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Time-domain Brillouin scattering (TDBS) uses ultrashort laser pulses to (i) generate coherent acoustic pulses of picoseconds duration in a solid sample and (ii) follow their propagation in order to image material inhomogeneities with the axial resolution that can be deeply sub-optical, to nm-scale, and the lateral one down to the optical diffraction limit (half the optical wavelength of the probe laser). TDBS permits highly resolved 3D-imaging of grains in polycrystalline transparent samples with unlimited lateral sizes and thicknesses of at least 10 μ m also when samples are orientationally textured and/or located in devices permitting access along one direction and from one side only. This optical technique presents, accordingly, clear advantages compared to any x-ray based computed tomography (neither back-projection algorithm nor multiple viewpoints of the sample are needed) and classical spectroscopic methods. Here, we applied TDBS to the 3D-imaging of a sample of polycrystalline water ice containing two high-pressure phases. The imaging, accomplished via a simultaneous detection of quasi-longitudinal and quasi-shear waves, provided shape, coordinates, phase content, and crystallographic orientation of resolved crystallites in a common coordinate system. Monitoring of acoustic pulses simultaneously propagating in two neighboring grains provided a new tool for the localization of grain boundaries. [ABSTRACT FROM AUTHOR]

The analytical theory for the directivity patterns of ultrasounds emitted from laser-irradiated interface between two isotropic solids is developed. It is valid for arbitrary combinations of transparent and opaque materials. The directivity patterns are derived both in two-dimensional and in three-dimensional geometries, by accounting for the specific features of the sound generation by the photo-induced mechanical stresses distributed in the volume, essential in the laser ultrasonics. In particular, the theory accounts for the contribution to the emitted propagating acoustic fields from the converted by the interface evanescent photo-generated compression-dilatation waves. The precise analytical solutions for the profiles of longitudinal and shear acoustic pulses emitted in different directions are proposed. The developed theory can be applied for dimensional scaling, optimization, and interpretation of the high-pressure laser ultrasonics experiments in diamond anvil cell. [ABSTRACT FROM AUTHOR]

We developed a multi-functional in situ measurement system under high pressure equipped with a laser ultrasonics (LU) system, Raman device, and laser heating system (LU-LH) in a diamond anvil cell (DAC). The system consists of four components: (1) a LU-DAC system (probe and pump lasers, photodetector, and oscilloscope) and DAC; (2) a fiber laser, which is designed to allow precise control of the total power in the range from 2 to 100Wby changing the diode current, for heating samples; (3) a spectrometer for measuring the temperature of the sample (using black body radiation), fluorescence spectrum (spectrum of the ruby for pressure measurement), and Raman scattering measurements inside a DAC under high pressure and high temperature (HPHT) conditions; and (4) an optical system to focus laser beams on the sample and image it in the DAC. The system is unique and allows us to do the following: (a) measure the shear and longitudinal velocities of non-transparent materials under HPHT; (b) measure temperature in a DAC under HPHT conditions using Planck's law; (c) measure pressure in a DAC using a Raman signal; and (d) measure acoustical properties of small flat specimens removed from the DAC after HPHT treatment. In this report, we demonstrate that the LU-LH-DAC system allows measurements of velocities of the skimming waves in iron at 2580 K and 22 GPa. [ABSTRACT FROM AUTHOR]

A two-dimensional imaging of a crack by nonlinear frequency-mixing photo-acoustic method is reported. The imaging contrast is due to nonlinear photo-thermo-acoustic processes taking place in case of simultaneous excitation by lasers of thermo-elastic and acoustic waves in the vicinity of the cracks. The images are obtained by scanning of two co-focused laser beams in region of crack location. The first cw laser beam, modulated in intensity at low frequency fL, generates a thermo-elastic wave, which is able to strongly periodically modulate the local crack rigidity up to complete closing/opening of the crack. The second cw laser beam, intensity modulated at much higher frequency fH, generates an acoustic wave incident on the breathing crack. The crack rigidity is also influenced by the stationary non-modulated inhomogeneous thermal stresses caused by the stationary heating of the sample by both lasers. The main contribution to imaging contrast comes from the strong dependence of the reflectivity of acoustic waves on the crack rigidity. The modulation of crack rigidity by thermo-elastic wave leads to the parametric modulation of the reflected acoustic waves and the generation in the spectrum of the acoustic field of the side lobes, which are separated from fH by the integer number of fL. Scan images of a crack with an amplitude dynamics up to 40 dB and a spatial resolution better than 100 μm, are obtained through the mapping of nonlinear side lobe amplitudes. The observed dependences of the images on the power of the lasers are discussed and the physical explanation of the appearance of crack phantom images at high level of optical excitation is proposed. For comparison and elucidation of spatial resolution issues related to this imaging method, the one-dimensional images, obtained with one laser beam focus position fixed and the second beam focus position scanning, are also presented and discussed. [ABSTRACT FROM AUTHOR]

Laser ultrasonics in a point-source-point-receiver configuration is applied for the evaluation of elastic properties of nontransparent materials in a diamond anvil cell at high pressures. Measurement of both longitudinal and shear acoustic wave velocities in an iron foil at pressures up to 23 GPa does not require any information in addition to the one obtained by all-optical pump-probe technique. [ABSTRACT FROM AUTHOR]

All-optical monitoring of the nonlinear motion of a surface-breaking crack is reported. Crack closing is induced by quasi-continuous laser heating, while Rayleigh surface acoustic pulses and bulk longitudinal surface skimming acoustic pulses are also generated and detected by lasers. By exploiting the strong dependence of the acoustic pulses reflection and transmission efficiency on the state-open or closed-of the contacts between the crack faces, the parametric modulation of ultrasonic pulses is achieved. It is observed that bulk acoustic waves skimming along the surface can be more sensitive to crack motion than Rayleigh surface waves. [ABSTRACT FROM AUTHOR]