Reflection Of Surface Acoustic Waves Research Articles

A high-performance lab-on-a-chip liquid sensor employing surface acoustic wave resonance: part II

We recently introduced an in-liquid sensing concept based on surface acoustic resonance (SAR) in a lab-on-a-chip resonant device with one electrical port. The 185 MHz one-port SAR sensor has a sensitivity comparable to other surface acoustic wave (SAW) in-liquid sensors, while offering a high quality factor (Q) in water, low impedance, and fairly low susceptibility to viscous damping. In this work, we present significant design and performance enhancements of the original sensor presented in part I. A novel ‘lateral energy confinement’ (LEC) design is introduced, where the spatially varying reflectivity of the SAW reflectors enables strong SAW localization inside the sensing domain at resonance. An improvement in mass-sensitivity greater than 100% at resonance is achieved, while the measurement noise stays below 0.5 ppm. Sensing performance was evaluated through real-time measurements of the binding of 40 nm neutravidin-coated SiO2 nanoparticles to a biotin-labeled lipid bilayer. Two complementary sensing parameters are studied, the shift of resonance frequency and the shift of conductance magnitude at resonance.

Development of chipless, wireless current sensor system based on giant magnetoimpedance magnetic sensor and surface acoustic wave transponder.

A chipless, wireless current sensor system was developed using a giant magnetoimpedance (GMI) magnetic sensor and one-port surface acoustic wave (SAW) reflective delay line for real-time power monitoring in a current-carrying conductor. The GMI sensor has a high-quality crystalline structure in each layer, which contributes to a high sensitivity and good linearity in a magnetic field of 3–16 Oe. A 400 MHz RF energy generated from the interdigital transducer (IDT)-type reflector on the one-port SAW delay line was used as an activation source for the GMI magnetic sensor. The one-port SAW delay line replaces the presently existing transceiver system, which is composed of thousands of transistors, thus enabling chipless and wireless operation. We confirmed a large variation in the amplitude of the SAW reflection peak with a change in the impedance of the GMI sensor caused by the current flow through the conductor. Good linearity and sensitivity of ~0.691 dB/A were observed for currents in the range 1–12 A. Coupling of Mode (COM) modeling and impedance matching analysis were also performed to predict the device performance in advance and these were compared with the experimental results.

Exciton-polariton gap soliton dynamics in moving acoustic square lattices

The modulation by a surface acoustic wave (SAW) provides a powerful tool for the formation of tunable lattices of exciton-polariton macroscopic quantum states (MQSs) in semiconductor microcavities. The MQSs were resonantly excited in an optical parametric oscillator configuration. We investigate the temporal dynamics of these lattices using time and spatially resolved photoluminescence (PL). Photoluminescence images of the MQSs clearly show the motion of the lattice at the acoustic velocity. Interestingly, the PL intensity emitted by the MQSs as well as their coherence length oscillate with the position of the lattice sites relative to the exciting laser beam. The coherence length and the PL intensity are correlated. The PL oscillation amplitude depends on both the intensity and the size of the exciting laser spot and increases considerably for excitation intensities close to the optical threshold power for the formation of the MQS. The oscillations are explained by a model that takes into account the combined effects of SAW reflections, which dynamically distort the amplitude of the potential, and the spatial phase of the acoustic lattice within the exciting laser spot. This paper could pave the way to tailor polariton-based light-emitting sources with intensity variations controlled by the SAWs.

Development of a highly effective multi-stage surface acoustic wave SU-8 microfluidic concentrator

As an important step in sample preparation, sample concentration step helps to improve sampling techniques that are often necessary to detect low levels of pathogenic bacteria in food safety inspections, reducing or completely eliminating the need for time-consuming cell enrichment process for detection. In this study, we investigated strategies to significantly enhance performances of a surface acoustic wave (SAW) microfluidic device to concentrate various particles, including microbeads, bacterial and mammalian cells in suspensions. A low acoustic energy loss material, SU-8, was used to fabricate the microfluidic concentrator, and SAW reflectors were designed to efficiently use the SAW generated acoustic energy for particle concentration. The concentration performance was further improved by using a two-stage concentration design. We demonstrated that the SU-8 microfluidic concentrator with a two-stage concentration configuration was able to concentrate 2.00μm bead samples at an overall concentration factor of ∼65-fold with a particle recovery efficiency of 81.9±5.9% at a inflow rate of 300μl/h. Similarly, when the concentrator was used to concentrate E. coli DH5α and NIH/3T3 cells, overall concentration factors of ∼59 and 75-fold with particle recovery efficiencies of 81.0±17.2% and 90.8±5.0% were obtained, respectively. These results show significant improvements upon those previously reported for either bacterial or mammalian cell concentration applications. In addition, the device fabrication process developed in this work is fully compatible with regular microfabrication processes, making this microfluidic concentrator readily integrable with other on-chip functionalities for cell manipulation and detection applications.

