Surface Acoustic Wave Velocity Research Articles

Metamaterial control of the surface acoustic wave streaming jet

The phenomenon of surface acoustic wave (SAW) streaming, where a streaming jet is created, occurs when an SAW propagating on the surface of a solid interacts with water, and underpins the increasingly important area of SAW microfluidics. A key characteristic of the streaming jet is the Rayleigh angle, i.e. the angle at which the jet is formed relative to the surface normal of the solid, which is determined by the ratio of the velocity of the acoustic wave in the fluid and in the solid. Although the ability to dynamically tune this angle would offer a novel tool for microfluidic control, the SAW velocity is normally fixed by the characteristics of the solid and liquid material properties. In this paper we show, using finite element method modelling, that changing the SAW Rayleigh wave phase velocity by patterning a metamaterial array, consisting of square annular holes, onto the surface of an SAW device can change the acoustic streaming Rayleigh angle by approximately a factor of two, in good agreement with calculations based on the change in velocity.

Remote measurement of material elastic property at elevated temperature with grating laser ultrasonic testing

ABSTRACT As a pivotal elastic property, elastic modulus plays a crucial role in characterising the mechanical performance of materials. To achieve the convenient and remote measurement of the elastic modulus of materials at elevated temperatures, a method based on diffracted grating laser-induced narrowband surface acoustic waves (SAW) is proposed. The design scheme of the diffraction grating is disclosed, and a CCD camera is deployed to confirm the generated grating pattern and determine the actual grating period. Delving into the viability of an effortless and reliable procedure for calculating elastic modulus is discussed. The elastic modulus determined by the SAW does not depend on the exact Poisson’s ratio, which can be calculated from the SAW velocity with a given, less precise value for Poisson’s ratio. Experimental results of the three metals are in line with the reference values, and the elastic modulus at elevated temperature is successfully measured remotely, thereby verifying the feasibility of the proposed method.

Growth of ferroelectric lithium niobate-tantalate LiNb(1-x)TaxO3 crystals

Congruent and stoichiometric ferroelectric lithium niobate-tantalate crystals LiNb(1-x)TaxO3 of different compositions were grown by the Czochralski method. The stoichiometric crystals were grown using the top seeded solution growth method using LiWO4, which increases the Li content in the melt and in the grown crystal. Also, the use of a solvent can significantly reduce the temperature of the melt. The composition of the crystals was studied by inductively coupled plasma mass spectrometry using laser ablation, which allow studying the change in the composition of the grown crystals along the length and diameter. It is demonstrated that the content of Ta in the crystals exceeds the content of Ta in the initial charge, i.e., in the process of crystal growth, the melt becomes depleted in Ta. It is shown that the Ta content decreases along the crystal length due to the decrease in the Ta content in the melt. In the LiNb0.88Ta0.12O3 crystals, the piezoelectric moduli d22 and d33, and the velocities of bulk and surface acoustic waves were measured. The values of piezoelectric moduli and acoustic wave velocities are shown to occupy an intermediate position between LiNbO3 and LiTaO3 crystals.

Asynchronous STW resonators with the high quality factor and reduced sizes

Surface transverse waves (STW) on quartz have the high propagation velocity, which is higher by 60% than velocity of the Rayleigh surface acoustic waves (SAW), high temperature stability comparable with temperature stability of SAW on quartz. Those advantages allow successfully use STW in the high frequency resonators. However, a large number of the electrodes 400 in the reflectors is required to ensure the high quality factor of the resonators. This leads to increase the resonators sizes, especially on the frequencies < 1000 MHz. The 500-1000 MHz STW resonators with high quality factor on 36°YX90° cut quartz and reduced size were presented. Use of the asynchronous topology, constructional, topological and technological optimization with a computer simulation on the base of a equivalent circuit model allowed obtain in the frequency range of 500-1000 MHz the high quality factor of 8600-9500, reduced sizes of the topology in comparison with the known prototypes and place the resonators in the 3×3×1.2 mm and 5×5×1.8 mm SMD packages. The developed STW resonators with high quality factor and reduced sizes will be widely used in the miniature low noise oscillators.

