We verify the reasonable existence of acoustic swirls in sessile droplets by a combination of simulations and experiments. The different periphery of the droplet meeting with SAWs will produce acoustic streaming phenomena with different intensities. The experiments demonstrate that acoustic swirls formed after SAWs encountering droplet boundaries will be more obvious. The acoustic swirls have strong stirring abilities to rapidly dissolve the yeast cell powder granules. Therefore, acoustic swirls are expected to be an effective means for rapid stirring of biomolecules and chemicals, providing a new approach to micro-stirring in biomedicine and chemistry.

A Composite Gas-Phase Explosives Sensor with Frequency-Resistance Dual-Signal Display Based on Surface Acoustic Wave

The surface acoustic wave (SAW) technology has been widely used for hazardous gas detection, but the low sensitivity, poor selectivity, long response/recovery time, and signal “false positives” of the sensors have limited its application for gas-phase explosives detection. In this paper, a composite gas-phase explosives sensor with frequency-resistance dual-signal display was designed by transferring 25 layers of graphene (rGO) in the sensing area of the SAW device and coupling p-aminothiophenol (PATP)-modified gold nanoparticles (AuNPs). The rGO:AuNP/PATP-SAW sensor shows signal response from two signal dimensions, frequency and resistance, to 0.1–80 ppm TNT gas with a low detection limit (DL) of 41.2 ppb. The composite sensor has a short response/recovery time (9.8 s/27.1 s) with good selectivity, repeatability, and stability. The excellent gas-sensitive performance of the rGO:AuNP/PATP-SAW sensor was attributed to the fact that PATP provides a large number of selective capture groups, which are enriched in the sensing area by the presence of multiple strong interactions with 2,4,6-trinitrotoluene (TNT) molecules. This work will provide scientific guidance for the engineering promotion of surface acoustic wave gas-phase explosives sensors.

Surface acoustic wave coupling between micromechanical resonators

The coupling of micro- or nanomechanical resonators via a shared substrate is intensively exploited to built systems for fundamental studies and practical applications. So far, the focus has been on devices operating in the kHz regime with a spring-like coupling. At resonance frequencies above several 10 MHz, wave propagation in the solid substrate becomes relevant. The resonators act as sources for surface acoustic waves (SAWs), and it is unknown how this affects the coupling between them. Here, we present a model for MHz frequency resonators interacting by SAWs, which agrees well with finite element method simulations and recent experiments of coupled micro-pillars. In contrast to the well-known strain-induced spring-like coupling, the coupling via SAWs is not only dispersive but also dissipative. This can be exploited to realize high quality phonon cavities, an alternative to acoustic radiation shielding by, e.g. phononic crystals.

Analysis of surface acoustic wave properties in CaYAl3O7 single crystal

The success on the growth of new piezoelectric materials allows sufficiently increase the operating temperature of the surface acoustic wave (SAW) devices from 300°C to 1000°C. A new calcium yttrium aluminate (CaYAl3O7) single crystal of the tetragonal symmetry has piezoelectric properties up to the temperature of 1000°C. The paper presents a numerical study of the surface acoustic wave properties in the crystal. The SAW velocity, electromechanical coupling coefficient and power flow angle are studied for different crystal cuts of CaYAl3O7. It is shown that the maximum value of SAW coupling coefficient (0.24%) is on the Z+60°-cut and wave propagation direction along the X-axis of the crystal. For the Z-cut and wave propagation direction along the X+45°-axis of crystal, the SAW coupling coefficient is equal to 0.2%. These two cuts of the crystal are potentially useful for SAW device applications.

Detecting Small Size Mass Loading Using Transversely Coupled SAW Resonator

In the detection of small size mass loading, such as a single cell, a micro droplet or an aerosol particle, the sensors with longitudinally coupled surface acoustic wave resonator (LC-SAWR) structure can hardly avoid waveform distortions. The relative size of mass loading to the sensitive surface of the detector is the main factor affecting the response of LC-SAWR. The smaller the relative size, the worse the waveform distortion. In order to avoid influences from the mass loading’s size, in this paper, a transversely coupled SAW resonator (TC-SAWR) was proposed in order to achieve high performance in sensing small size mass loadings. For the design and simulation of TC-SAWR, the two-dimensional coupling of model (2D-COM) theory and finite element method (FEM) were used in this work. In the experiment, SiO2 was deposited on the sensor’s surface as a small size mass loading. The results from simulation and experiment mutually demonstrated the advantage of TC-SAWR to conquer waveform distortion in the detection of small size mass loading.

