Acoustic Wave Vector Research Articles

Self-Biased Magnetic Field Sensors Based on Surface Acoustic Waves through Angle-Dependent Magnetoacoustic Coupling

Surface-acoustic-wave- (SAW) based devices emerge as promising technology in magnetic field sensing by integrating a magnetostrictive layer with the giant \ensuremath{\Delta}E/\ensuremath{\Delta}G effect. However, almost all SAW magnetic field sensors require a bias field to obtain high sensitivity. In addition, the true nature of magnetoacoustic coupling still presents a major challenge in understanding and designing this kind of device. Here, a dynamic magnetoelastic model for the \ensuremath{\Delta}E/\ensuremath{\Delta}G effect is established in consideration of the important role of the dipole-dipole interaction. The model is also implemented in finite-element-method software to calculate the resonance-frequency responses of multiple fabricated sensors with different \ensuremath{\psi} angles between the acoustic wave vector and the induced uniaxial magnetic anisotropy. The measured results are in excellent agreement with the simulated ones. A strong resonance-frequency sensitivity (${S}_{\mathrm{RF}}$) of 630.4 kHz/Oe is achieved at zero bias field for the device with an optimized \ensuremath{\psi} angle. Furthermore, the ${S}_{\mathrm{RF}}$ measurements along different directions verify its vector-sensing capability.

Nonsymmetric spin pumping in a multiferroic heterostructure

We present spin pumping experiments in Co/Pt bilayers grown on Al2O3(0001) and on ferroelectric Y-cut LiNbO3 substrates. Spin pumping is triggered by resonant ferromagnetic resonance induced by surface acoustic waves. We observe that spin pumping efficiency varies when both the magnetization vector and the acoustic wave vector are reversed in Pt/Co/LiNbO3. This phenomenon is not observed in Pt/Co/Al2O3. We propose that the in-plane electric polarization of LiNbO3 is at the origin of the observed phenomenon. These observations open up the perspective of an electric field control of spin pumping efficiency.

Induced Hall-Like Current by Acoustic Phonons in Semiconductor Fluorinated Carbon Nanotube

We show that Hall-like current can be induced by acoustic phonons in a nondegenerate, semiconductor fluorine-doped single-walled carbon nanotube (FSWCNT) using a tractable analytical approach in the hypersound regime (q is the modulus of the acoustic wavevector and is the electron mean free path). We observed a strong dependence of the Hall-like current on the magnetic field, H, the acoustic wave frequency, , the temperature, T, the overlapping integral, , and the acoustic wavenumber, q. Qualitatively, the Hall-like current exists even if the relaxation time does not depend on the carrier energy but has a strong spatial dispersion, and gives different results compared to that obtained in bulk semiconductors. For and , the Hall-like current is in the absence of an electric field and in the presence of an electric field at 300 K. Similarly, the surface electric field due to the Hall-like current is in the absence of an external electric field. In the presence of an external electric field, and for at 300 K. q and can be used to tune the Hall-like current and of the FSWCNT. This offers the potential for room temperature application as an acoustic switch or transistor, as well as a material for ultrasound current source density imaging (UCSDI) and AE hydrophone device in biomedical engineering.

Fluctuating Elasticity Fails to Capture Anomalous Sound Scattering in Amorphous Solids.

The fluctuating elasticity (FE) model, introduced phenomenologically and developed by Schirmacher [J. Non-Cryst. Solids 357, 518 (2011)JNCSBJ0022-309310.1016/j.jnoncrysol.2010.07.052], is today the only theoretical framework available to analyze low-temperature elastic acoustic scattering in glasses. Its existing formulations, which neglect the tensorial nature of elasticity and exclude long-range disorder correlations, predict that the acoustic damping coefficients obey the standard Rayleigh scaling law: Γ∼k^{d+1}, with k the acoustic wave vector, in dimension d. However, recent numerical data, supported by the analysis of existing experimental results, show that Γ does not obey this scaling law but Γ∼-k^{d+1}lnk. Here we analyze in detail how a fully tensorial FE model can be constructed as a long wavelength approximation of the elastic response of the discrete, atomistic, problem. We show that, although it incorporates all long-range correlations, it fails to capture the observed damping in two respects: (i)it misses the anomalous scaling, and predicts the standard Rayleigh law; (ii)it grossly underestimates the amplitude of scattering by about 2 orders of magnitude. This brings clear evidence that the small scale nonaffine displacement fields, although not simply reducible to local defects, play a crucial role in acoustic wave scattering and hence cannot be ignored.

