Vitali Gusev

Generation of acoustical phonons by femtosecond laser pulses in GaAs in the presence of external electric field

Experimental results on the generation and the detection by fs laser pulses of the acoustical phonons at frequencies from tens to hundreds GHz in GaAs in the presence of external electric field are presented. The influence of the magnitude and the direction of the applied electric field on the parameters of ps ultrasound is investigated. Results obtained in GaAs and low‐temperature GaAs are compared. The experimental opportunities to discriminate the acoustical phonons produced by laser‐induced inverse piezoelectrical effect and the acoustical phonons due to the thermoelastic mechanism and to the mechanism of electron‐phonon deformation potential are discussed. This study was supported by ANR project BLAN06‐3‐136284.

Numerical simulation and experimental study on surface acoustic waves interacting with cracks heated by scanning heating laser source

The influence on surface acoustic waves (SAWs) propagate through the micro-crack with partial closure is presented in this paper. Heating brought by laser irradiation causes the thermal expansion of the sample, which leads to partial closure of the micro-crack. The partial closure of crack impacts the transmission efficiency of acoustic pulses strongly. Detected SAWs signals are different when heating laser irradiates different regions, when the middle region of the crack is heated, the amplitude of SAWs signals reach the maximum value. Based on this, the experimental system for detecting micro-crack is set up. The crack can be detected effectively by scanning the laser heating source. The finite element method is applied to simulate the temperature rise and relative displacement of crack edges caused by laser irradiation. The relative displacement change with different location of heating laser source is also calculated. The results of numerical simulation and experiment coincide with each other.

On the optical generation and detection of high frequency ultrasounds: thermal and non‐thermal processes

We report on the results of the generation and detection by femtosecond laser pulses of the acoustic waves at frequencies of tens to hundreds GHz in semiconductors and in oxides compounds exhibiting phase transition. We focus first on the generation mechanisms involved to achieve opto‐acoustic transformations. Particular attention will be paid to the cases where classical thermal effects (thermoelastic coupling) drive the mechanism of generation of acoustic phonons and those where non‐thermal effects become significant and sometimes dominant sources of acoustic phonons field. In the latter cases, we will especially focus on the phonons generation based on photo‐induced modifications of microscopic internal electric fields (potential deformation) and also on the use of photoexcited carriers dynamics (carriers recombination) as a tuning parameter of the photo‐generated ultrasounds spectrum. Secondly, we give the examples demonstrating that the choice of optical frequency for ultrasound detection influences ...

Acoustic waves propagation along mechanically free surface of unconsolidated granular porous media

Unconsolidated granular materials exhibit strong dependence of elastic properties on pressure due to high sensitivity of the intergrain contacts to magnitude of loading. As a consequence the gravity field makes these granular assemblages highly elastically inhomogeneous particularly near mechanically free surface. Theoretical and experimental investigations conducted recently have demonstrated that multiple waveguide surface acoustic modes propagate along the free surface of the solid‐state skeleton in the disordered air‐saturated granular packings. These modes are localized near the surface. In ordered granular packing the acoustic waves can travel along the surface inside the horizontal channels, which are localized beneath the surface. The waves of different frequencies are travelling at different depths. The anisotropy of these inhomogeneous granular phononic crystals has an important influence of the existence of the waveguide modes. The experiments with acoustic waves traveling in the vicinity of a ...

Measurement of the velocity dispersion and attenuation in a liquid metal at GHz frequencies

Ultrashort optical pulses are used to excite and interferometrically detect picosecond longitudinal acoustic pulses in thin films of liquid mercury sandwiched between sapphire plates. By analysing consecutive acoustic echoes we derive the dispersion of the ultrasonic attenuation and sound velocity for this liquid at frequencies up to 10 GHz. Two types of optical detection, from the same side of the film as the excitation light and from the opposite side to the excitation light, are presented. Significant effects of structural relaxation are observed and are compared to a simple model that indicates the presence of picosecond relaxation times in mercury.

