Transmission Of Surface Acoustic Waves Research Articles

Surface acoustic wave devices on bulk ZnO crystals at low temperature

Surface acoustic wave (SAW) devices based on thin films of ZnO are a well established technology. However, SAW devices on bulk ZnO crystals are not practical at room temperature due to the significant damping caused by finite electrical conductivity of the crystal. Here, by operating at low temperatures, we demonstrate effective SAW devices on the (0001) surface of bulk ZnO crystals, including a delay line operating at SAW wavelengths of λ = 4 and 6 μm and a one-port resonator at a wavelength of λ = 1.6 μm. We find that the SAW velocity is temperature dependent, reaching v ≃ 2.68 km/s at 10 mK. Our resonator reaches a maximum quality factor of Qi ≃ 1.5 × 105, demonstrating that bulk ZnO is highly viable for low temperature SAW applications. The performance of the devices is strongly correlated with the bulk conductivity, which quenches SAW transmission above 200 K.

Microfluidic pumping through miniaturized channels driven by ultra-high frequency surface acoustic waves

Surface acoustic waves (SAWs) are an effective means to pump fluids through microchannel arrays within fully portable systems. The SAW-driven acoustic counterflow pumping process relies on a cascade phenomenon consisting of SAW transmission through the microchannel, SAW-driven fluid atomization, and subsequent coalescence. Here, we investigate miniaturization of device design, and study both SAW transmission through microchannels and the onset of SAW-driven atomization up to the ultra-high-frequency regime. Within the frequency range from 47.8 MHz to 754 MHz, we show that the acoustic power required to initiate SAW atomization remains constant, while transmission through microchannels is most effective when the channel widths w ≳ 10 λ, where λ is the SAW wavelength. By exploiting the enhanced SAW transmission through narrower channels at ultra-high frequencies, we discuss the relevant frequency-dependent length scales and demonstrate the scaling down of internal flow patterns and discuss their impact on device miniaturization strategies.

Realizability of a bolometer based on a surface-acoustic-wave delay line with periodic reflecting structures

Transmission of surface acoustic waves through a delay line with periodic reflecting structures forming a Fabry-Perot resonator is studied. An expression for the wave transmission coefficient in such a structure is derived. It is demonstrated that a pair of strongly reflecting gratings can become completely transparent, i.e., the blooming effect takes place with the blooming area of an exponentially small width. The effect of temperature on the frequency response curve of a delay line with a Fabry-Perot resonator is experimentally investigated. The investigated effect is shown to be useful for fabrication of a bolometer detecting electromagnetic radiation.

UV epoxy bonding and enhanced SAW transmission for applications in acoustofluidic integration

Surface acoustic waves (SAWs) are highly attractive as a means of fluidic and particulate manipulation in Lab-on-a-Chip systems. However, standard acoustofluidic fabrication practices rely heavily on the use of elastomeric materials such as PDMS which are inherently ill-suited for conveyance of SAWs as they introduce severe acoustic attenuation. Here, we explore the use of a low-viscosity UV epoxy resin for room temperature bonding of piezoelectric SAW substrates with standard micromachined supersaturates such as Pyrex and Silicon. The bonding methodology is straightforward and allows for reliable production of sub-micron bonds that are capable of enduring the high surface strains and accelerations typical of SAW propagation. Transmission in devices prepared with this approach show as much as 20 dB of improvement compared to devices fabricated using the standard PDMS elastomer. The method is further used in the fabrication of closed-channel SAW pumping concepts for applications in micro-scale flow control.

UV epoxy bonding for enhanced SAW transmission and microscale acoustofluidic integration

Surface acoustic waves (SAWs) are appealing as a means to manipulate fluids within lab-on-a-chip systems. However, current acoustofluidic devices almost universally rely on elastomeric materials, especially PDMS, that are inherently ill-suited for conveyance of elastic energy due to their strong attenuation properties. Here, we explore the use of a low-viscosity UV epoxy resin for room temperature bonding of lithium niobate (LiNbO(3)), the most widely used anisotropic piezoelectric substrate used in the generation of SAWs, to standard micromachined superstrates such as Pyrex® and silicon. The bonding methodology is straightforward and allows for reliable production of sub-micron bonds that are capable of enduring the high surface strains and accelerations needed for conveyance of SAWs. Devices prepared with this approach display as much as two orders of magnitude, or 20 dB, improvement in SAW transmission compared to those fabricated using the standard PDMS elastomer. This enhancement enables a broad range of applications in acoustofluidics that are consistent with the low power requirements of portable battery-driven circuits and the development of genuinely portable lab-on-a-chip devices. The method is exemplified in the fabrication of a closed-loop bidirectional SAW pumping concept with applications in micro-scale flow control, and represents the first demonstration of closed channel SAW pumping in a bonded glass/LiNbO(3) device.

