Longitudinal Acoustic Waves Research Articles

Generation and Propagation of Superhigh-Frequency Bulk Acoustic Waves in GaAs

Coherent super-high-frequency (SHF) vibrations provide an excellent tool for the modulation and control of excitations in semiconductors. Here, we investigate the piezoelectric generation and propagation of longitudinal bulk acoustic waves (LBAWs) with frequencies up to 20 GHz in GaAs crystals using bulk acoustic wave resonators (BAWRs) based on piezoelectric thin ZnO films. We show that the electro-acoustic conversion efficiency of the BAWRs depends sensitively on the sputtering conditions of the ZnO films. The BAWRs were then used for the study of the propagation properties of the LBAWs in GaAs in the frequency and temperature ranges from 1 to 20 GHz and 10 and 300 K, respectively, which have so far not been experimentally accessed. We found that the intrinsic acoustic absorption of GaAs in the temperature range from 80 K to 300 K is dominated by scattering with thermal phonons. At lower temperatures, in contrast, the intrinsic acoustic absorption saturates at a frequency-dependent value. Experiments carried out with different propagation lengths indicate that the saturation is associated with losses during reflection at the sample boundaries. We also demonstrate devices with high quality factor fabricated on top of acoustic Bragg-reflectors. The results presented here prove the feasibility of high-quality acoustic resonators embedding GaAs-based nanostructures, thus opening the way for the modulation and control of their properties by electrically excited SHF LBAWs.

Magnetosonic Waves in a Two-Dimensional Electron Fermi Liquid

The properties of highly viscous fluids at high frequencies become similar to those of amorphous solids. In particular, the propagation of not only longitudinal acoustic waves (plasmons in the case of an electron fluid), but also transverse acoustic waves associated with shear deformations becomes possible. In this work, the formation of transverse acoustic waves at high frequencies in a two-dimensional electron fluid in a magnetic field is studied. Consideration is performed within Landau’s Fermi-liquid model. It is shown that the dynamics of Fermi-liquid excitations is described by hydrodynamic equations at a rather strong quasiparticle interaction. The Navier–Stokes equation and expressions for high-frequency shear viscosity coefficients are derived. Based on the equations obtained, dispersion laws are calculated for transverse and longitudinal magnetosonic waves. It is shown that the cyclotron frequency entering the viscosity coefficients and the dispersion relation of transverse magnetosonic waves is renormalized and typically becomes lower than the ordinary cyclotron frequency determining cyclotron resonance. The latter fact was apparently observed in the photoresistance of high-mobility GaAs quantum wells in which two-dimensional electrons form a viscous fluid.

Reflection and Refraction of Harmonic Plane Acoustic Waves by a Plane Interface of Media with Arbitrary Acoustic Properties

We propose universal formulas for the amplitudes of longitudinal and transverse plane acoustic waves, which are reflected and refracted by a plane interface of two media. The formulas are valid for liquid and gaseous media, as well as for elastic isotropic media. Advantages of these formulas are their relative clarity compared with the well-known and commonly accepted formulas, as well as a possibility to pass over from one scattering problem to another by simply replacing the corresponding wave numbers. Despite the well-known methods which are used in the calculations and rather have a methodical character, we manage to find a new type of surface waves at the interface of elastic media. These waves are characterized either by ratios of the wave numbers of transverse waves and densities, or by only the wave numbers of longitudinal waves and the ratio of the densities of media.

Simulation of Acoustic Propagation in 3D Viscoelastic Inhomogeneous Media with Application to Medical Shear Wave Elastography

This paper proposes a new numerical framework to simulate ultrasound wave propagation in 3D viscoelastic heterogeneous media based on the elastodynamic wave equation including a 3D second-order time-domain perfectly matched layer formulation. A finite difference discretization of this formulation is presented, along with a stability analysis. The resulting model is capable of simulating 3D shear and longitudinal acoustic waves for arbitrary source geometries and excitations, together with arbitrary initial and boundary conditions. A simulation example is provided to show the application of our proposed method for tissue elastography imaging. Specifically, the propagation of shear waves from a localized lesion to the surrounding normal tissue is examined.

