Surface Acoustic Wave Field Research Articles

Forward and inverse problems for surface acoustic waves in anisotropic media: A Ritz–Rayleigh method based approach

We show that the Ritz–Rayleigh method can be used for calculation of velocity of surface acoustic waves (SAWs) propagating in a general direction of an anisotropic medium of arbitrary symmetry class. The main advantage of this method is that expanding the displacement field of SAW into a fixed functional basis transforms the calculation of SAW velocities into a simple linear eigenvalue problem. The correctness and reliability of the proposed approach are verified on experimental SAW data obtained for generally oriented planes of an indium phosphide single crystal. The same experimental datasets are then used to discuss the invertibility of the method, i.e. the possibility of determination of elastic coefficients from SAW measurements in general directions. It is shown that the SAW data obtained on a single generally oriented plane are sufficient for such an inverse calculation for a cubic material only if they are complemented by measurements of velocities of bulk quasi-longitudinal (qL) waves propagating along the same free surface. Moreover, when the SAW and qL data are available from three almost perpendicular faces of a single specimen, the complete elastic tensor (21 independent constants) can be inversely determined, without considering a priori any symmetry constraints to the material.

VERIFICATION OF THE HELIOSEISMOLOGY TRAVEL-TIME MEASUREMENT TECHNIQUE AND THE INVERSION PROCEDURE FOR SOUND SPEED USING ARTIFICIAL DATA

We performed three-dimensional numerical simulations of the solar surface acoustic wave field for the quiet Sun and for three models with different localized sound-speed perturbations in the interior with deep, shallow, and two-layer structures. We used the simulated data generated by two solar acoustics codes that employ the same standard solar model as a background model, but utilize different integration techniques and different models of stochastic wave excitation. Acoustic travel times were measured using a time-distance helioseismology technique, and compared with predictions from ray theory frequently used for helioseismic travel-time inversions. It is found that the measured travel-time shifts agree well with the helioseismic theory for sound-speed perturbations, and for the measurement procedure with and without phase-speed filtering of the oscillation signals. This testing verifies the whole measuring-filtering-inversion procedure for static sound-speed anomalies with small amplitude inside the Sun outside regions of strong magnetic field. It is shown that the phase-speed filtering, frequently used to extract specific wave packets and improve the signal-to-noise ratio, does not introduce significant systematic errors. Results of the sound-speed inversion procedure show good agreement with the perturbation models in all cases. Due to its smoothing nature, the inversion procedure may overestimate sound-speed variations in regions with sharp gradients of the sound-speed profile.

Principles of X-ray diffractometry of surface acoustic waves

Theoretical fundamentals of modern methods of X-ray diffractometry of surface acoustic waves (SAW) are considered briefly. X-ray diffraction on SAW-modulated crystals under total external reflection conditions and the Bragg conditions for the YZ-cut of the LiNbO3 crystal is considered. Agreement of theoretical and experimental results makes it possible to use them for SAW diagnostics. Possibilities and limitations of listed methods for determining the SAW field parameters are discussed.

Separation of Escherichia coli bacteria from peripheral blood mononuclear cells using standing surface acoustic waves.

A microfluidic device was developed to separate heterogeneous particle or cell mixtures in a continuous flow using acoustophoresis. In this device, two identical surface acoustic waves (SAWs) generated by interdigital transducers (IDTs) propagated toward a microchannel, which accordingly built up a standing surface acoustic wave (SSAW) field across the channel. A numerical model, coupling a piezoelectric effect in the solid substrate and acoustic pressure in the fluid, was developed to provide a better understanding of SSAW-based particle manipulation. It was found that the pressure nodes across the channel were individual planes perpendicular to the solid substrate. In the separation experiments, two side sheath flows hydrodynamically focused the injected particle or cell mixtures into a very narrow stream along the centerline. Particles flowing through the SSAW field experienced an acoustic radiation force that highly depends on the particle properties. As a result, dissimilar particles or cells were laterally attracted toward the pressure nodes at different magnitudes, and were eventually switched to different outlets. Two types of fluorescent microspheres with different sizes were successfully separated using the developed device. In addition, Escherichia coli bacteria premixed in peripheral blood mononuclear cells (PBMCs) were also efficiently isolated using the SSAW-base separation technique. Flow cytometric analysis on the collected samples found that the purity of separated E. coli bacteria was 95.65%.

