Vitalyi Gusev

Acoustic streaming in closed thermoacoustic devices

A derivation of acoustic streaming in a steady-state thermoacoustic device is presented in the case of zero second-order time-averaged mass flux across the resonator section (nonlooped device). This yields analytical expressions for the time-independent second-order velocity, pressure gradient, and time-averaged mass flux in a fluid supporting a temperature gradient and confined between widely to closely separated solid boundaries, both in the parallel plate and in the cylindrical tube geometries (two-dimensional problem). From this, streaming can be evaluated in a thermoacoustic stack, regenerator, pulse tube, main resonator of a thermoacoustic device, or in any closed tube that supports a mean temperature gradient, providing only that the acoustic pressure, the longitudinal derivative of the pressure, and the mean temperature variation are known.

Theory of Scholte, leaky Rayleigh, and lateral wave excitation via the laser-induced thermoelastic effect

The analysis of the laser‐induced thermoelastic excitation of acoustic waves propagating along a plane interface between two elastic media is presented. The general solution for the interface motion is derived. The detailed description of the liquid–solid interface motion caused by the photoexcited leaky Rayleigh, Scholte and lateral wave in the liquid is given both in frequency and time domain. The presented theory predicts that laser‐induced thermoelastic stresses in the liquid and the solid can contribute in phase to the excitation of a Scholte wave and that the lateral wave excitation is suppressed when the light penetration depth and Scholte wave penetration depth in the liquid are equal. The obtained analytical solutions provide necessary theoretical background for the optimization of the laser‐induced generation of interface waves in experiments.

Physical mechanisms of coherent acoustic phonons generation by ultrafast laser action

In this review we address the microscopic mechanisms that are involved in the photogeneration processes of GHz-THz coherent acoustic phonons (CAP) induced by an ultrafast laser pulse. Understanding and describing the underlying physics is necessary indeed for improving the future sources of coherent acoustic phonons useful for the non-destructive testing optoacoustic techniques. Getting more physical insights on these processes also opens new perspectives for the emerging field of the opto-mechanics where lattice motions (surface and/or interfaces ultrafast displacements, nanostructures resonances) are controlled by light. We will then remind the basics of electron-phonon and photon-phonon couplings by discussing the deformation potential mechanism, the thermoelasticity, the inverse piezoelectric effect and the electrostriction in condensed matter. Metals, semiconductors and oxide materials will be discussed. The contribution of all these mechanisms in the photogeneration process of sound will be illustrated over several examples coming from the rich literature.

Dispersion of nonlinearity, nonlinear dispersion, and absorption of sound in micro-inhomogeneous materials

New evolution equations for nonlinear acoustic waves in micro-inhomogeneous media, which take into account relaxation processes, are derived. The proposed theory provides the description of such physical effects as frequency-dependent nonlinear absorption of sound, nonlinearity of its velocity dispersion, and dispersion of the nonlinear acoustic parameters of micro-inhomogeneous materials. The theory predicts that, depending on the ratio of the characteristic relaxation time to the wave period, nonlinearity can grow or diminish with increasing frequency, while an increase in wave amplitude can lead to a rise or fall of the propagation velocity. In the limiting cases where the relaxation processes are instantaneous or quasi-frozen, analytical solutions of the nonlinear equations are found and analyzed.

Observation of the “Luxemburg–Gorky effect” for elastic waves

An experimental observation of a new nonlinear-modulation effect for longitudinal elastic waves is reported. The phenomenon is a direct elastic wave analogy with the so-called Luxemburg-Gorky (L-G) effect known over 60 years for radio waves propagating in the ionosphere. The effect consists of the appearance of modulation of a weaker initially non-modulated wave propagating in a nonlinear medium in the presence of an amplitude-modulated stronger wave that produces perturbations in the medium properties on the scale of its modulation frequency. The reported transfer of modulation from one elastic wave to another was observed in a resonator cut of a glass rod containing a few small cracks. Presence of such a small damage drastically enhances the material nonlinearity compared to elastic atomic nonlinearity of homogeneous solids, so that the pronounced L-G type cross-modulation could be observed at strain magnitude in the stronger wave down to 10(-7) and smaller. Main features of the effect are pointed out and physical mechanism of the observed phenomena is discussed.

