Interaction Of Surface Acoustic Waves Research Articles

Finite-Difference Simulation of the Interaction of Surface Acoustic Waves with Partially Closed Surface-Breaking Cracks

Simulations by the finite-difference time-domain method were performed to model the interaction of laser-excited broadband surface acoustic wave (SAW) pulses with a partially closed surface-breaking crack, prepared by stress-induced impulsive fracture. One- and two-parameter models were developed to study the role played by the interfacial stress of the crack. Good agreement with laser-scanning nondestructive evaluation was found for the measured reflected and transmitted SAW velocity profiles. Even the single-parameter reduced-stress model, based on the same softening of all interface stress-tensor components, provided a reasonable description, consistent with the two-parameter simulations treating the tensile and shear interfacial stress contributions separately. In addition, the enhancement of the velocity profile at the front edge of the crack was calculated for the one- and two-parameter stress models and compared with experimentally observed enhancement effects.

Interaction of a Contact Resonance of Microspheres with Surface Acoustic Waves

We study the interaction of surface acoustic waves (SAWs) with a contact-based vibrational resonance of 1 μm silica microspheres forming a two-dimensional granular crystal adhered to a substrate. The laser-induced transient grating technique is used to excite SAWs and measure their dispersion. The measured dispersion curves exhibit "avoided crossing" behavior due to the hybridization of the SAWs with the microsphere resonance. We compare the measured dispersion curves with those predicted by our analytical model and find excellent agreement. The approach presented can be used to study the contact mechanics and adhesion of micro- and nanoparticles as well as the dynamics of microscale granular crystals.

Frequency effect on streaming phenomenon induced by Rayleigh surface acoustic wave in microdroplets

Acoustic streaming of ink particles inside a water microdroplet generated by a surface acoustic wave (SAW) has been studied numerically using a finite volume numerical method and these results have been verified using experimental measurements. Effects of SAW excitation frequency, droplet volume, and radio-frequency (RF) power are investigated, and it has been shown that SAW excitation frequency influences the SAW attenuation length, lSAW, and hence the acoustic energy absorbed by liquid. It has also been observed that an increase of excitation frequency generally enhances the SAW streaming behavior. However, when the frequency exceeds a critical value that depends on the RF power applied to the SAW device, weaker acoustic streaming is observed resulting in less effective acoustic mixing inside the droplet. This critical value is characterised by a dimensionless ratio of droplet radius to SAW attenuation length, i.e., Rd/lSAW. With a mean value of Rd/lSAW ≈ 1, a fast and efficient mixing can be induced, even at the lowest RF power of 0.05 mW studied in this paper. On the other hand, for the Rd/lSAW ratios much larger than ∼1, significant decreases in streaming velocities were observed, resulting in a transition from regular (strong) to irregular (weak) mixing/flow. This is attributed to an increased absorption rate of acoustic wave energy that leaks into the liquid, resulting in a reduction of the acoustic energy radiated away from the SAW interaction region towards the droplet free surface. It has been demonstrated in this study that a fast and efficient mixing process with a smaller RF power could be achieved if the ratio of Rd/lSAW ≤ 1 in the SAW-droplet based microfluidics.

Finite element modeling of the interaction of laser-generated ultrasound with a surface-breaking notch in an elastic plate

The interaction of surface acoustic waves generated by laser line source in the thermoelastic regime with surface notches are investigated. The finite element method is used to establish the model of the transient displacement field for surface notches with various depths and orientation. The magnitude of the signal enhancement in the near field and the mechanism by which this occurs are explained. The positions of notches were evaluated by the reflected Rayleigh wave. The depths and orientations of the notches were also determined using a shear wave that was generated through mode conversion of a surface acoustic wave at the notch tip. The results agree with previously published experimental measurements.

Nonlinear reflection of surface acoustic waves from contact lines

We present the results of an experimental investigation of nonlinear processes accompanying the interaction of surface acoustic waves (SAWs) with real cracking defects and contact lines. The dynamic characteristics of the nonlinear reflection of higher harmonics are presented. It is shown that the third harmonic generation has a pronounced threshold character, the dynamic characteristics exhibit hysteresis, and the efficiency of nonlinear response depends on the duration of SAW action on the defect. The results lead to a conclusion that the nonlinear reflection of SAWs from a contact line provides an adequate model of reflection from a real cracking defect.