Acousto-electric transport in epitaxial monolayer graphene on SiC

We report on the piezoelectric excitation and acoustic charge transport by gigahertz surface acoustic waves (SAWs) in epitaxial monolayer graphene (EG) on SiC. The GHz SAWs frequencies were generated by interdigital transducers fabricated on a piezoelectric island on the SiC substrate. Acoustic transport studies in a Hall bar geometry show that the propagating SAW field transports carriers in EG, the transport direction being determined by the direction of the acoustic beam. Carrier transport is driven by drift in the piezoelectric field induced by the SAW in EG.

SURFACE-ACOUSTIC-WAVE FIELD AND ACOUSTO-OPTIC INTERACTION IN AlN/128-DEG-ROTATED Y-CUT X-PROPAGATION LiNbO3

The efficiency of guided-wave acousto-optic (AO) interaction in AlN /128-deg-rotated Y-cut X-propagation lithium niobate (128-deg YX- LiNbO 3) structure is analyzed theoretically for the first time by determining the overlap integral between the optical and the surface acoustic wave (SAW) field distribution. The results show that the use of an AlN film can increase the phase velocity of SAW, the electromechanical coupling coefficient and the diffraction efficiency of AO interaction. A maximum of 9.33% for the electromechanical coupling coefficient is obtained when the normalized thickness of AlN film equals 0.09. The diffraction efficiency has a significant improvement when the normalized thickness of AlN film is increased from 0 to 0.05. And, the improvement for the TM polarization is more evident than that for the TE polarization. However, for a well-concentrated optical waveguide, the use of an AlN film reduces the diffraction efficiency of the TM polarization when the SAW frequency is low.

On-chip manipulation of single microparticles, cells, and organisms using surface acoustic waves

Techniques that can dexterously manipulate single particles, cells, and organisms are invaluable for many applications in biology, chemistry, engineering, and physics. Here, we demonstrate standing surface acoustic wave based "acoustic tweezers" that can trap and manipulate single microparticles, cells, and entire organisms (i.e., Caenorhabditis elegans) in a single-layer microfluidic chip. Our acoustic tweezers utilize the wide resonance band of chirped interdigital transducers to achieve real-time control of a standing surface acoustic wave field, which enables flexible manipulation of most known microparticles. The power density required by our acoustic device is significantly lower than its optical counterparts (10,000,000 times less than optical tweezers and 100 times less than optoelectronic tweezers), which renders the technique more biocompatible and amenable to miniaturization. Cell-viability tests were conducted to verify the tweezers' compatibility with biological objects. With its advantages in biocompatibility, miniaturization, and versatility, the acoustic tweezers presented here will become a powerful tool for many disciplines of science and engineering.

Programmable manipulation of microbubbles in microfluidic systems with surface acoustic wave

Programmable microfluidic systems for bioparticles manipulation that could enable automated biological analysis and diagnostics have the potential to revolutionize a wide range of applications in life sciences research, clinical diagnostics, and drug discovery. Several methods such as those based on hydrodynamic force, magnetic tweezers, and dielectrophoresis can manipulate the movement of the bioparticles successfully. However, these methods must be constructed the flow structure, magnetic wire, and electrode in the microchannel respectively, which not only limits the flexibility of manipulation but also makes the sample more likely to be contaminated. In this paper, a programmable microfluidic device is developed to manipulate the microbubbles along an arbitrary trajectory in the microchannel without any structure by introducing phase-shifts to a planar standing surface acoustic wave field. The microbubbles can be transported in a square trajectory, circular trajectory and even helix trajectory via adjusting the relative phase between the excitation signals controlled by LabVIEW program. The results reveal that the microbubbles can be transported over a predetermined distance continuously until they reach the targeted locations. This acoustic programmable microfluidic device would have potential applications on drug screening, cell studies, and other biomedical applications.

Tunable patterning of microparticles and cells using standing surface acoustic waves.

We have developed an acoustic-based tunable patterning technique by which microparticles or cells can be arranged into reconfigurable patterns in microfluidic channels. In our approach, we use pairs of slanted-finger interdigital transducers (SFITs) to generate a tunable standing surface acoustic wave field, which in turn patterns microparticles or cells in one- or two-dimensional arrays inside the microfluidic channels--all without the assistance of fluidic flow. By tuning the frequency of the input signal applied to the SFITs, we have shown that the cell pattern can be controlled with tunability of up to 72%. This acoustic-based tunable patterning technique has the advantages of wide tunability, non-invasiveness, and ease of integration to lab-on-a-chip systems, and shall be valuable in many biological and colloidal studies.

