Evaluation of Homogeneity and Elastic Properties of Solid Argon at High Pressures Using Picosecond Laser Ultrasonic Interferometry

Abstract:

In picosecond ultrasonic interferometry [1], femto- or picosecond pump laser pulses are first used to generate acoustic pulses ranging from several to a few tens of nanometres length, thanks to the optoacoustic transduction in a light absorbing generator. Time-delayed femto- or picosecond probe laser pulses are then used to follow the propagation of these ultrashort acoustic pulses through a transparent medium that is in contact with the generator surface. The transient signal thus contains, at each moment in time, information on the local elastic, optical and elasto-optical properties of the tested material at the position where the laser-generated picosecond acoustic pulse is located during its propagation in the sample depth. Hence, the technique allows evaluation of sound velocities and elastic anisotropy of micro-crystallites composing a transparent material compressed to high pressures in a diamond anvil cell (DAC). This interferometry technique also helps to understand the micro-crystallite orientations in a case of elastically anisotropic material. Here we report the preliminary results of picosecond ultrasonic interferometry applied to the evaluation of homogeneities and elastic properties of polycrystalline solid argon compressed to 10 GPa and 15 GPa. In comparison with the earlier reported experiments on H₂O ice at Mbar pressures [2], more efforts were spent to the evaluation of the lateral microstructure of the sample at high pressures, i.e., to inhomogeneities along the surface of the optoacoustic generator, rather than to the in-depth imaging along the axis of the DAC. The lateral imaging is performed over a distance of 60 – 90 µm, nearly corresponding to the complete sample diameter. In addition to the presence of expected lateral inhomogeneities the obtained results demonstrate important changes in their distribution upon pressure increase from 10 to 15 GPa. On the basis of the analysis of the statistic probability in the detection of differently oriented micro-crystallites in the argon samples, we discuss the possible physical reasons of the experimental observations.

This work was supported by the ANR BLANC 2011 program (project LUDACism).

[1] Thomsen et al. Phys. Rev. B 34, 4129 (1986)

[2] Nikitin et. al., Sci. Rep. 5, 9352 (2015)