Evidence for vortex transport by surface acoustic waves in a high‐Tc superconductor

The interaction of surface acoustic waves (SAW) with the magnetic vortex system in YBa2Cu3O7 is investigated. A 100nm YBCO film is deposited on a piezoelectric substrate and structured for electrical 4‐point measurements. Interdigital transducers are fabricated on the same substrate. When applying an external magnetic field perpendicular to the film surface a SAW‐induced dc‐voltage perpendicular to the acoustic sound‐path is observed. This is interpreted as consequence of directed dragging of vortices by the SAW induced dynamic pinning structure [1]. The piezoacoustic wave acts as a conveyor for mobile flux quanta. Additional ac‐dc‐conversion as result of the nonlinear current‐voltage characteristics close to the superconducting transition temperature can be resolved and separated. In order to observe the sound‐induced vortex motion directly, the flux distribution is analyzed magnetooptically. Magnetooptic imaging allows for time resolved analysis of flux distribution. Quantitative analysis of changes in the magnetization distribution when acoustic driving fields are applied is carried out. The influence of piezoacoustic waves on the pinning properties and sound‐induced depinning is discussed. [1] F. Jachmann and C. Hucho, Sol. State Comm., 142 (4) (2007), 212

Design of III-Nitride multi-layer structures for optical and surface acoustic wave interaction

Research efforts into Surface Acoustic Waves (SAW) devices built on III-Nitrides has recently significantly intensified because of the increasing availability of good-quality epitaxial material. Owing to their strong piezoelectricity and high acoustic velocity, high-Al-content materials are of particular interest in high-speed applications (GHz electronics). For optoelectronics applications III-Nitrides cover a wide range of wavelengths from ultra-violet (AlN) to blue (GaN) to infra-red (InN); thus providing great flexibility in the choice of operating wavelength. III-Nitrides, therefore, become particularly attractive for the development of acousto-optic modulators where the SAW is used to create a controllable (moving) optical grating, causing the diffraction of the Optical Guided Wave (OGW). The interaction is governed by the momentum and energy conservation and importantly by the field overlap of the two waves. We present here design guidelines and modelling results for the development of III-Nitride-based multi-layer structures to support both OGW and SAW. The purpose of this study is to attempt to optimise the device efficiency compared to single GaN-layer structures typically used in the literature where only the SAW characteristics are optimised.

Interaction of surface acoustic waves with moving vortex structures in superconducting films

A method is proposed for describing a moving film vortex structure and its interaction with surface acoustic waves. It is shown that the moving vortex structure can amplify (generate) surface acoustic waves. In contrast to a similar effect in semiconductor films, this effect can appear when the velocity of the vortex structure is much lower than the velocity of the surface acoustic waves. A unidirectional collective mode is shown to exist in the moving vortex structure. This mode gives rise to an acoustic analogue of the diode effect that is resonant in the velocity of the vortex structure. This acoustic effect is manifested as an anomalous attenuation of the surface acoustic waves in the direction of the vortex-structure motion and as the absence of this attenuation for the propagation in the opposite direction.

Numerical investigation of a piezoelectric surface acoustic wave interaction with a one-dimensional channel

We investigate the propagation of a piezoelectric surface acoustic wave (SAW) across a GaAs/Al$_X$Ga$_{1-X}$As heterostructure surface, on which there is fixed a metallic split-gate. Our method is based on a finite element formulation of the underlying equations of motion, and is performed in three-dimensions fully incorporating the geometry and material composition of the substrate and gates. We demonstrate attenuation of the SAW amplitude as a result of the presence of both mechanical and electrical gates on the surface. We show that the incorporation of a simple model for the screening by the two-dimensional electron gas (2DEG), results in a total electric potential modulation that suggests a mechanism for the capture and release of electrons by the SAW. Our simulations suggest the absence of any significant turbulence in the SAW motion which could hamper the operation of SAW based quantum devices of a more complex geometry.

Interaction of Surface Acoustic Waves with Ultraviolet Light

Fabrication and functional performance of highly sensitive ultraviolet detectors based on zinc oxide (ZnO) thin film SAW device and a ZnO/LiNbO 3 layered structure is described. A 37 MHz bulk LiNbO 3 surface acoustic wave (SAW) filter with a 71 to 200 nm-thick ZnO over layer exhibits a downshift in the frequency with ultraviolet (UV) light due to acoustoelectric interactions between the photo-generated carriers in the semiconducting ZnO over layer and the surface acoustic waves. In contrast, a 36 MHz ZnO thin film SAW delay-line fabricated on a 8 μ m thick insulating ZnO film exhibited an upshift in the frequency. A linear change in frequency shift with UV intensity is found useful for the fabrication of wireless UV sensors.