Efficient photon detectors using surface acoustic waves

An efficient photon detector based on the ambipolar transport of photogenerated electrons and holes by surface acoustic waves (SAWs) has been investigated. In the experiments, the photogenerated electrons and holes are separated and transported in a GaAs channel and electrically detected by a lateral p–i–n junction. We show that the acoustic transport efficiency improves by using biased metallic guides along the SAW beam to create independent transport channels for electrons and holes. By optimizing the photon absorption efficiency and the amplitude of the SAW field, we demonstrated overall transport efficiencies above 85% for transport lengths exceeding 200 μ m .

Improvement of Detection System for a Fast-Scanning and Phase-Sensitive Laser Probe for Surface Acoustic Wave Devices in 2 GHz Range

This paper describes recent advance in the detection system for the fast-scanning and phase-sensitive laser probing system for surface acoustic wave (SAW) devices operating in a 2 GHz range. Electric and electro-optic components were examined, and high sensitivity and high signal–noise ratio were achieved in a 2 GHz range without reducing the signal bandwidth. The developed detection system was installed to the laser probe system, and its performance was demonstrated by the SAW field visualisation of a shear-horizontal SAW device operating in a 2 GHz range.

Surface electrical charge and field analysis of single-electrode type interdigital transducers with bus-bar

A single-electrode type interdigital transducer (SIDT) is extensively used to excite surface acoustic waves (SAWs) on piezoelectric substrates because of its simple structure and fabrication process. In a SIDT a bus-bar is used for feeding in electrical voltage. However, systematic analyses about bus-bar effects of SIDT are still sparse among the existing studies concerning the propagation of acoustic wave of SIDT. In this paper, a two-dimensional interface Green's function method is adopted for calculating the electrical charge density on the electrode surfaces and acoustic wave fields in a Y-cut Z-propagation LiNbO3 substrate excited by SIDTs with a bus-bar. The computation results show that the charge density associated with SAW cannot be neglected; the bus-bar has essential effects on the excited SAW fields and the SAW field generally diverges strongly due to the influence of the bus-bar. Approaches for reducing this effect are discussed.

Field analysis and calculation of interdigital transducers with arbitrary finger shapes

The interdigital transducer (IDT) is a key component in surface-acoustic wave (SAW) and acousto-optical devices and has extensive applications in signal processing and optical communication at present. Properties of the acoustic field are mainly dominated by the geometric shapes of the electrodes (i.e. fingers) of the IDT and the piezoelectric characteristics of the substrate. However, the studies on excitation and acoustic wave field characteristics of IDTs are still not matured or perfect. In this paper, the 2D interface Green's function method (GFM) for simulating numerically the SAW field of IDTs with arbitrary finger shapes is presented. The electric charge densities originated from the electrostatic field and the generated SAW on the IDT electrodes are calculated first using the 2D interface GFM. Then the charge density distribution as the distributed source of SAW used to calculate the SAW field. IDT with arbitrary finger shapes can be treated. As an example, the properties of a SAW field generated by focused IDTs with the shape of concentric circular arc fingers on Y–Z LiNbO3 and c-oriented PZT substrates are also analysed and discussed. The method can be applied to numerical analysis of IDTs with arbitrary finger shapes, such as broadband chirp transducers, curved-finger transducers and finger-length weighted transducers.

Focusing of Surface Acoustic Wave on a Piezoelectric Crystal

We investigate the focusing phenomena of a surface acoustic wave (SAW)field generated by a circular-arc inter-digital transducer (IDT) on apiezoelectric crystal. A rigorous vector field theory of surfaceexcitation on the crystal we developed previously is used to evaluatethe convergent SAW field instead of the prevalent scalar angularspectrum used in optics. The theoretical results show that theanisotropy of a medium has great impact on the focusing properties ofthe acoustic beams, such as focal length and symmetrical distributionsnear the focus. A dark field method is used in experiment to observe thefocusing of the SAW field optically. Although the convergent phenomena ofSAW field on the anisotropic media or piezoelectric crystals are verycomplicated, the experimental data are in agreement with those from therigorous theory.