The BAW and SAW coupling analysis of patterned piezoelectric films as inter-digital transducers on diamond substrates using delta-function model

Patterned piezoelectric films as inter-digital transducer (PP-IDT) structures on diamond substrates have been attracting extensive attention due to the increased working frequency, electromechanical coupling, and quality factor of piezoelectric devices. In this study, by conceiving every single patterned piezoelectric IDT bar as the longitudinal bulk acoustic wave (BAW) vibrating source, we study how the BAW excites and couples with the surface acoustic wave (SAW) on the surface of diamond substrates and resonates simultaneously. The classic delta-function model of the SAW and electric impedance theory of the film bulk acoustic resonator were innovatively combined to develop this conception. Resonating frequency and corresponding piezoelectric IDT thickness were calculated using MATLAB software based on the developed delta-function model. The resonance results coincided well with finite element method simulations, validating this conception and the developed delta-function model for PP-IDT structures.

Phase Nonreciprocity of Microwave‐Frequency Surface Acoustic Waves in Hybrid Heterostructures with Magnetoelastic Coupling

AbstractMagnetoelastic coupling is considered as one of the most reliable method to induce nonreciprocity of propagation losses of microwave‐frequency surface acoustic waves (SAW) and other acoustic modes propagating in nonmagnetic‐ferromagnetic heterostructures. Here, it is demonstrated theoretically that magnetoelastic coupling can also induce phase nonreciprocity of SAW, which is necessary for the development of SAW circulators and other nonreciprocal solid‐state‐acoustic devices. In contrast to previous studies, induction of the phase nonreciprocity requires the coupling of SAW to a strongly nonreciprocal spin wave (SW), having the nonreciprocal splitting of the SW spectrum much larger than the strength of the magnetoelastic coupling, which, in turn, should be much larger than the geometric mean of the SW and SAW damping rates. In this case, the hybridized SAW in the spectral region between the magnetoelastic gaps demonstrate significant phase nonreciprocity, retaining, at the same time, propagation losses that are close to those of unhybridized SAW. Possible practical realization of nonreciprocal SAW phase shifters and SAW‐ring‐based circulators based on hybridized waves in acoustic crystal and synthetic antiferromagnetic heterostructures is discussed.

Laser-Ultrasonic Characterization of Strongly Anisotropic Materials by Transient Grating Spectroscopy

Transient grating spectroscopy (TGS) is a laser-ultrasonic method allowing measurement of the surface acoustic wave (SAW) velocity in an examined material for a given direction of the wave vector. We explore the capability of TGS for determination of shear elastic coefficients ( $$c^\prime$$ and $$c_{44}$$ ) of strongly anisotropic cubic materials. TGS is tested on a set of single crystals with an anisotropy factor up to $$A=25$$ . Using a numerical simulation based on a Ritz-Rayleigh approach, we show that strong anisotropy may lead to significant coupling of SAWs with bulk shear waves, which complicates TGS measurements in specific directions. Based on the obtained TGS data, we discuss the possibility of also using the TGS technique for assessing the longitudinal elastic coefficient ( $$c_L$$ ). Despite the energy focusing and other effects originating from the strong anisotropy, the TGS method can be used to reliably determine the directional dependence of the SAW velocity in these materials, and the resulting experimental datasets are sufficient for inverse determination of both the soft shear elastic constant ( $$c^\prime$$ ) and the hard shear elastic constant ( $$c_{44}$$ ). The longitudinal coefficient can be determined with lower accuracy. TGS is a suitable experimental tool for contactless characterization of strongly anisotropic materials.

Nonreciprocal surface acoustic wave propagation via magneto-rotation coupling.

A fundamental form of magnon-phonon interaction is an intrinsic property of magnetic materials, the "magnetoelastic coupling." This form of interaction has been the basis for describing magnetostrictive materials and their applications, where strain induces changes of internal magnetic fields. Different from the magnetoelastic coupling, more than 40 years ago, it was proposed that surface acoustic waves may induce surface magnons via rotational motion of the lattice in anisotropic magnets. However, a signature of this magnon-phonon coupling mechanism, termed magneto-rotation coupling, has been elusive. Here, we report the first observation and theoretical framework of the magneto-rotation coupling in a perpendicularly anisotropic film Ta/CoFeB(1.6 nanometers)/MgO, which consequently induces nonreciprocal acoustic wave attenuation with an unprecedented ratio of up to 100% rectification at a theoretically predicted optimized condition. Our work not only experimentally demonstrates a fundamentally new path for investigating magnon-phonon coupling but also justifies the feasibility of the magneto-rotation coupling application.