High-frequency transport inp-typeSi∕Si0.87Ge0.13heterostructures studied with surface acoustic waves in the quantum Hall regime

The interaction of surface acoustic waves (SAWs) with $p$-type $\mathrm{Si}∕{\mathrm{Si}}_{0.87}{\mathrm{Ge}}_{0.13}$ heterostructures has been studied for SAW frequencies of $30--300\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$. For temperatures in the range $0.7<T<1.6\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and magnetic fields up to $7\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, the SAW attenuation coefficient $\ensuremath{\Gamma}$ and velocity change $\ensuremath{\Delta}V∕V$ were found to oscillate with filling factor. Both the real ${\ensuremath{\sigma}}_{1}$ and imaginary ${\ensuremath{\sigma}}_{2}$ components of the high-frequency conductivity have been determined and compared with quasi-dc magnetoresistance measurements at temperatures down to $33\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$. By analyzing the ratio of ${\ensuremath{\sigma}}_{1}$ to ${\ensuremath{\sigma}}_{2}$, carrier localization can be followed as a function of temperature and magnetic field. At $T=0.7\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the variations of $\ensuremath{\Gamma}$, $\ensuremath{\Delta}V∕V$, and ${\ensuremath{\sigma}}_{1}$ with SAW intensity have been studied and can be explained by heating of the two-dimensional hole gas by the SAW electric field. Energy relaxation is found to be dominated by acoustic phonon deformation potential scattering with weak screening.

Minimum-loss short reflectors on 128/spl deg/ LiNbO/sub 3/

We consider the interaction of surface acoustic waves (SAWs) with short electrode gratings encompassing only few electrodes on 128/spl deg/ lithium niobate (LiNbO/sub 3/). The qualifications of the reflectors are evaluated by comparing the part of incident SAW energy scattered by the structure into the bulk to the energy reflected back as a SAW.

Remote collection and measurement of photogenerated carriers swept by surface acoustic waves in GaN

The interaction of surface acoustic waves and photogenerated carriers in GaN has been used for the fabrication of a remote ultraviolet detector where the carrier collector electrode is far away from the illuminated region. In this device, the recombination of the photogenerated carriers at the region where they are created is prevented by the potential fields associated with the acoustic wave, and the carriers are swept by the acoustic wave to the collector electrode. This effect is strongly dependent on the frequency and power of the acoustic waves and therefore of the input radio frequency signal. New optoelectronic devices based on the combination of the acoustic and electronic properties of the semiconductors can be envisaged.

Interaction of surface acoustic waves and ultraviolet light in ZnO films

The frequency response of a 37 MHz bulk LiNbO3 surface acoustic wave (SAW) filter with a 200-nm-thick ZnO overlayer exhibited a downshift in the frequency with ultraviolet (UV) light due to acoustoelectric interactions between the photo-generated carriers in the semiconducting ZnO and the surface acoustic waves. In contrast, a 36 MHz ZnO thin film SAW delay-line with insulating ZnO films exhibited an upshift in the frequency. The response was more pronounced at higher harmonics (130–315 MHz) and was attributed to changes in the elastic/dielectric properties in the upper surface layer of ZnO. A linear change in the frequency with UV intensity shows immense applicability for wireless ultraviolet sensor applications.

Finite element analysis of Rayleigh wave interaction with finite-size, surface-breaking cracks

The interaction of surface acoustic waves with finite-size, surface-breaking, semi-circular cracks is studied numerically, and experimentally. We focus on the behavior of the reflection coefficient of the Rayleigh wave from such cracks in the far field of the crack, when the depth of the crack is comparable to the wavelength of the interrogating surface wave. The cases in which the depth of the crack is much smaller or much larger compared to the wavelength have been extensively investigated by many authors and are not considered here except for validating the numerical and experimental results in these regimes. The theoretical, finite element, and experimental results presented are in very good agreement over the range were the crack depth is much smaller or much larger compared to the wavelength of the incident Rayleigh wave. In the transition regime, between these two limiting cases, only the finite element and experimental data show good agreement since the theoretical predictions are no longer applicable. In the high crack depth to wavelength ratio ( a/ λ R) regime, the finite element and experimental results close to the crack approach the limiting value of the reflection coefficient from a 90° corner.

Real-time, frequency-translated holographic visualization of surface acoustic wave interactions with surface-breaking defects

A real-time, frequency-translated holographic imaging system has been developed by use of bacteriorhodopsin film. The system provides a capability for imaging surface acoustic waves and has been utilized to detect and characterize surface-breaking defects through near-field ultrasonic scattering effects. Frequency-plane filtering was used to discriminate between ultrasonic standing-wave and near-field scattering features, dramatically enhancing the holographic visualization of the defect sites. A detailed description of the system is presented, along with representative holographic images showing the interaction of surface acoustic waves with surface-breaking cracks and small notches in aluminum and titanium substrates.