Klein-Gordon Representation of Acoustic Waves and Topological Origin of Surface Acoustic Modes.

Recently, it was shown that surface electromagnetic waves at interfaces between continuous homogeneous media (e.g., surface plasmon-polaritons at metal-dielectric interfaces) have a topological origin [K. Y. Bliokh etal., Nat. Commun. 10, 580 (2019)NCAOBW2041-172310.1038/s41467-019-08397-6]. This is explained by the nontrivial topology of the non-Hermitian photon helicity operator in the Weyl-like representation of Maxwell equations. Here we analyze another type of classical waves: longitudinal acoustic waves corresponding to spinless phonons. We show that surface acoustic waves, which appear at interfaces between media with opposite-sign densities, can be explained by similar topological features and the bulk-boundary correspondence. However, in contrast to photons, the topological properties of sound waves originate from the non-Hermitian four-momentum operator in the Klein-Gordon representation of acoustic fields.

Determination of acousto-optical effect maxima for optically isotropic crystalline material on the example of GaP cubic crystal

The peculiarities of the acousto-optic interaction geometry optimization in the optically isotropic crystalline material are demonstrated on the example of GaP cubic crystal. The calculation algorithm takes into account the change of the optical indicatrix form during acoustic wave propagation through the crystal. On this basis, the highest achievable value of the acousto-optic figure of merit M2=98·10−15 s3/kg is determined by the extreme surfaces method. This value is achieved in the case of the isotropic diffraction of the light beam on the longitudinal acoustic wave. The approach developed in this work can be used for optimization of the acousto-optic interaction in other optically isotropic crystalline materials.

Laser-ultrasonic nondestructive evaluation of porosity in particulate reinforced metal-matrix composites

In this work, we propose the laser-ultrasonic method for nondestructive evaluation of porosity in particulate reinforced metal-matrix composites fabricated by stir and in-situ reactive casting techniques. The method is based on the influence of porosity on dispersion of the phase velocity of longitudinal acoustic waves, which is measured by the broadband acoustic spectroscopy with laser excitation of ultrasound (laser-ultrasonic spectroscopy). We studied stir cast hypereutectic aluminum-silicon alloy A336 matrix composites reinforced with the SiC micro particles (3.3–13.5 vol%) and in-situ reactive cast Al/Al3Ti composites reinforced with the Al3Ti intermetallic particles (4–11.5 vol%). In the spectral range of 3–40 MHz, the phase-velocity dispersion in both types of composites was observed: the high-frequency velocity in the range of 20–40 MHz increases with the increase of the reinforcement content independent of porosity, whereas the low-frequency velocity in the range of 3–10 MHz decreases with the increase of porosity independent of the reinforcement content. As a result, the relative dispersion grows up with the increase in the composite porosity independent of the variation in the reinforcement content. The empirical dependence between the porosity in a scanning composite region and the relative phase-velocity dispersion in this region is approximated by the same unified function. For the first time, such unified porosity-phase velocity functional relationship is obtained for particulate reinforced metal-matrix composites completely different in fabrication techniques as well as in chemical composition and elastic properties of reinforcing particles.

Acousto-optic modulators driven by longitudinal leaky surface acoustic waves on LiNbO3 thin-plate bonded structures

To obtain an acousto-optic modulator (AOM) driven by a longitudinal leaky surface acoustic wave (LLSAW) with a low driving voltage and a high diffraction efficiency, proton-exchanged planar optical waveguide-type AOMs with structures consisting of an X-cut 36°Y-propagating LiNbO3 (LN) thin-plate bonded to a AT-cut 45°X-propagating quartz or c-plane sapphire (Al2O3) substrate were fabricated and their diffraction properties evaluated. The maximum diffraction efficiency in the zeroth transverse electric (TE0) mode optical guided wave with an optical wavelength of 0.633 μm and the driving voltage required by the LLSAWs of the AOMs were 91% and 5.1 VRMS for LN/quartz and 60% and 11 VRMS for LN/Al2O3, respectively. The diffraction properties of the LLSAWs on the bonded structures were improved compared with those on a single LN substrate.