Locally resonant surface acoustic wave band gaps in a two-dimensional phononic crystal of pillars on a surface

We investigate theoretically the propagation of acoustic waves in a two-dimensional array of cylindrical pillars on the surface of a semi-infinite substrate. Through the computation of the band structure of the periodic array and of the transmission of waves through a finite length array, we show that the phononic crystal can support a number of surface propagating modes in the nonradiative region of the substrate, or sound cone, as limited by the slowest bulk acoustic wave. The modal shape and the polarization of these guided modes are more complex than those of classical surface waves propagating on a homogeneous surface. Significantly, an in-plane polarized wave and a transverse wave with sagittal polarization appear that are not supported by the free surface. In the band structure, guided modes define band gaps that appear at frequencies markedly lower than those expected from the Bragg interference condition. We identify them as originating from local resonances of the individual cylindrical pillars and show their dependence on the geometrical parameters, in particular with the height of the pillars. The transmission of surface acoustic waves across a finite array of pillars shows the signature of the locally resonant band gaps for surface modes and their dependence on the symmetry of the source and its polarization. Numerical simulations are performed by using the finite element method and considering silicon pillars on a silicon substrate.

ZnO nanoparticle surface acoustic wave UV sensor

The response to ultraviolet illumination of ZnO nanoparticles deposited on LiNbO3 substrate was investigated using surface acoustic waves (SAWs) in the wide range of UV wavelengths from 280 to 375 nm. Deposition of ZnO nanoparticles caused a SAW transmission loss of 27 dB at 64 MHz due to the acoustoelectric attenuation. Acoustoelectric change in the SAW velocity by 3.78×10−4 under 375 nm illumination led to downshift in transmitted SAW phase by 5.5° at UV power density of 691 μW/cm2. The spectral measurements show the peak response at 345 nm with corresponding sensitivity on the order of 2.8 ppm/(μW/cm2).

Monitoring of surface processes with laser-excited surface acoustic waves and dual-probe-beam detection

Laser generated broadband surface acoustic wave (SAW) pulses were used for studying surface dynamic processes. The changes in a liquid layer deposited on a Si substrate and the ablation damage of Si wafers were monitored with SAWs. The SAW amplitude, pulse shape and spectrum change after propagation through the area with the liquid layer. These changes can be used for real-time monitoring of the evaporation process and determination of the parameters of the layer. In the laser ablation with a sequence of pulses the etching depth and the extent of the damaged area increase with the number of pulses, affecting transmission of SAWs. A model taking into account dimensions of the ablation groove was developed and used for assessment of the ablation process.

Phases of the SAW reflection and transmission coefficients for short reflectors on 128/spl deg/ LiNbO/sub 3/

We study numerically the phase of surface acoustic waves reflected by or transmitted through short reflectors comprising only 1-3 aluminium electrodes on 128 degree YX-cut lithium niobate (LiNbO3). The electrodes have a finite thickness, and they are either open-circuited or grounded. The center-to-center distance between adjacent electrodes d corresponds roughly either to half of the characteristic wavelength d proportional to lambda0/2 or to d proportional to lambda0, for the reflectors operating at the fundamental and second harmonic modes, respectively. We use software based on the finite-element and boundary-element methods (FEM/BEM) for numerical experiments with a tailored test structure having 3 interdigital transducers (IDTs), simulating experimental conditions with an incident wave and reflected and transmitted surface acoustic wave (SAW). Using artificial enhancement of time resolution in conjunction with the fast Fourier transform (FFT) and time-gating, calculation of the Y-parameters in a relatively wide frequency range allows us to determine the phase of the reflection and transmission coefficients.