Fabrication and characterization of PZT/TiOx/SiO2/SiNx/SiO2/Si structure for acousto-optic device applications

In this study, we fabricated the PZT/TiOx/SiO2/SiNx/SiO2/Si structure for acousto-optic (AO) device applications using the interaction of surface acoustic wave (SAW) and optical beam. SAW was generated using the inter-digital transducer (IDT) on PZT film, and optical beam (He-Ne laser) was guided in SiNx film using prism-coupling method. Two SiO2 layers were used for optical cladding layer, and TiOx for the buffer layer to prevent the inter-diffusion between PZT and SiO2 layers. In this structure, we measured insertion loss of - 7dB at minimum point, resonance frequency of 17.6 MHz and sound wave velocity of 8800 m/s under impedance matching condition. We also measured optical propagation loss of the SiNx-film in both SiNx/SiO2/Si and PZT/TiOx/SiO2/SiNx/SiO2/Si structure for confirming the applicability of SiNx film to the waveguide layer.

Interaction of Surface Acoustic Waves, Electrons, and Light

The interaction of surface acoustic waves with free carriers in semiconductor nanostructures has turned out to yield a powerful tool not only for the investigation of the dynamic conductivity of such quantum systems. The latter has been shown in the study of the dynamics of the fractional and integer quantum Hall effect and many other interesting physical phenomena. The interaction is based on a relaxation type and impedance matching effect. However - to make practical use of this strong interaction, the electromechanical coupling coefficients of state-of-the-art semiconductor layered systems are too small. A hybrid technique, merging the strong piezoelectricity of LiNbO3 or similar substrates with the excellent electronic properties of band gap engineered semiconductor quantum wells tackles this problem. Based on this new hybridization technique, several acoustoelectric high frequency devices have been realized. But also optically generated free electrons and holes in a semiconductor efficiently interact with the piezoelectric fields and potentials accompanying the surface wave. Those are able to field-ionize optically generated excitons leading to an acoustically induced quenching of the photoluminescence of a semiconductor quantum well, and to a system in which photonic signals can be efficiently converted into spatially separated electrons and holes which then can be transported over macroscopic distances along the quantum well. Finally - at a predetermined time and location on the sample – they can be reassembled into photonic signals. But also much simpler photonic devices can be realized using surface acoustic waves on semiconductor samples. For instance, we report on a simple, yet efficient camera type of device for pattern recognition and image processing.

Advances in Surface Acoustic Wave Technology, Systems and Applications - 2

Coupling-of modes analyis of SAW devices, V. Plessky and J. Koskela theory and applications of Green's functions, A.R. Baghai-Wadji new piezoelectric substrates for SAW devices, J. Kosinski pseudo and high velocity pseudo SAWs, M.P. da Cunha SAW devices beyond 5 GHz, H. Odagawa and K. Yamanouchi wireless SAW identification and sensor systems, F. Schmidt and G. Scholl interaction of surface acoustic waves, electrons and light, A. Wixforth.

INTERACTION OF SURFACE ACOUSTIC WAVES, ELECTRONS, AND LIGHT

The interaction of surface acoustic waves with free carriers in semiconductor nanostructures has turned out to yield a powerful tool not only for the investigation of the dynamic conductivity of such quantum systems. The latter has been shown in the study of the dynamics of the fractional and integer quantum Hall effect and many other interesting physical phenomena. The interaction is based on a relaxation type and impedance matching effect. However - to make practical use of this strong interaction, the electromechanical coupling coefficients of state-of-the-art semiconductor layered systems are too small. A hybrid technique, merging the strong piezoelectricity of LiNbO 3 or similar substrates with the excellent electronic properties of band gap engineered semiconductor quantum wells tackles this problem. Based on this new hybridization technique, several acoustoelectric high frequency devices have been realized. But also optically generated free electrons and holes in a semiconductor efficiently interact with the piezoelectric fields and potentials accompanying the surface wave. Those are able to field-ionize optically generated excitons leading to an acoustically induced quenching of the photoluminescence of a semiconductor quantum well, and to a system in which photonic signals can be efficiently converted into spatially separated electrons and holes which then can be transported over macroscopic distances along the quantum well. Finally - at a predetermined time and location on the sample – they can be reassembled into photonic signals. But also much simpler photonic devices can be realized using surface acoustic waves on semiconductor samples. For instance, we report on a simple, yet efficient camera type of device for pattern recognition and image processing.