Reflection of surface acoustic waves in the bulk of a hexagonal single crystal

It has been shown that surface acoustic waves in some hexagonal crystals can include not only the waves outgoing from the surface into the bulk of the crystal but also the reflected waves, i.e., those coming from the bulk of an infinite single crystal. The outgoing and reflected waves decay exponentially with the distance from the surface. It has been found that the reflected wave can exist if the velocity of its propagation is below the critical value, which does not exist for some crystals. The numerical calculations have shown in what real hexagonal crystals the reflected wave can exist. The values of the critical velocity have been found for a number of hexagonal crystals.

Anisotropy effects in the reflection of surface acoustic waves from obstacles

The paper studies the difference between the reflection coefficients of a surface acoustic wave incident on a groove or strip along the inverse directions. This difference occurs because of the crystallographic anisotropy of the substrate and/or the asymmetry of the shape of the obstacle. The numerical computations are carried out by the finite element method. A perfectly matched layer is implemented to truncate the computational domain. Some general properties of the transmission and reflection coefficients of surface acoustic waves are also discussed.

Enhancement of fatigue crack monitoring by surface acoustic wave reflection and modulation in a space-cycle-load domain: an imaging approach

The surface wave acoustic method is applied to the in situ ultrasonic monitoring (during fatigue) of fatigue crack initiation and evolution. A small low-frequency periodic loading is also applied, resulting in a nonlinear modulation of reflected pulses. The acoustic wave reflections are collected for: each experimental cycle; a range of applied fatigue and modulation load levels; and a range of spatial propagation positions, and are presented in image form to aid pattern identification. Salient features of the image are then extracted and processed to evaluate the initiation time of the crack, its opening/closing behavior and its subsequent size evolution until sample failure. This image representation improves the visibility of the small-crack reflectivity pattern and thus enhances the detectability of small cracks.

Magnetically programmable surface acoustic wave radio frequency identification tags

A reconfigurable surface acoustic wave reflector using an integrated magnetoresistive bit was fabricated and evaluated for use in programmable radio frequency identification tags. It is shown that two distinct reflectivities can be achieved depending on the magnetic state of the bit. The experimental results are compared with theoretical calculations of optimal reflectivities achievable from resistively loaded surface acoustic wave transducers.

Surface acoustic wave scattering from steps, grooves, and strips on piezoelectric substrates

The paper studies, by the finite element method, the reflection of surface acoustic waves from single obstacles of regular shapes on the surface of piezoelectric materials. The so-called perfectly matched layer is used to truncate the computational domain. The following types of imperfections are considered: single steps, grooves, and projections, as well as metallic strips overlaying the substrate or inset into it. The absolute values and the phases of the reflection coefficients are computed for YZ and 128°YX LiNbO3 substrates as functions of the height-to-wavelength and the width-to-wavelength ratios. In addition, the reflectivity of gratings comprising a finite number of grooves or electrodes is computed and compared with the analytic estimations based on the coupling-of-modes theory.

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.

Surface acoustic waves propagating on microstructured phononic crystals

We investigate the interaction between high frequency surface acoustic waves and periodic microstructured patterns that form phononic crystals. The experimental method combines an optical pump-probe setup with interferometric detection and provides picosecond temporal and micron spatial resolutions. Surface acoustic waves with frequency components up to 1.3 GHz are imaged in real-time propagating over the periodic metamaterial. We used a DRIE (deep reactive ion etching) process to fabricate 2D air-silicon phononic crystals in the form of a square lattice. We present real-time animations of surface acoustic waves scattered by the phononic crystals. In particular we describe the frequency and angular dependence of the surface acoustic wave reflection from a 2D phononic crystal boundary. Fourier analysis allows us to reveal details of the acoustic band structure including gaps. The presence of such phononic band gaps enables us to visualize surface acoustic waves in waveguides, cavities and other phononic circuits at GHz frequencies.

3P2b-6 Imaging of surface acoustic wave reflection and focusing using circular, ellipsoidal and parabolic boundaries(Poster Session)

3P2b-6 Imaging of surface acoustic wave reflection and focusing using circular, ellipsoidal and parabolic boundaries(Poster Session)

Technique and results of measuring surface-acoustic-wave reflection coefficients with the use of laser sounding

A technique of measuring the surface-acoustic-wave (SAW) reflection with the use of a laser sounding facility containing a reference diffraction grating is described. The characteristics of the SAW reflections from the edges of YX-cut quartz-crystal substrates with the end faces beveled at various angles are measured. In addition, the frequency dependence of the magnitude of the SAW reflection coefficient for reflection from periodic gratings in the form of lithium niobate metal strips are measured.