The generation of large-amplitude surface acoustic waves induced by a moving laser source

This letter focuses on the non-contact generation of surface acoustic waves by using a continuous wave laser moving along the sample surface. The desired large-amplitude surface acoustic waves can be generated efficiently if the laser moving speed matches the Rayleigh value, which is a typical characteristic of the continuous laser moving excitation method. The different amplitudes of surface acoustic waves were generated at different laser moving speeds, and the largest amplitude occurred when the laser moving speed approached the surface acoustic wave velocity of the material. Subsequently, the influence of laser moving distance on the wave’s amplitude under Rayleigh resonance conditions was investigated. The surface acoustic waves’ amplitude grew linearly with the laser’s moving distance in the elastic range. In brief, this paper provides a novel method to excite large amplitude surface acoustic waves by simply changing the moving speed and moving distance of the light.

Improved Crystallinity of Annealed 0002 AlN Films on Sapphire Substrate

AlN is a piezoelectric material used in telecommunication applications due to its high surface acoustic wave (SAW) velocity, stability, and mechanical strength. Its performance is linked to film quality, and one method to achieve high-quality films goes through the process of annealing. Consequently, c-orientated AlN film with a thickness of 1.1 μm deposited on sapphire was annealed at temperatures of 1100 °C and 1150 °C in a N2 controlled atmosphere. This was compared to annealing at 1100 °C, 1450 °C, and 1700 °C with N2 flow in an open atmosphere environment. Sample rotation studies revealed a significant impact on the ⍵-2θ X-ray rocking curve. A slight variation in the film crystallinity across the wafer was observed. After the annealing, it was found that the lattice parameter c was increased by up to 2%, whereas the screw dislocation density dropped from 3.31 × 1010 to 0.478 × 1010 cm-2, and the full width at half maximum (FWHM) of reflection (0002) was reduced from 1.16° to 0.41° at 1450 °C. It was shown that annealing in a N2-controlled atmosphere plays a major role in reducing the oxidation of the AlN film, which is important for acoustic wave devices where the electrodes are placed directly on the piezoelectric substrate. The face-to-face arrangement of the samples could further reduce this oxidation effect.

Measurement of temperature dependent shear modulus using frequency dependent SAWs ToF delay in laser-induced temperature gradient

Phase velocity of surface acoustic waves(SAWs) is frequency dependent when propagate on medium with inhomogeneous elasticity over depth. In this work, frequency dependent Time-of-Flight (ToF) variation of SAWs induced by a local and dynamic heating was applied for measurement of temperature dependent shear modulus. Laser-generated broad-band SAWs propagated through the material with elastic properties and density modified by dynamic inhomogeneous temperature field induced by a millisecond laser heating. Sample with spatial dependent material properties introduces phase velocity dispersion in the SAW propagation. As consequence, ToF of SAW becomes frequency dependent. Frequency dependent ToF variation curves at two time instants respected to laser heating were measured by time–frequency analysis together with a differential technique. Free fitted parameters, temperature dependent shear modulus and surface temperature distribution were evaluated by solving the inverse problem by fitting the experimental ToF variation curves into the theoretical ones by means of the differential evolution method. The inversed temperature dependent parameter of shear modulus of Ti–6Al–4V alloy was in good agreement with literature value.

Coexistence of two hole phases in high-quality p−GaAs/AlGaAs in the vicinity of Landau-level filling factors ν=1 and ν=(1/3)

We focused on the transverse AC magnetoconductance of a high-mobility $p$-GaAs/AlGaAs quantum well $(p=1.2\ifmmode\times\else\texttimes\fi{}{10}^{11}{\mathrm{cm}}^{\ensuremath{-}2})$ in the vicinity of two values of the Landau-level filling factor $\ensuremath{\nu}$: $\ensuremath{\nu}=1$ (integer quantum Hall effect) and $\ensuremath{\nu}=1/3$ (fractional quantum Hall effect). The complex transverse AC conductance ${\ensuremath{\sigma}}_{xx}^{\mathrm{AC}}(\ensuremath{\omega})$ was found from simultaneous measurements of attenuation and velocity of surface acoustic waves propagating along the interface between a piezoelectric crystal and the two-dimensional hole system under investigation. We analyzed both the real and imaginary parts of the hole conductance and compared the similarities and differences between the results for filling factor 1 and filling factor 1/3. Both to the left and to the right of these values maxima of a specific shape, ``wings,'' arose in the $\ensuremath{\sigma}(\ensuremath{\nu})$ dependences at those two $\ensuremath{\nu}$. Analysis of the results of our acoustic measurements at different temperatures and surface acoustic wave frequencies allowed us to attribute these wings to the formation of collective localized states, namely, the domains of a pinned Wigner crystal, i.e., a Wigner solid. While the Wigner solid has been observed in 2D hole systems previously, we were able to detect it at the highest hole density and, therefore, the lowest hole-hole interaction reported.