Longitudinal leaky surface acoustic wave with low attenuation on LiTaO3 or LiNbO3 thin plate bonded to quartz substrate

To obtain a bonded structure with low attenuation for longitudinal leaky surface acoustic waves (LLSAWs), the propagation and resonance properties on a LiTaO3 (LT) or LiNbO3 thin plate bonded to an X-cut quartz substrate were theoretically analyzed. The attenuation of an X-cut 31°Y-propagating LT (X31°Y-LT)/X32°Y-quartz (X32°Y-Q) was calculated to be 0.0005 dB/λ at the normalized LT thin plate thickness h/λ = 0.062 (λ: wavelength) and was lower than that on an X31°Y-LT/AT45°X-Q. Using a finite element method, for the X31°Y-LT/X32°Y-Q, the admittance ratio and Q factor were improved to 120 dB and 53 400 from 62 dB and 1000 for the X31°Y-LT/AT45°X-Q, respectively. Then, the propagation and resonance properties were measured. For the X31°Y-LT/X32°Y-Q, the measured electromechanical coupling factor (K2) and Q factor increased to 5.6% and 280 from 1.8% and 32 for the single LT, respectively. The temperature coefficient of frequency of the LLSAW was measured to be −26.2 ppm °C−1.

Analysis of longitudinal leaky surface acoustic wave propagation characteristics on a piezoelectric ScAlN layer/sapphire or quartz substrate

Longitudinal leaky surface acoustic wave (LLSAW) modes with a high phase velocity are suitable for high-frequency SAW filter applications. However, their attenuation is higher than that of other SAW modes, causing a small Q factor. On the other hand, a low attenuation and high coupling factor K2 in LLSAWs on a piezoelectric thin plate/high-velocity sapphire or quartz substrate have been demonstrated theoretically and experimentally. We expected that LLSAWs propagating on a highly piezoelectric ScAlN layer/sapphire or quartz substrate also have a high phase velocity, high K2 and low attenuation. In this study, the characteristics of LLSAWs on ScAlN layer/sapphire or quartz substrates were theoretically investigated. The cut angle and SAW propagation direction of quartz substrates strongly affect the attenuation of LLSAWs on the layered structure. We found that a LLSAW resonator with (0° 90° 90°) Sc0.4Al0.6N layer/X-cut 30° Y propagation quartz has a high Q factor and high K2, which is adequate for SAW filter applications.

Observational and theoretical validation of longitudinal type surface acoustic wave devices

The authors have proposed longitudinal type surface acoustic wave (SAW) devices comprising a LiNbO3 thin plate, acoustic mirror, handle substrate and interdigital transducers (IDT). This paper describes the observational and theoretical validation of the proposed devices by using a laser probe system and finite element analysis (FEA). It is shown that the laser probe system based on the Sagnac interferometer is effective for direct observation of the devices operated at even 2 GHz range through the observation of SAW field distribution of the existence and lack of acoustic mirror, their comparison with FEA results and changes of the transverse mode pattern. In addition, a phenomenon of SAW radiation normal to the busbars from IDT is found both observationally and theoretically, and a possible radiation mechanism explaining the results is also discussed.

Tailoring one-dimensional layered metamaterials to achieve unidirectional transmission and reflection

We investigate elastic-wave propagation in a spatially dispersive multilayered, totally passive metamaterial system. At oblique incidence a longitudinal (acoustic) wave can convert to transverse in the solid material comprising the layers, but when the incident wave enters the multilayer from a solid as opposed to a liquid medium, the incident transverse component supported by the solid medium indirectly causes the longitudinal transmission response to be greatly modified, with a similar result for the transverse wave exiting the multilayer into a solid medium in response to an incident longitudinal wave. The conversion between longitudinal and transverse waves is found to lead to the emulation of a characteristic nonreciprocal phenomenon at some frequencies: a directionality in the transmission response, sometimes simultaneously with the reflection response. The directionality can be exploited, for example, in the construction of antiseismic structures or breakwater structures. The inclusion of gain and loss elements can strongly enhance the directionality. Periodicity-breaking defects can cause a great variability in the response, enabling the use of devices based on this phenomenon as sensors.