Valley Acoustoelectric Effect.

We report on the novel valley acoustoelectric effect, which can arise in a 2D material, like a transition metal dichalcogenide monolayer, residing on a piezoelectric substrate. The essence of this effect lies in the emergence of a drag electric current (and a spin current) due to a propagating surface acoustic wave. This current consists of three contributions, one independent of the valley index and proportional to the acoustic wave vector, the other arising due to the trigonal warping of the electron dispersion, and the third one is due to the Berry phase, which Bloch electrons acquire traveling along the crystal. As a result, there appear components of the current orthogonal to the acoustic wave vector. Further, we build an angular pattern, encompassing nontrivial topological properties of the acoustoelectric current, and suggest a way to run and measure the conventional diffusive, warping, and acoustoelectric valley Hall currents independently. We develop a theory, which opens a way to manipulate valley transport by acoustic methods, expanding the applicability of valleytronic effects on acoustoelectronic devices.

Dependence of acoustic anomalies on chemical composition in strontium barium niobate crystals (from conventional ferroelectric to relaxor) probed by Brillouin light scattering

Brillouin spectra in a series of strontium barium niobate crystals SrxBa1-xNb2O6 of different chemical composition (x = 0.3, 0.5, 0.6, 0.75) are investigated in various spectral geometries and in a wide temperature ranges including ferroelectric and paraelectric phases. It is shown that for all crystals studied, a pronounced temperature dependences of position and width of the Brillouin lines are clearly manifested only when the acoustic wave vector is parallel to the polar axis Z. Exponential temperature dependence of anomalies of elastic modules was obtained. The exponent rate decreases with increasing of disordering in crystals under study.

Acoustic nonreciprocity in Coriolis mean flow systems.

One way to break acoustic reciprocity is to have a moving wave propagation medium. If the acoustic wave vector and the moving fluid velocity are collinear, the wave vector shift caused by the fluid flow can be used to break. In this paper, an alternative approach is investigated in which the fluid velocity enters the differential equation of the system as a cross product term with the wave vector. A circular field where the fluid velocity increases radially has a Coriolis acceleration term. In such a system, the acoustic wave enters from the central wall and exits from the perimeter wall. In this paper, the differential equation is solved numerically and the effect of fluid velocity on the nonreciprocity factor is examined.

Breaking time reversal symmetry with coriolis mean flow systems

One way to break time reversal symmetry is to have a moving wave propagation medium. If the acoustic wave vector and the moving fluid velocity are collinear, such as the one created by Fluery et al. (Science, 343, 2014) we can use the wave vector shift caused by the fluid flow to break reciprocity. An alternative approach we have taken is to use a fluid velocity field which enters the differential equation of the system as a cross product term with the wave vector. A circular field in which the fluid velocity increases radially has a Coriolis acceleration term. In this system, the acoustic wave enters from the central wall and exits from the perimeter wall. Equations of conservation of mass and momentum, after linearization can be simplified to: (∂/∂t+V.▽)P+ρC2▽.(u+V) = 0, ρ∂/∂t(u+V)+ρ(V.▽)u+ρ(u.▽)V+ρ(V.▽)V+▽P = 0, in which u is the particle acoustic velocity and V = rωeθis the fluid velocity due to the rotational velocity field. It can be shown that the third term in the second equation can be simplified as:ρωek×u. This term Coriolis acceleration induces nonreciprocity. In this work, we solved the differential equation numerically and investigated the effect of fluid velocity on the nonreciprocity factor.