Optical characterization of the acoustic response in a nanostructure using the transient reflection matrix formalism

The transient reflectometry and transient interferometry are the most commonly used techniques of picosecond acoustics for the study of isotropic planar stratified nanostructures. Nevertheless when anisotropy is present in the sample, the standard techniques have to be completed by transient polarimetry. The reflection properties of an anisotropic sample at oblique incidence are completely determined by the 2x2 reflection matrix (RM): R=[rpp,rps;rsp,rss]. Considering that the transient acoustic phenomena induce a perturbation ΔR of the reflection matrix, we demonstrate that the transient reflection matrix (TRM): ΔR⋅R‐1, where R‐1 is the inverse of the reflection matrix, can be completely determined experimentally using the three techniques: transient reflectometry, interferometry and polarimetry (TRIP). In particular, the off‐diagonal components of the TRM can be determined by transient polarimetry measurements only. Moreover, theoretical calculations of the TRM point up the close relation between the off...

Opto‐acousto‐optic evaluation of the physical properties of nanoporous materials

The porous materials with the characteristic dimensions of the pores from few nanometers up to a few hundred of nanometers find applications in microelectronic industry (as low‐k materials), in photovoltaics and for developing of effective chemical sensors. When the pores are ordered in a spatially periodic structure, these systems present photonic and/or phononic properties which are of a prime interest in applied optics and telecommunication (light and/or phonons spectrum control). Here we report how the methods of picosecond laser ultrasonics based on the generation and detection by lasers of the acoustic waves with frequencies in the band of 10 GHz ‐ 1 THz (with the lengths of hundreds of nanometers down to few nanometers) are applied for the evaluation of the mechanical and optical properties of these materials.

Optical detection of longitudinal and shear acoustic waves with laser picosecond acoustics

The absorption of picosecond light pulses in a medium can generate sub‐THz acoustic waves. These cause a transient optical reflectance change that can be monitored by delayed probe light pulses. This technique, termed laser picosecond acoustics, can be used for the nondestructive evaluation of the physical properties of thin films and substrates. This paper describes a general method for quantitative analysis of such reflectance changes. It is applicable to multiple anisotropic layers that may be opaque or transparent. Longitudinal or shear acoustic waves propagating along the stacking direction of the multilayers modulate the dielectric permittivity anisotropically and inhomogeneously through the photoelastic effect, through local rotation, or through the surface and interface displacements. We describe how the optical reflectance for obliquely incident probe light can be calculated for the modulated medium. We then demonstrate the method with reference to experimental results for a sample consisting of ...

Linear and non linear acoustic waves in macroscopically inhomogeneous unconsolidated granular crystals

Ordered unconsolidated structure of spherical beads in the absence of external loading except the gravity field constitutes a macroscopically inhomogeneous and strongly nonlinear phononic crystal. We report the experimental and theoretical investigation of linear and nonlinear acoustic phenomena in these granular crystals of finite thickness along the gravity direction. In particular the dependence of the resonance frequencies on the thickness of the crystal (the number of layers) is evaluated. The linear transmission of acoustic waves from the bottom to the free surface of the granular crystal exhibits complicated features. It is compared to the linear transmission through the same structure but with a macroscopically homogeneous static stress. Some frequency regions of the acoustic response function are shown to be insensitive to the transition from an inhomogeneous static stress (gravity induced) to a homogeneous one (additional external load). The nonlinear acoustic phenomena of resonance frequency sh...

Application of nonlinear laser photoacoustic technique to crack detection

Nonlinear acoustics is a promising method for non‐destructive testing (NDT) as it allows to improve the sensitivity and contrast of defects detection. The application of contactless laser photoacoustic technique for the tasks of nonlinear acoustics looks very promising. Meanwhile, real examples of industrial systems using nonlinear photoacoustics are still absent. In this work nonlinear photoacoustic response of artificially prepared cracks has been studied. Acoustic vibrations have been excited through the loASAHGRcal heating caused by the absorption of a focused beam of visible or near‐infrared laser radiation. Several methods have been applied for the detection of surface displacement including piezo‐transducers, air‐coupled transducers and optical laser interferometry. It has been shown, that nonlinear acoustic response increases drastically when both pump and probe points are localized near the crack. For this reason, the last two methods allowing local probing look very promising. Two dimensional scans of generation and detection points on the sample surface could be used to obtain the images of crack at new spectral components induced by crack nonlinearity. The goal of this work is to find optimal methods of the excitation and detection of nonlinear photoacoustic response of the crack. It should make the technique attractive for the industrial applications.