Microfluidic Bulk-Modulus Measurement by a Nanowavelength Longitudinal-Acoustic-Wave Microsensor in the Nonreflective Regime

Investigating the physical mechanisms of interaction between acoustic waves and fluids is significant for the detection of the properties of liquids. Researchers in this field have mainly used shear-mode acoustic wave sensors to measure fluid density and viscosity, but not the bulk modulus. Using an immersed gigahertz-range microsensor, one can obtain compressional information about a liquid from the observed propagation of longitudinal acoustic waves in the nonreflective regime. This work fills a gap by enabling measurement of a liquid's bulk modulus in a microfluidic setting, with an eye toward physiological fluids in particular.

Fidelity of the dual-pulse phase-OTDR response to spatially distributed external perturbation

In the present paper, we perform, for the first time, analysis of the dual-pulse phase-optical time-domain reflectometer (OTDR) response fidelity to spatially distributed external perturbation. The fidelity includes the concepts of response linearity with respect to the external action and the variance of this response as it contains random contributions. Generally, the response of a dual-pulse phase-OTDR configuration to external phase perturbation, provided the demodulation of the backscattered signal has been performed, consists of linear and nonlinear contributions. The linear part of the response is produced due to the difference in the optical paths of the fields backscattered by different scattering fiber segments and is usually considered as a useful signal. The nonlinear part of the response is produced by the scattering fiber segments themselves and is usually considered as random distortions that degrade the phase-OTDR response fidelity. However, in cases when the shape of the external perturbation is known, e.g. the external perturbation is a travelling acoustic wave or uniform stretching of a sensitive fiber, the average characteristics of the random nonlinear part of the phase-OTDR response can be calculated and additional information about the external perturbation can be extracted.In this paper we theoretically calculated the total response of the dual-pulse phase-OTDR to external perturbation in the form of a longitudinal acoustic wave with small amplitude depending on the ratio of the acoustic wavelength and the length of the scattering fiber segment. Due to the influence of the scattering segments, this response always contains random components. Using the theory of speckles, we analyzed the probability density function (PDF) of this random response and its variance.We also performed theoretical and experimental consideration of the important special case of uniform strain distribution along the sensitive fiber occupied by the dual-pulse. It is shown that in this case the contributions of the scattering fiber segments to the phase-OTDR response are linear on average, and therefore the total phase-OTDR response is linear on average; the experimental results are in good agreement with theoretical predictions.

Verification of the Kramers-Kronig relations between ultrasonic attenuation and phase velocity in a finite spectral range for CFRP composites

The aim of this work is to verify applicability of the Kramers-Kronig relations between the attenuation coefficient and phase velocity of longitudinal acoustic waves in carbon fiber reinforced plastic (CFRP) composites in a finite megahertz frequency range. To measure these characteristics, the method of broadband acoustic spectroscopy with a laser source of ultrasound is used. We have experimentally shown that absolute attenuation in CFRPs is determined by both absorption in a polymer matrix and scattering on carbon fibers and gas pores of several tenths of microns in size. We have also found that the increasing composite porosity leads to the decrease in the absolute value of the ultrasonic velocity and to the increase in its relative dispersion in the entire studied frequency range of 1–10 MHz. The experimental results have shown that for the studied CFRPs in this range, features of the mechanism causing attenuation and dispersion of longitudinal acoustic waves by propagation in composites do not influence on applicability of the local Kramers-Kronig relations.

Investigation on Characteristics of Longitudinal Acoustic Wave Propagation in Coal Mine Drill Rods

In order to investigate the feasibility of drill rods used for wireless data transmission in coal mine, the properties of longitudinal acoustic wave propagation in drill string are studied. The results show that: the spectral properties of drill string for coal mine show like a comb filter with wide passbands; when single drill rod become longer, numbers of passbands become more, the width of total passband become narrower; bigger diameter of drill rod makes the width of total passband narrower and amplitudes lower; for integral type and joint-connection type drill rod with same diameter and length, they have little difference in total passband width; the drill strings with different total length have almost same spectral characteristics. In the drill strings investigated, the drill strings composed by drill rods with 42mm diameter and 1m length has the widest passband which occupy 70% of the whole frequency scope. All results indicate that drill rods can be used as channel in data transmission in coal mine gallery.