Diagnosis of fire damage depth in mortar using nonlinear harmonic method and scanning airborne ultrasound technique

Abstract This paper reports the effectiveness of a new fire depth diagnosis of mortars using airborne ultrasonic source scanning method and non-linear harmonic method(NHM). We propose a model of fire damage mortar in which Young's modulus gradually decreases from the surface layer at high temperature during fire. In addition, based on the model, we also propose to diagnose the depth of fire damage from changes in the propagation velocity of surface acoustic waves. In addition, in order to verify the theory of the proposed method, the propagation speed of surface acoustic waves propagating in the fire damage mortar is simulated by FEM analysis, and the possibility of diagnosing the depth of fire damage is obtained. Finally, we measure the propagation velocity of surface acoustic waves using an experimental sample simulating the fire damage mortar model described above, and obtain results with almost the same tendency as that of FEM analysis.

Influence of top-coat thickness of thermal barrier coating on time–frequency domain and dispersion characteristics of laser-induced surface acoustic wave

Thermal barrier coating is commonly used to insulate the hot end components of aero engine and gas turbine in order to meet extreme high temperature service conditions. The performance of the thermal barrier coating will be significantly impacted by thinning or even spalling during the service process. Given the uniqueness of thermal barrier coating in structure and composition materials, and the fact that laser-induced surface acoustic waves are extremely sensitive to material surface characteristic parameters, laser-induced ultrasonic technology is used to detect the thermal barrier coating in this paper. To characterize the size and defect of thermal barrier coating, the excitation mechanism and propagation law of surface acoustic waves are very crucial. Firstly, the surface acoustic wave propagation law is investigated using spectral analysis and wavelet transform. Then, to study the influence of top-coat thickness on laser-induced surface acoustic wave, the surface acoustic wave velocity dispersion curves of thermal barrier coating with different top-coat thicknesses are extracted using wavelet analysis. The simulative and experimental results indicate that the time–frequency domain and dispersion characteristics of laser-induced surface acoustic wave in multilayered heterostructure are strongly affected by the top-coat thickness. By the establishment of the relationship between phase velocity of laser-induced surface acoustic wave and top-coat thickness, a theoretical basis is provided to characterize the top-coat thickness of multilayered heterostructure.

Impact of Spin-Wave Dispersion on Surface-Acoustic-Wave Velocity

The dependence of the velocity of surface acoustic wave (SAW) as a function of an external applied magnetic field is investigated in a Fe thin film epitaxially grown on a piezoelectric GaAs substrate. The SAW velocity is observed to strongly depend on both the amplitude and direction of the magnetic field. To interpret the experimental data a phenomenological approach to the relative change in SAW velocity is implemented. We find that the experimental velocity variation can be well reproduced provided that the spin wave dispersion is taken into account. The validity of this phenomenological model is attested by the comparison with a quasi-exact magnetoelastic one.

Dispersion of surface acoustic waves in thin films at extreme wavelength-to-thickness ratios.

Surface acoustic waves (SAWs) are sensitive to the presence of a layer on the surface of a material, even if this layer is extremely thin compared to their wavelengths. Given the very slow propagation velocities of SAWs compared to electromagnetic waves, their wavelengths are on the order of 40 μm for acoustic frequencies on the order of 100 MHz. However, it has been shown that these waves are dispersive for coatings whose thicknesses are more than 1000 times smaller than their wavelength. This sensitivity is verified by studying the dispersion of SAWs for a frequency range between 90 and 260 MHz.

Development of a Broadband (100-240 MHz) Surface Acoustic Wave Emitter Devoted to the Non-Destructive Characterization of Sub-Micrometric Thin Films.