Boundary scattering of phonons: Specularity of a randomly rough surface in the small-perturbation limit

Scattering of normally incident longitudinal and transverse acoustic waves by a randomly rough surface of an elastically isotropic solid is analyzed within the small perturbation approach. In the limiting case of a large correlation length $L$ compared with the acoustic wavelength, the specularity reduction is given by $4\eta^2k^2$, where $\eta$ is the RMS roughness and $k$ is the acoustic wavevector, which is in agreement with the well-known Kirchhoff approximation result often referred to as Ziman's equation [J. M. Ziman, Electrons and Phonons (Clarendon Press, Oxford, 1960)]. In the opposite limiting case of a small correlation length, the specularity reduction is found to be proportional to $\eta^2k^4L^2$, with the fourth power dependence on frequency as in Rayleigh scattering. Numerical calculations for a Gaussian autocorrelation function of surface roughness connect these limiting cases and reveal a maximum of diffuse scattering at an intermediate value of $L$. This maximum becomes increasingly pronounced for the incident longitudinal wave as the Poisson's ratio of the medium approaches 1/2 as a result of increased scattering into transverse and Rayleigh surface waves. The results indicate that thermal transport models using Ziman's formula are likely to overestimate the heat flux dissipation due to boundary scattering, whereas modeling interface roughness as atomic disorder is likely to underestimate scattering.

Structure of nanoparticles in transformer oil-based magnetic fluids, anisotropy of acoustic attenuation

The anisotropy of acoustic attenuation in transformer oil-based magnetic fluids upon the external magnetic field was studied to discover the structure of nanoparticles. When a magnetic field is increased, the interaction between the external magnetic field and the magnetic moments of the nanoparticles leads to the aggregation of magnetic nanoparticles and following clusters formation. However, the temperature of magnetic fluids and the concentration of nanoparticles also have very important influence on the structural changes. The measurement of the dependence of the acoustic attenuation on the angle between the magnetic field direction and acoustic wave vector (anisotropy) can give the useful information about the structure of magnetic nanoparticles formations. In the present, the results of anisotropy measurements of the transformer oil-based magnetic fluids are described and using appropriate theory the basic parameters of clusters are calculated. On the basis of the performed calculations, the proportion of the acoustic wave energy used for excitation of the translational and rotational degrees of freedom was also established.

New Method to Calculate Spatial Distribution of Acousto-Optic Figure of Merit in Crystals

We propose a novel method to calculate acousto-optic gure of merit in crystals. Calculations are performed in laboratory coordinate system where Z′-axis is collinear with wave vector of ultrasound and the Fresnel equation is considered as an equation on the third component of refractive index vector. The method is applicable to both uniaxial and biaxial crystals. In this paper, we compared obtained values of acousto-optic gure of merit with values from literature data for uniaxial crystals such as paratellurite, lithium niobate, tellurium and for biaxial crystals such as lead and strontium tetraborates. Calculations in paratellurite were carried out for slow-shear acoustic wave propagating along [110] crystal axis. In lithium niobate crystal, we perform comparison with results for geometry of acousto-optic interaction where acoustic wave vector forms 88◦ angle with X crystal axis and 150.4◦ angle with Z crystal axis. In tellurium crystal, we investigate geometries applied in infrared de ectors. In SrB4O7 and PbB4O7 crystals we analyze acousto-optic characteristics of slow-shear mode propagating along [100] crystal axis. Spatial distributions of acousto-optic gure of merit and acoustic frequency for the mentioned acousto-optic interaction geometries are presented.