Local Kramers–Kronig Relations between the Attenuation Coefficient and Phase Velocity of Longitudinal Ultrasonic Waves in Polymer Composites

The relationship between the experimentally obtained attenuation coefficient and dispersion of the phase velocity of longitudinal ultrasonic waves in polymer composite materials has been analyzed. The laser optoacoustic method has been used to measure the ultrasonic attenuation and velocity in a wide frequency range. Verification of the Kramers–Kronig relations for ultrasound-scattering and absorbing carbon fiber reinforced plastic (CFRP) composites has been performed using the analysis of the relationship between the frequency dependences of the attenuation coefficient and phase velocity of longitudinal acoustic waves in samples. We have shown that the Kramers–Kronig relations between the ultrasonic attenuation and phase velocity are valid within the 1–10 MHz frequency range without regard to a particular mechanism of the decay in the energy of an initial acoustic wave during its propagation in a composite.

Anderson Photon-Phonon Colocalization in Certain Random Superlattices.

Fundamental observations in physics ranging from gravitational wave detection to laser cooling of a nanomechanical oscillator into its quantum ground state rely on the interaction between the optical and the mechanical degrees of freedom. A key parameter to engineer this interaction is the spatial overlap between the two fields, optimized in carefully designed resonators on a case-by-case basis. Disorder is an alternative strategy to confine light and sound at the nanoscale. However, it lacks an apriori mechanism guaranteeing a high degree of colocalization due to the inherently complex nature of the underlying interference processes. Here, we propose a way to address this challenge by using GaAs/AlAs vertical distributed Bragg reflectors with embedded geometrical disorder. Because of a remarkable coincidence in the physical parameters governing light and motion propagation in these two materials, the equations for both longitudinal acoustic waves and normal-incidence light become practically equivalent for excitations of the same wavelength. This guarantees spatial overlap between the electromagnetic and displacement fields of specific photon-phonon pairs, leading to strong light-matter interaction. In particular, a statistical enhancement in the vacuum optomechanical coupling rate, g_{o}, is found, making this system a promising candidate to explore Anderson localization of high frequency (∼20 GHz) phonons enabled by cavity optomechanics. The colocalization effect shown here unlocks the access to unexplored localization phenomena and the engineering of light-matter interactions mediated by Anderson-localized states.

Oblique propagation of longitudinal spin-electron acoustic waves under the influence of the Coulomb exchange interaction and the quantum Bohm potential

The influence of the exchange interaction on the properties of spin-electron acoustic waves at the oblique propagation of waves relative to the external magnetic field in the magnetically ordered metals is studied. The spectra of the Langmuir wave and the Trivelpiece-Gould wave is also considered. It is well-known that there are two branches of the spectrum of the spin-electron acoustic waves in this regime. The change of their properties under the influence of the exchange interaction is studied. The quantum Bohm potential is included either. The exchange interaction and the quantum Bohm potential give opposite contributions, but they do not compensate each other since they have different dependences on the wave vector. This competition creates a non-monotonical behavior of the Trivelpiece-Gould wave spectrum. The concavity changes in the monotonic spectra of the Langmuir wave and the SEAWs are found.

Stimulated Brillouin scattering of terahertz electromagnetic pulses in paraelectrics

The nonlinear acoustoelectromagnetic phenomena in the terahertz (THz) range in the crystalline paraelectric SrTiO3 are investigated theoretically. The goal is to investigate an appearance of short THz electromagnetic (EM) pulses. The moderate cooling to the temperature T ≈ 77 K is considered. The resonant three-wave interaction of two counterpropagating EM THz waves with the longitudinal acoustic wave of the difference frequency is under investigation that is called the stimulated Brillouin scattering. This resonant interaction is due to the quadratic nonlinearity called electrostriction. The appearance of short EM pulses is possible when the input amplitudes of the signal EM wave are comparable with the pump amplitude and the duration of the input pulses is quite long. In the temporal scale associated with the EM wave propagation within a crystal, the cubic EM nonlinearity and the EM wave dispersion affect this three-wave resonant interaction. Under the development of the resonant nonlinear interaction, the modulation instability occurs that results in the complex and even chaotic wave modulation.