In the ultrasonic non-destructive evaluation of thin films, it is essential to have ultrasonic transducers that are able to generate surface acoustic waves (SAW) of suitably high frequencies in a wide frequency range of between ten and several hundred megahertz. If the characterization is carried out with the transducer in contact with the sample, it is also necessary that the transducers provide a high level of mechanical displacement (>100 s pm). This level allows the wave to cross the transducer–sample interface and propagate over the distance of a few millimeters on the sample and be properly detected. In this paper, an emitter transducer formed of interdigitated chirp electrodes deposited on 128° Y-cut LiNbO3 is proposed. It is shown that this solution efficiently enables the generation of SAW (displacement level up to 1 nm) in a frequency range of between 100 and 240 MHz. The electrical characterization and a displacement field analysis of SAW by laser Doppler vibrometry are presented. The transducer’s significant unidirectionality is demonstrated. Finally, the characterization of two titanium thin films deposited on silicon is presented as an example. A meaningful SAW velocity dispersion (~10 m/s) is obtained, which allows for the precise estimation (5% of relative error) of the submicrometer thickness of the layers (20 and 50 nm).

Theoretical investigation of the piezoelectric and surface acoustic wave properties of GeSn alloys

The piezoelectric and dielectric constants of Ge1-xSnx with varying concentrations (x∈(0–1.0) were calculated using the first-principles method based on the density functional theory, and the surface acoustic wave (SAW) properties such as phase velocity, electromechanical coupling coefficient k2, and power flow angle (PFA) were obtained for Ge0.5Sn0.5 from using the Christopher equations in different orientations (0–90°, 0–90°, and 0–180°). The results demonstrate that the piezoelectric and dielectric constants of Ge1-xSnx deviate from Vegard's law when the x is varied, and the piezoelectric constant value reaches a maximum when x = 0.5. Additionally, the SAW velocity of Ge1-xSnx can be tuned by changing the Sn concentration. It can be seen from the contour maps of Ge0.5Sn0.5 at orientations (45°, 0–90°, and 0–180°), many orientations exhibit outstanding SAW performance. Orientations with large k2 and zero PFA are selected as promising candidates for fabricating SAW devices, which has important significance for experimental research.

Evaluation of Adhesion Properties of Thin Film Structure through Surface Acoustic Wave Dispersion Simulation.

A theoretical simulation study of the dispersion characteristic of the surface acoustic wave (Rayleigh wave) was conducted by modeling the adhesion interlayer with stiffness coefficients to evaluate the bonding properties of nano-scale thin film structures. For experimental validation, a set of thin film specimens were fabricated—637 nm, 628 nm, 637 nm, 600 nm, and 600 nm thick titanium (Ti) films were deposited on silicon (Si) (100) substrate using a DC Magnetron sputtering process with DC power from 28.8 W, 57.6 W, 86.4 W, 115.2 W, and 144 W. The thicknesses of the Ti films were measured using a scanning electron microscope (SEM). Surface acoustic wave velocity for each of the manufactured thin film specimens was measured by using a V(z) curve technique of a Scanning Acoustic Microscope. The measured velocity, transducer frequency, and thickness of the film were applied to dispersion characteristic simulation for a given stiffness coefficient to calculate adhesion strength of each specimen. To verify the simulation result, the adhesion force of each specimen was measured using a nano-scratch test and then compared with the calculated values from the dispersion characteristic simulation. The value of adhesion strength from the dispersion characteristic simulation and the value of adhesion force of the nano-scratch test were found to have a similar tendency according to the process variable of the thin film. The results demonstrated that the adhesion strength of a thin film could be evaluated quantitatively by calculating the dispersion characteristics with the adhesion interlayer stiffness model.

Firmness evaluation of watermelon and melon using velocity dispersion of surface-acoustic-wave

The surface-acoustic-wave (SAW) velocity was measured in the frequency range of 400–7000 Hz in watermelon and melon to evaluate the firmness nondestructively. We showed that the positive velocity dispersion (velocity increases with increasing frequency) observed was caused by the fruit structure, which consists of a hard pericarp and underlying soft flesh. In watermelon, the low-frequency limit of the velocity dispersion curve observed for the pericarp predicted the SAW velocity in watermelon flesh, which was measured to be independent of frequency. In melon, the positive velocity dispersions observed for the pericarp as well as the flesh suggested a distribution of elasticity in the depth direction. Ripening for fourteen days caused a decrease in the SAW velocity by 34%–57% depending on the frequency. The present results demonstrate that the SAW velocity dispersion is a good measure of the firmness and ripening of fruits.