Theoretical analysis of novel Brillouin scattering properties in photonic crystal fibers based on silica rod model

Based on an approximate model for acoustic waves in a silica rod with circular cross section, we theoretically investigated the dispersion properties of the hybrid acoustical modes and the acousto-optical interactions in the small core photonic crystal fiber. The calculation result shows that the dual-peaked Brillouin scattering gain coefficient spectrum is caused by the mode coupling of the hybrid acoustical modes in the small fiber core. Together with the dispersion properties of the acoustical modes, the influence of the pump wavelength and temperature on the dual-peaked Brillouin scattering gain coefficient spectrum are also calculated via their influences on the acoustic wave vector and material parameters. Our theoretical results agree with the experimental results well. And we also discuss the limit of application of this model through comparing the theoretical and experimental results.

Alignment of particles in microfluidic systems using standing surface acoustic waves

We report on the use of standing surface acoustic waves, formed on a single-crystal piezoelectric substrate, to organize micron-scale latex particles into an array comprising a series of lines in an adjacent microfluidic system. The lines of particles are formed parallel to the substrate surface and perpendicular to the surface acoustic wave vector. They extend across the width of the acoustic beam aperture, with a periodicity of one-half the surface acoustic wavelength. The position and spacing of the particle arrays can be altered by adjusting the acoustic wave frequency within the device passband. We discuss the mechanism responsible for the formation of the lines, which could be widely applicable to the alignment of microscopic objects held in suspension.

Flow acoustics in periodic structures

A recent paper [A.A. Krokhin, J. Arriaga, L.N. Gumen, Speed of sound in periodic elastic composites, Phys. Rev. Lett. 91 (2004) 264302-1–4] addresses the speed of sound in periodic elastic composites (phononic crystals) with particular emphasis to the case where air bubbles are present in water and arranged periodically. In such periodically arranged mixtures, the well-known phenomena of the drop of the speed of sound may occur and applications related to, e.g., sound-beam focusing and acoustic surgery are possible [F. Cervera, L. Sanchez, J.V. Sanchez-Perez, R. Martinez-Sala, C. Rubio, F. Meseguer, C. Lopez, D. Caballero, J. Sanchez-Dehesa, Phys. Rev. Lett. 88 (2002) 023902]. In this paper, the analysis is extended theoretically to include cases where a background flow in a periodic structure is maintained. Calculations of dispersion relations and group velocities are presented in cases with one- and two-dimensional material periodicity for background flow values in the range: 0−1 m/s. Materials considered in the calculations are periodic water–air mixtures. It is shown that acoustic waves couple to the group velocities only if the (acoustic) wave vector has a component along the background flow velocity direction.

Effect of acoustic flexural waves in a tilted superstructure fiber grating

This study demonstrates that the cladding modes of a tilted superstructure fiber grating can be coupled back to the core mode to create induced channels by acousto-optic interactions when the acoustic wave traveling along the fiber axis vibrates the fiber. This phenomenon is based on the acoustic wave vector matching the difference between the core mode and the cladding mode wave vector. Acoustic power levels can be used to control the reflectivity and number of the induced wavelength channels. Moreover, the wavelength location of the induced channel can be tuned by varying the acoustic wave frequency. Thus, the proposed device may provide a switchable multiwavelength comb filter for applications in wavelength-division multiplexing systems, fiber lasers, or fiber sensors.

Acousto-optic-induced cladding-mode reflection in a blazed-superstructure fiber grating

The coupling phenomenon between the cladding mode and the core mode in a blazed-superstructure fiber grating is experimentally demonstrated in this letter by applying both transverse and longitudinal acoustic waves. This is based on the difference between the cladding-mode wave vector and the core-mode wave vector, which is equivalent to the acoustic wave vector. By this means, a reflectivity-tunable multiwavelength switchable comb filter would be developed for the applications in fiber optics.