Examination of surface acoustic wave reflections by observing acoustoelectric current generation under pulse modulation

The authors investigate surface acoustic wave (SAW) reflections using the frequency dependence of the acoustoelectric current through a one-dimensional channel. By pulse modulating the transducer power with a short pulse width (0.2μs) and matching the pulse repetition period to the delay time of a specific reflection path, the authors detect individual reflected SAW wave packets. Fourier transform results show that reflections from the sample edge behind the transducer and Bragg reflections from the second transducer have the strongest effects. The reflection power coefficients for these are estimated to be ≈0.29 and ≈0.99, respectively.

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.

P1-36 Visualization of the reflection of surface acoustic waves at surface discontinuities by the photoelastic method

P1-36 Visualization of the reflection of surface acoustic waves at surface discontinuities by the photoelastic method

Second-harmonic reflectors on 128 degrees LiNbO3.

In this work, we study theoretically the operation of long surface acoustic wave reflectors, comprising a large number of electrodes, at the fundamental and second harmonic frequencies on the 128 degrees LiNbO3 substrate for various electrode thicknesses and metallization ratios. Numerical simulations utilizing tailored test structures and time gating indicate that the reflectivity of the second-harmonic reflectors can be very high for certain geometries. Furthermore, our simulations suggest that inside the stopband the total losses for the second harmonic are of the same order as those for operation at the fundamental harmonic.

Micromechanical precision pressure sensor incorporating SAW delay lines

We have developed a wireless surface acoustic wave (SAW) pressure sensor operating in the pressure range of 0 Pa to 250 kPa. In order to minimize the temperature sensitivity the pressure sensor is made of on an all-quartz package (AQP), which has been designed with the Finite Element Method. The package of the pressure sensor consists of a diaphragm and a cover, both made of conventional Y-cut quartz. A blind-hole was structured into the sensor cover. By attaching the cover and the diaphragm with an epoxy-adhesive, this blind-hole forms a closed cavity. The SAW element is a reflective delay line (RDL), working at 434 MHz. The RDL consists of ten reflectors and extends over the whole diaphragm. The pressure is determined by evaluating the change of the carrier phase shifts of the reflected impulses at the reflectors. We show that it is possible to minimize the temperature sensitivity and to achieve good linearity by proper positioning of the SAW reflectors. The measurements of the SAW pressure sensor show a deviation from linearity of less than ±0,7%. The temperature dependence is almost negligible in the range from-20°C to 100°C. The objective of this paper is to provide a deeper insight into the behaviour of SAW propagation on pre-stressed substrates. To do so, we start with investigating the behavior of SAWs on stress-free substrates followed by an analysis of SAW propagation on pre-stressed substrates. Further, the requirements on suitable substrate materials for the AQP are specified. Finally, we take advantage of the method of differences to compensate for the temperature dependence of our pressure sensor.

SAW-based radio sensor systems

Surface acoustic wave (SAW) devices can be used as identification and sensor elements (SAW transponders) for mea- suring physical quantities such as temperature, pressure, torque, acceleration, humidity, etc., that do not need any power supply and may be accessed wirelessly. The complete wireless sensor system consists of such a SAW transponder and a local radar transceiver. An RF burst transmitted by the radar transceiver is received by the antenna of the SAW transponder. The passive transponder responds with an RF signal—like a radar echo—which can be received by the front-end of the local transceiver. Amplitude, frequency, phase and time of arrival of this RF response signal carry information about the SAW reflection and propagation mechanisms which in many cases can be directly attributed to the sensor effect for a certain measurand. Usually no intersymbol interference (ISI) due to environmental echoes occur, due to the high delay time of the SAW transponder in the order of some s. The present work reviews the operating principles of such sensor systems and their state-of-the-art performance by way of some examples which include the wireless measurement of temperature, pressure, torque, acceleration, tire-road friction, magnetic field, and water content of soil. Index Terms—Acceleration, local radar transceiver, magnetic field, passive SAW transponder for sensing, pressure, temper- ature, tire-road friction, torque, water content of soil, wireless measurement.

Effects of Dielectric Overlay upon Surface Acoustic Wave Propagation under Metallic Grating Structure

This paper describes surface acoustic wave (SAW) propagation under a periodic metallic grating structure overlaid with an SiO2 film. An analysis was made by modifying the software FEMSDA, in which the finite element method is applied to both the grating electrodes and SiO2 overlay. For Rayleigh-type SAW propagating on 128°YX-LiNbO3, changes in the reflection coefficient and effective velocity are calculated as a function of the SiO2 film thickness overlaying an Al grating structure. It is shown that the SiO2 film overlaying the top surface of the grating electrodes affects both the SAW reflection coefficient and effective SAW velocity more significantly than the film overlaying the electrode gaps.