Study of electronic band structure in a 75-nm-wide n-GaAs/AlGaAs quantum well by surface acoustic waves

From measured magnetic field dependences of attenuation and velocity of surface acoustic waves (SAW), we calculated the real part of electron the conductivity in a 75-nm-wide n-GaAs/AlGaAs quantum well in magnetic fields of up to 18 T at frequencies of 30–300 MHz in the temperature range 20–500 mK. We observed slow magneto-oscillations of the conductivity depended on SAW intensity and occurred due to elastic intersubband scattering between the symmetric (S) and antisymmetric (AS) subbands formed by Coulomb repulsion between the electrons. Our theoretical description of the magneto-oscillations allowed estimating the quantum and the intersubband scattering times. The experimentally obtained subband splitting value 0.42 meV was close to the theoretically found 0.57 meV. Additionally, in tilted magnetic fields of 0.5–1.5, T we observed crossings of the Landau levels between the S and AS subbands. According to the developed theory, the crossings at the in-plane field below 1.5 T occur due to different dependences of cyclotron energies in the subbands on the in-plane field. Namely, the cyclotron energy in the S (AS) subband decreases (increases) with the rising of the in-plane field. Work supported by “BASIS,” NSF DMR-1644779 and DMR-1420541, GBMF4420, and State of Florida.]

Measuring Velocity, Attenuation, and Reflection in Surface Acoustic Wave Cavities Through Acoustic Fabry-Pérot Spectra.

Surface acoustic wave (SAW) cavities have been widely applied as electronic bandpass filters, sensors, microfluidic tweezers, and, in recent years, as devices for coupling with quantum systems. Here we propose a novel method of analyzing acoustic Fabry-Pérot spectra, by analogy with optical cavities, to determine the free surface velocity and attenuation of SAW waves, as well as the reflection of interdigital transducers (IDTs), all of which are crucial design parameters. In our experiment, two-port SAW resonators, consisting of two IDTs laterally separated by a free surface cavity length, are used to generate SAWs on 128° Y-X lithium niobate that are trapped between the two IDTs which also act as Bragg reflectors. Resonant cavity peaks can be observed through the electrical S11 (reflection) spectrum measured on one IDT. The free spectral range and linewidths of cavity peaks are then measured to obtain the free surface SAW velocity, SAW propagation attenuation coefficient, and IDT reflection phase and amplitude. Our method of analyzing Fabry-Pérot spectra provides an intuitive approach for determining key characteristics of SAW waves and cavities.

Analysis of thin layers using surface acoustic wave-photonic devices in silicon-on-insulator.

The analysis of thin layers deposited on various substrates is widely employed in thickness monitoring, materials research and development and quality control. Measurements are often performed based on changes to acoustic resonance frequencies of quartz micro-balance devices. The technique is extremely sensitive, but it is restricted to hundreds of MHz frequencies and requires electrical connectivity. In this work we propose and demonstrate the analysis of elastic properties of thin layers deposited on surface acoustic wave-photonic devices in standard silicon-on-insulator. The devices operate at 2.4 GHz frequency, and their interfaces are fiber-optic. The radio-frequency transfer functions of the devices are modified by sub-percent level changes to the group velocity of surface acoustic waves following deposition of layers. Layers of aluminum oxide and germanium sulfide of thickness between 10-80 nm are characterized. The analysis provides estimates for Young's modulus of the layers.

A nonlinear analysis of surface acoustic waves in isotropic elastic solids

With the fast evolution of wireless and networking communication technology, applications of surface acoustic wave (SAW), or Rayleigh wave, resonators are proliferating with fast shrinking sizes and increasing frequencies. It is inevitable that the smaller resonators will be under a strong electric field with induced large deformation, which has to be described in wave propagation equations with the consideration of nonlinearity. In this study, the formal nonlinear equations of motion are constructed by introducing the nonlinear constitutive relation and strain components in a standard procedure, and the equations are simplified by the extended Galerkin method through the elimination of harmonics. The wave velocity of the nonlinear SAW is obtained from approximated nonlinear equations and boundary conditions through a rigorous solution procedure. It is shown that if the amplitude is small enough, the nonlinear results are consistent with the linear results, demonstrating an alternative procedure for nonlinear analysis of SAW devices working in nonlinear state.