Giant ultrafast photo-induced shear strain in ferroelectric BiFeO3

Generation of strain using light is a key issue for future development of ultrasonic devices. Up to now, photo-induced GHz-THz acoustic phonons have been mainly explored in metals and semiconductors, and in artificial nanostructures to enhance their phononic emission. However, despite their inherent strong polarization (providing natural asymmetry) and superior piezoelectric properties, ferroelectric oxides have been only poorly regarded. Here, by using ultrafast optical pump-probe measurements, we show that photogeneration/photodetection of coherent phonons in BiFeO3 ferroelectric leads, at room temperature, to the largest intensity ratio ever reported of GHz transverse acoustic wave versus the longitudinal one. It is found that the major mechanism involved corresponds to screening of the internal electric fields by light-induced charges, which in turn induces stress by inverse piezoelectric effect. This giant opto-acoustic response opens new perspectives for the use of ferroelectric oxides in ultrahigh frequency acoustic devices and the development of new GHz-THz acoustic sources.

Laser picosecond acoustics in isotropic and anisotropic materials

We present experimental results concerning the laser generation of picosecond acoustic pulses and their propagation in isotropic and anisotropic materials. We make use of a conventional reflectance detection technique as well as interferometric detection to probe the real and imaginary changes in reflectance. We also demonstrate the detection of transverse acoustic waves by mode conversion at an interface between an isotropic polycrystalline film and an anisotropic substrate.

Nanoscale noncontact subsurface investigations of mechanical and optical properties of nanoporous low-k material thin film.

Revealing defects and inhomogeneities of physical and chemical properties beneath a surface or an interface with in-depth nanometric resolution plays a pivotal role for a high degree of reliability in nanomanufacturing processes and in materials science more generally. (1, 2) Nanoscale noncontact depth profiling of mechanical and optical properties of transparent sub-micrometric low-k material film exhibiting inhomogeneities is here achieved by picosecond acoustics interferometry. On the basis of the optical detection through the time-resolved Brillouin scattering of the propagation of a picosecond acoustic pulse, depth profiles of acoustical velocity and optical refractive index are measured simultaneously with spatial resolution of tens of nanometers. Furthermore, measuring the magnitude of this Brillouin signal provides an original method for depth profiling of photoelastic moduli. This development of a new opto-acoustical nanometrology paves the way for in-depth inspection and for subsurface nanoscale imaging of inorganic- and organic-based materials.

Acoustic power flow measurement in a thermoacoustic resonator by means of laser Doppler anemometry (L.D.A.) and microphonic measurement

Abstract Acoustic power flow measurements in the resonator of a thermoacoustic refrigerator are described. The technique of measurement is based on particle velocity measurement by laser Doppler anemometry (L.D.A.) together with microphonic acoustic pressure measurement. The calibration procedure is explained and results of measurements are compared with analytical results. The L.D.A. technique permits the measurement of acoustic power flow at almost any position and for almost any working frequency in the resonator of thermoacoustic devices.

Depth-profiling of elastic inhomogeneities in transparent nanoporous low-k materials by picosecond ultrasonic interferometry

We achieve depth-profiling of the elasticity of a thin transparent film of a nanoporous low-k material using picosecond acoustic interferometry. The variation in the material properties with depth is extracted from time-resolved femtosecond optical reflectivity measurements. More than 40% of the variation in the longitudinal elastic modulus between the front and the back surfaces of an 800 nm thick nanoporous layer is mapped with a 40 nm spatial resolution. We attribute this variation to the spatially inhomogeneous UV curing of the film during fabrication.