Transmission of surface acoustic waves through the interface based on discontinuity of electrical boundary conditions on surface of potassium niobate

This paper presents theoretical investigation of the propagation of surface acoustic waves (SAWs) across the boundary between metallized (electrically shorted) and unmetallized (electrically open) regions on the surface of potassium niobate crystals. Potassium niobate is a very strong piezoelectric material and has the interesting property that only one type of SAW, namely a Rayleigh wave, can exist on unmetallized surface, where as two types of SAWs, namely Rayleigh and Bleustein–Gulyaev (BG), can exist on a metallized surface. Analysis shows that the Rayleigh wave propagates through the interface with very little change in amplitude or polarization. On the other hand, almost total reflection of the BG wave is expected. Details of the theoretical analysis and calculated results will be presented.

Investigations on temperature controlled monolithic integrated surface acoustic wave (SAW) gas sensors

The paper presents examinations on temperature behavior of mass sensitive surface acoustic wave (SAW) gas sensors. The investigations were realized on dual-delay-line SAW-devices based on AlN as piezoelectric layer. In order to measure temperature, a thin film resistor was placed on the surface of the SAW-device. To heat up the sensor device a monolithic integrated polysilicon heating resistor was used. As mass sensitive layers different polymers were deposited using a spin-on-process. For the detection of aromatic compounds like benzene, toluene and xylene suitable imprinted polymers were used. To examine sensitivity and selectivity of alcoholic compounds a polyurethane was deposited as mass sensitive layer. In our investigations, the effect of gas adsorption on the SAW transmission behavior was studied. Different atmospheres caused varying damping behavior and thus energy losses of the transmission line influenced the surface temperature of the mass sensitive layer. Growing demands on quality standards demand improvements in sensitivity, selectivity and stability of gas sensors. Because of the temperature influence on physical properties of surface acoustic waves, the temperature behavior of the SAW-devices was examined. Further on, the effects of cross sensitivity were studied according to different temperature behavior of adsorption. As a result, a higher sensitivity and stability could be achieved by controlling and changing temperature during the measurement in comparison to uncontrolled devices.

X-ray imaging of surface acoustic waves generated in semiconductor crystals by an external transducer

We demonstrate successful x-ray-diffraction imaging of high-frequency (0.29-GHz) surface acoustic waves (SAWs) propagating in semiconductor crystals (Si and GaAs) with no deposited electrodes on top of them. Experiments were performed at the ID19 beamline of the European Synchrotron Radiation Facility (Grenoble). We used the stroboscopic x-ray topography technique, in which x-ray bursts coming from the storage ring are synchronized with the excited acoustic waves. This technique is able to visualize individual acoustic wave fronts of traveling SAWs and their distortions caused by phonon scattering. In order to generate SAW in semiconductor crystals the latter were coupled to LiNbO3-based SAW transducers via contact liquids. The maximum SAW transmission is achieved for evanescent waves under optimized coupling conditions at the liquid/sample interface. The technique developed opens a way to directly study phonon interaction with defects in nonpiezoelectric and weakly piezoelectric crystals.

Propagation of surface acoustic waves in a GaAs/AlAs/GaAs heterostructure and micro-beams

The dispersion of the Rayleigh wave in a strain-unrelaxed GaAs/AlAs/GaAs heterostructure is evaluated. We determine the wavelengths at which the velocity of surface acoustic waves (SAWs) is maximum or comparable to that in GaAs. The information is utilized to design interdigital transducers to investigate the SAW transmission properties when GaAs micro-beams are realized in the SAW propagation path by selectively etching the buried AlAs layer. A frequency shift as well as an attenuation of the SAW transmission peak is observed for certain SAW wavelengths within the range of the enhanced SAW velocity in the unpatterned heterostructure.

Giant Surface Acoustic Wave Attenuation in the Quantum Hall Regime Induced by a DC Current

The attenuation and the velocity shift of surface acoustic waves (SAWs) in a GaAs–Al0.3Ga0.7As asymmetric double quantum well structure are investigated under nonequilibrium conditions. When the electron density is in the vicinity of the occupation threshold of the second subband, the SAW transmission in the quantum Hall regime is suppressed over the entire magnetic field range with a large magnitude. The suppression is quenched in the presence of a dc current whenever the Fermi level lies just above the Landau gaps associated with the first subband, resulting in marked oscillations in the SAW transmission.