Visualization of short surface acoustic waves by stroboscopic x-ray topography: analysis of contrast

In this paper the capabilities of stroboscopic x-ray topography to visualize the propagation of short (<10 μm) surface acoustic waves (SAW) in single crystals are analysed. Experiments with LiNbO3 crystals under SAW excitation were carried out at the ID19 beam line of the European synchrotron radiation facility. The developed theoretical analysis of the SAW-induced contrast in Bragg scattering geometry helped to optimize experimental conditions and receive high quality images with well-resolved individual acoustic wave fronts. The basic contrast mechanism is related to the SAW-induced corrugation of atomic planes that causes the focusing of x-rays. The focal distance is directly proportional to the SAW wavelength and inversely proportional to the deformation amplitude at the crystal surface. Simulations of diffraction contrast allowed us to conclude that the best contrast is achieved when the acoustic wave vector is perpendicular to the x-ray scattering plane and the sample/film distance equals 2–4 focal distances.

Mechanisms of ultrasonic modulation of multiply scattered coherent light: a Monte Carlo model

A Monte Carlo model of the ultrasonic modulation of multiply scattered coherent light in scattering media is provided. The model is based on two mechanisms: the ultrasonic modulation of the index of refraction, which causes a modulation of the optical path lengths between consecutive scattering events, and the ultrasonic modulation of the displacements of scatterers, which causes a modulation of optical path lengths with each scattering event. Multiply scattered light accumulates modulated optical path lengths along its path. Consequently, the intensity of the speckles that are formed by the multiply scattered light is modulated. The contribution from the index of refraction is comparable with the contribution from displacement when the acoustic-wave vector is less than a critical fraction of the transport mean free path and becomes increasingly greater than the contribution from displacement beyond this critical point. This Monte Carlo model agrees well with an independent analytical model for isotropically scattering media. Both mechanisms are coherent phenomena, requiring the use of a coherent light source.

Backscattering enhancements due to reflection of meridional leaky Rayleigh waves at the blunt truncation of a tilted solid cylinder in water: Observations and theory

Leaky surface waves once launched on an elastic object in water will undergo partial reflection at the truncations of the object. Under certain conditions, such reflections can radiate locally flat acoustic wavefronts with a reversal of the acoustic wave vector, and thus a large backscattered signal is produced. A stainless-steel circular rod with flat ends was ensonified through an angular region appropriate for launching meridional Rayleigh waves, and the resulting backscattering enhancement is consistent with the hypothesized end-reflection process. The observed backscattering peak is in general agreement with predictions from a convolution formulation [P. L. Marston, J. Acoust. Soc. Am. 102, 358–369 (1997)]. The tilt of the cylinder was varied to explore the peak’s width, and the results are consistent with an extension of the theory for the dephasing of the Rayleigh wave given here. The amplitude measurements used ultrasonic tone bursts having ka of 50 and 83, and the enhancement is predicted to be significant over a wide range of ka.

Meridional leaky ray amplitudes for tilted cylinders and high-frequency backscattering enhancements for finite cylinders

Consider the meridional plane which contains the axis of a circular cylinder and the incident acoustic wave vector. The meridional leaky rays which lie in this plane may be excited for certain tilt angles. Radiation resulting from the reflection of such rays off truncations on a cylinder gives strong high-frequency backscattering enhancements [Kaduchak et al., J. Acoust. Soc. Am. 100, 67–71 (1996)]. The present work gives ray approximations for such backscattering contributions and for the related contributions to scattering by tilted infinite cylinders. These are evaluated using a general convolution integral formulation [P. L. Marston, J. Acoust. Soc. Am. 100, 2820(A) (1996)]. The infinite cylinder results for radiation by Rayleigh waves on a solid cylinder agree with the partial-wave series solution. The truncation enhancement for a finite cylinder causes a backward-directed three-dimensional wavefront possessing a vanishing Gaussian curvature. The amplitude from the far-field propagation integral associated with the caustic due to this wavefront can significantly exceed the amplitude for reflection off a rigid sphere having the same radius as the cylinder. The analysis does not rely on thin shell assumptions. [Work supported by the Office of Naval Research.]