Effect of Pb(Zr0.52Ti0.48)O3 thin films on electron transport at the LaAlO3/SrTiO3 interface by surface acoustic waves

Surface acoustic waves (SAWs) are capable tools for providing mechanical control over the electronic properties of functional materials. Coupling SAWs with the LaAlO3/SrTiO3 (LAO/STO) conducting interface is particularly interesting as this interface exhibits extraordinary features, such as high mobility at low temperature, ferromagnetism, and superconductivity below 200 mK. For SAW generation, piezoelectricity is indispensable, and due to lack of that in the LAO/STO system, a 200 nm thick Pb(Zr0.52Ti0.48)O3 (PZT) film was grown on top of LAO. SAW excitation and propagation was demonstrated on a PZT/LAO/STO multilayer structure. We further employed SAWs in order to transport free electrons confined to the LAO/STO interface, detected as an acoustoelectric voltage at room temperature. Electrical characterization of the interface was carried out by Van der Pauw measurements. We found that having a PZT layer on top of LAO/STO considerably degraded the interfacial conductivity. The degradation became more pronounced at low temperatures. We attribute these effects to the filling of oxygen vacancies due to interlayer oxygen migration, combined with carrier freeze-out at low temperatures.

Acoustic ferromagnetic resonance and spin pumping induced by surface acoustic waves

Voltage induced magnetization dynamics of magnetic thin films is a valuable tool to study anisotropic fields, exchange couplings, magnetization damping and spin pumping mechanism. A particularly well established technique is the ferromagnetic resonance (FMR) generated by the coupling of microwave photons and magnetization eigenmodes in the GHz range. Here we review the basic concepts of the so-called acoustic-FMR (a-FMR) induced by the coupling of surface acoustic waves (SAW) and magnetization of thin films. Interestingly, additional to the benefits of the microwave excited FMR technique, the coupling between SAW and magnetization also offers fertile ground to study magnon-phonon and spin rotation couplings. We describe the in-plane magnetic field angle dependence of the a-FMR by measuring the absorption / transmission of SAW and the attenuation of SAW in the presence of rotational motion of the lattice, and show the consequent generation of spin current by acoustic spin pumping.

Theoretical analysis of high electromechanical coupling surface acoustic wave propagating on lead-free Na0.5Bi0.5TiO3–BaTiO3 single crystal

The propagation properties of surface acoustic wave (SAW) on the Na0.5Bi0.5TiO3–BaTiO3 (NBBT) substrate with different crystal cuts were theoretically analyzed by Finite Element Method (FEM). The calculated results, for the NBBT with the optimized electrode type and thickness, indicate the existence of Rayleigh and Sezawa modes with high electromechanical coupling factor K2 ~18% and ~6.8% respectively, which are considerably higher than that of the conventional lithium niobate (LiNbO3) single crystal for SAW applications. The research provides a comprehensive understanding of the SAW propagation characteristics of lead-free NBBT, and explores the promising applications of NBBT in wideband SAW devices.

Wide-Band Nonreciprocity of Surface Acoustic Waves Induced by Magnetoelastic Coupling with a Synthetic Antiferromagnet

One of the major challenges in surface acoustic wave (SAW) technology, which is widely used in modern signal processing, is the absence of wave nonreciprocity (one-way propagation). To tackle this problem, the authors propose an approach based on magnetoelastic coupling of SAWs to spin waves possessing a suitable spectrum. This approach allows the creation of nonreciprocal SAW devices with almost no limitations on the working-frequency band (in contrast to the severe band limitations of traditional approaches), to realize the full potential of magnetoelastically induced SAW nonreciprocity. These results could become a breakthrough step toward nonreciprocal SAW signal processing.

Low-loss waveguides on Y-cut thin film lithium niobate: towards acousto-optic applications.

We investigate the dependence of photonic waveguide propagation loss on the thickness of the buried oxide layer in Y-cut lithium niobate on insulator substrate to identify trade-offs between optical losses and electromechanical coupling of surface acoustic wave (SAW) devices for acousto-optic applications. Simulations show that a thicker oxide layer reduces the waveguide loss but lowers the electromechanical coupling coefficient of the SAW device. Optical racetrack resonators with different lengths were fabricated by argon plasma etching to experimentally extract waveguide losses. By increasing the oxide layer thickness from 1 µm to 2 µm, we were able to reduce propagation loss of 2 µm (1 µm) wide waveguide from 1.85 dB/cm (3 dB/cm) to as low as 0.37 dB/cm (0.77 dB/cm). Resonators with a quality factor greater than 1 million were demonstrated as well. An oxide thickness of approximately 1.5 µm is sufficient to significantly reduce propagation loss, due to leakage into the substrate and simultaneously attain good electromechanical coupling in acoustic devices. This work not only provides insights on the design and realization of low-loss photonic waveguides in lithium niobate, but also, most importantly, offers experimental evidence of how the oxide thickness directly impacts losses and guides its selection for the synthesis of high-performance acousto-optic devices in Y-cut lithium niobate on insulator.