Influence of the second subband on thesurface acoustic wave transmission in the quantum Hall regime

The attenuation of surface acoustic waves (SAWs) propagatingthrough an asymmetric two-layer system is investigated in thequantum Hall regime. The position of the Fermi level is variedin the vicinity of the population threshold of the secondsubband by changing the electron density and by injecting a dccurrent. For large currents, the quantum Hall effect (QHE)breaks down. The SAW attenuation in the breakdown regime revealsthat the vanishing conductivity of the QHE is maintained in theinterior of the sample despite its absence from the resistance.The inhomogeneous breakdown of the QHE becomes more apparentwhen the Fermi level approaches the second subbandlevel. When the electron population in the second subband issignificantly large, the SAW transmission develops giganticstructures at the quantum Hall states. A drastic variation ofthe screening properties associated with the relocation ofelectrons between the two layers is responsible for thebehaviour. The observation of a hysteretic behaviour evidencesthat the SAW-generated electric fields are strong enough toinfluence the interlayer charge transfer.

Acoustic wave–liquid interactions

Surface acoustic waves (SAWs) are one of a broad class of acoustic wave (AW) techniques that have been applied to the study of physical changes at the solid–liquid interface. Other examples include shear horizontally polarised SAWs (SH-SAWs), acoustic plate modes, Love waves and quartz crystal microbalances (QCMs). Several factors motivate and favour these techniques. The sensing surface is highly mass sensitive, it is accessible and can be chemically modified, and it provides a rapid in situ method for studying dynamic chemical and biochemical changes. Moreover, for a Newtonian fluid, the AW only entrains fluid within a penetration depth of the interface, so that the technique truly probes interfacial changes. However, many studies of liquids using these acoustic techniques have been limited to fixed pools of liquid in contact with a device surface. In this work, a damped harmonic oscillator model is described, providing a unified view of the mass damping of the shear motion in SAW, SH-SAW, and QCM AW systems by finite thickness loadings of viscoelastic fluids. The simplicity of the model also allows the effect of fluid slip at the solid–liquid interface to be examined. In the limit of small relaxation time and thick fluid coating, the model recovers the expected limit with acoustic devices acting as sensors of the device area coated by the fluid. In the large relaxation time limit, the fluid acts as an amorphous solid and the influence of the acoustic shearing motion is able to extend to the free surface of the fluid, thus inducing shear wave resonances. To complement the theory, an experiment is described which uses pulses of high frequency (169 MHz) Rayleigh SAWs to probe a small stripe of a viscous fluid (polydimethylsiloxane [PDMS] oil with a viscosity between 10 000 and 100 000 centistokes [cSt]) as it dynamically evolves in shape. The cross-sectional profile of the liquid is a well-defined spherical cap shape and this is recorded using a video based interferometry arrangement. This allows a range of geometrical parameters to be obtained and correlated with the acoustic signals. The changing geometry of the stripe does not simply decrease the magnitude of the SAW transmission as the fluid wets a progressively larger area of the surface, but also specific significant attenuations are observed. Interpreting these attenuations within the damped harmonic oscillator model, data from a range of experiments can all be fitted by relaxation rates obtained from the viscosity and a high frequency shear modulus of μ=(1.5±0.1) GN m −2. Hence, with this type of viscous fluid, the acoustic method is not acting as a simple mass sensor, although it is probing the elasticity via the shear modulus.

Scanning phononic lattices with ultrasound

A method for probing the elastic properties of newly developed periodic structures using acoustic waves is introduced. Highly anisotropic transmission of surface acoustic waves is observed by continuously scanning the wave vector angle. Preliminary models of wave propagation through multilayers and two-dimensional lattices explain some of the experimental features, while other features can be attributed to the resonant excitation of interface waves. {copyright} {ital 1999} {ital The American Physical Society}

Reflection and transmission of surface acoustic waves at a boundary between metallized and free surfaces on an anisotropic substrate

AbstractThis paper considers the case where the regions of conductor‐deposited surface and the free surface are bordered by a straight line. The reflection and transmission characteristics of the surface acoustic wave are approximately analyzed, which is incident to the boundary. In the proposed analysis, the incident, reflected and transmitted fields near the straight‐line boundary are represented by the linear sums of surface waves, which are a three‐dimensional surface wave in each region with the real and complex wavenumbers, in general. The wavenumber of the surface wave on the substrate is decomposed into the longitudinal wavenumber in parallel to the straight‐line boundary and the wavenumber in the perpendicular direction. The reflection and the transmission coefficients are determined so that the discontinuity of the field near the straight‐line boundary is minimized. As a result, the analytic formula is derived for the reflection and the transmission coefficients, using only the transverse wavenumber of the surface wave. By this elaboration, a simple and approximate method of analysis is presented, which can be applied in general to the arbitrary anisotropic substrate.