A numerical method for calculating the dynamic stress in SAW devices

SAW-induced stress-migration, also known as acoustomigration is a well-known phenomenon in the field of surface acoustic wave (SAW) devices. In order to improve the power durability of SAW devices a better understanding of the acoustomigration mechanism as well as a detailed knowledge of the stress distribution in the SAW devices is essential. We present a numerical method to calculate the absolute values of the dynamic stress in the metallization for a given driving condition and frequency at each point in the metal structure. To present the device we use a P-matrix based model, which yields the acoustic power distribution and the resulting energy density in the device for a given driving level. For the calculation of the relative stress values we use the finite element method. To calculate quantitatively the distribution of the stress values we use the relationship between the energy density distribution and the sum of the strain–stress products. This method is a very flexible tool for the stress analysis and the detailed investigation of the driving forces of acoustomigration and the development of power durable SAW devices.

Investigation of surface acoustic wave fields in silicon crystals by x-ray diffraction: A dynamical theory approach

X-ray diffraction spectra at different x-ray energies from a Si crystal subjected to a deformation produced by surface acoustic wave propagation have been modeled using the general framework of dynamical diffraction theory. The simulations have been successfully compared with the corresponding experimental results confirming the accuracy of the elastic model describing the acoustic wave fields inside the crystal.

Stern–Gerlach spin filter using surface acoustic waves

We propose the ambipolar carrier transport by surface acoustic waves (SAWs) in a semiconductor quantum well (QW) for the realization of the Stern–Gerlach (SG) experiment in the solid phase. The well-defined and very low carrier velocity in the moving SAW field leads to a large deflection angle and thus to efficient spin separation, even for the weak field gradients and short (μm-long) interaction lengths that can be produced by micromagnets. The feasibility of a SG spin filter is discussed for different QW materials.

Band mixing and ambipolar transport by surface acoustic waves in GaAs quantum wells

The interaction of strong surface acoustic wave (SAW) fields with the electronic band structure of GaAs quantum wells (QW's) is investigated using spatially resolved photoluminescence (PL) spectroscopy. The optical studies are accompanied by k.p and tight-binding (TB) calculations of the SAW effects on the electronic band structure. The SAW induces a time-dependent coupling between the heavy- (hh) and light-hole (1h) states in the valence band of the QW's, which leads to an anticrossing of their energy levels for high SAW intensities. The coupling alters the strength and polarization of the optical transitions and can be reproduced by calculations of the optical transition matrix elements. Spatially resolved PL measurements of the SAW-induced ambipolar transport of electrons and holes provide evidence of a reduction of the transport efficiency for high SAW fields, which is attributed to a decrease of the hole mobility as the hh and 1h levels approach each other. This conclusion is supported by TB calculations that show a significant enhancement of the heavy-hole effective mass under these conditions. In addition, the mobility may also be reduced by the squeezing of the wave functions towards the QW interfaces induced by strong piezoelectric fields, which makes the transport more sensitive to potential fluctuations induced by interface roughness and defects in the barrier layers.

X-ray Bragg diffraction from langasite crystal modulated by surface acoustic wave

X-ray diffraction on the X cut of a langasite crystal (La3Ga5SiO14) modulated by a Λ=12 μm Rayleigh surface acoustic wave (SAW) has been studied at the ESRF synchrotron radiation source. Due to the sinusoidal modulation of the crystal lattice involved by the SAW diffraction satellites appear on the rocking curve, with their number, angular positions, and intensities depending on the amplitude and wavelength of the ultrasonic superlattice. Full extinction of a specific satellite could be performed by adjusting the acoustic amplitude. It is shown that x-ray diffraction can be used to study surface acoustic wave field distributions in crystals.

Spatio-temporal gradient signal processing for reconstructing scattered wave fields

This paper presents a subspace signal processing method which detects the scattered sound waves based on the spatio-temporal gradient analysis over the Lamb wave field. The proposed method has an ability to classify the surface acoustic wave field through the rank of the covariance matrix defined over the four-dimensional vector space which is spanned by the following components: a vertical displacement, its vertical velocity, and a pair of shear strains of the plate surface. The covariance matrix provides the information about cracks. When a unique sound source exists on the surface without any cracks or scatterers, the rank of the covariance matrix is reduced to two. Increasing the rank up to three, however, one or more scatterers exist on the surface. Therefore, by the singular value decomposition (SVD) of the covariance matrix, optimal, orthonormal bases are obtained. In formulating the scattered wave field by the unknown object, its best reconstruction, in the LMS sense, can be obtained by the sum of orthogonal vector spaces. The computational process in a wave field near the cracks is discussed and their physical meanings are investigated through DTD-simulations and acoustic experiments.