Effect of LiNbO3 polarity on the structural, optical and acoustic properties of epitaxial ZnO and MgxZn1−xO films

The effect of LiNbO3 substrate polarization on a charge density at the interface with epitaxial ZnO and MgxZn1−xO films was studied in comparison with ZnO films on dielectric sapphire substrates. An excitonic emission and optical properties such as band gap energy, refraction index, absorption and reflectivity of the ZnO/LiNbO3 structures were studied. The effect of interface charges on the performance of surface acoustic waves with frequency of 2.7 GHz was investigated, as well. The stimulated cavity mode emission was observed due to attained quasi-phase matching conditions for thin ZnO films on sapphire at high laser excitation power. The positive polarization of the ferroelectric substrate induced the band bending of the semiconductor layer, resulting in the accumulation of the negative charges at the ferroelectric/semiconductor interface, which induced an increased reflectivity, a generation of oxygen emission lines in PL spectra under laser excitation, and an increased electromechanical coupling of surface acoustic waves. The negative polarization of the substrate increased atomic peening during sputtering deposition, which induced deteriorated quality of texture and in-plane orientation and increased the residual stresses in ZnO and MgxZn1−xO films.

Wide Band Low Noise Love Wave Magnetic Field Sensor System

We present a comprehensive study of a magnetic sensor system that benefits from a new technique to substantially increase the magnetoelastic coupling of surface acoustic waves (SAW). The device uses shear horizontal acoustic surface waves that are guided by a fused silica layer with an amorphous magnetostrictive FeCoSiB thin film on top. The velocity of these so-called Love waves follows the magnetoelastically-induced changes of the shear modulus according to the magnetic field present. The SAW sensor is operated in a delay line configuration at approximately 150 MHz and translates the magnetic field to a time delay and a related phase shift. The fundamentals of this sensor concept are motivated by magnetic and mechanical simulations. They are experimentally verified using customized low-noise readout electronics. With an extremely low magnetic noise level of ≈100 pT/sqrt{{rm{Hz}}}, a bandwidth of 50 kHz and a dynamic range of 120 dB, this magnetic field sensor system shows outstanding characteristics. A range of additional measures to further increase the sensitivity are investigated with simulations.

Confinement of surface waves at the air-water interface to control aerosol size and dispersity

The precise control over the size and dispersity of droplets, produced within aerosols, is of great interest across many manufacturing, food, cosmetic, and medical industries. Amongst these applications, the delivery of new classes of high value drugs to the lungs has recently attracted significant attention from pharmaceutical companies. This is commonly achieved through the mechanical excitation of surface waves at the air liquid interface of a parent liquid volume. Previous studies have established a correlation between the wavelength on the surface of liquid and the final aerosol size. In this work, we show that the droplet size distribution of aerosols can be controlled by constraining the liquid inside micron-sized cavities and coupling surface acoustic waves into different volumes of liquid inside micro-grids. In particular, we show that by reducing the characteristic physical confinement size (i.e., either the initial liquid volume or the cavities’ diameters), higher harmonics of capillary waves are revealed with a consequent reduction of both aerosol mean size and dispersity. In doing so, we provide a new method for the generation and fine control of aerosols’ sizes distribution.

Effect of magnetoelastic film thickness on power absorption in acoustically driven ferromagnetic resonance

Surface acoustic waves (SAWs) traveling on the surface of a piezoelectric substrate are capable of exciting magnetoelastic ferromagnets into resonance. In this work, we explore the effects of magnetoelastic film thickness on the coupling of SAWs into such magnetic thin films. We find that power absorption as a function of film thickness begins to saturate above thicknesses of ≈20 nm. This is contrary to current models that predict an exponential increase of the absorption as a function of increasing film thickness. We show that the saturation happens due to an increase in the damping of the film beyond this thickness range.

A highly attenuating and frequency tailorable annular hole phononic crystal for surface acoustic waves

Surface acoustic wave (SAW) devices are widely used for signal processing, sensing and increasingly for lab-on-a-chip applications. Phononic crystals can control the propagation of SAW, analogous to photonic crystals, enabling components such as waveguides and cavities. Here we present an approach for the realisation of robust, tailorable SAW phononic crystals, based on annular holes patterned in a SAW substrate. Using simulations and experiments, we show that this geometry supports local resonances which create highly attenuating phononic bandgaps at frequencies with negligible coupling of SAWs into other modes, even for relatively shallow features. The enormous bandgap attenuation is up to an order-of-magnitude larger than that achieved with a pillar phononic crystal of the same size, enabling effective phononic crystals to be made up of smaller numbers of elements. This work transforms the ability to exploit phononic crystals for developing novel SAW device concepts, mirroring contemporary progress in photonic crystals.