Picosecond acoustics: a new way to access elastic properties of materials at pressure and temperature conditions of planetary interiors

Abstract

Picosecond acoustics is an optical pump-probe technique allowing to access thermoelastic properties and sound velocities of a large variety of materials under extreme conditions. Coupled with diamond anvil cells and laser heating, picosecond acoustic measurements offer the possibility to probe materials over a pressure and temperature range directly pertinent for the deep planetary interiors. In this paper we highlight the capabilities and versatility of this technique by presenting some recent applications on materials of geophysical interest. All the independent components of the elastic tensor of MgO are simultaneously determined Picosecond acoustics: elastic properties under extreme conditions by measurements on single crystal at ambient conditions. Compressional sound velocity is measured at high pressure on an iron-carbon alloy and on polycrystalline argon. First laser heating test measurements performed on molybdenum at high pressure are also presented. These examples demonstrate that picosecond acoustics is a valuable alternative to already existing techniques for determining the physical properties of samples under extreme pressure and temperature conditions.

Picosecond acoustics on iron and nickel in an internal-resistive- heated diamond anvil cell to 50 GPa and 2000 K

“…Previous studies of this method have successfully measured the signals under the core pressure conditions of Earth and Venus 17,65,66 . In addition, acoustic pulses do not disappear under almost all temperature conditions of Mars, Mercury, and the Moon 34 . Since the internal-resistive heating can cover a much wider P-T range than the picosecond acoustic method combined with the laser heating technique 30 , this method can provide the sound velocity data for solid Fe alloys to investigate solid cores of terrestrial bodies.…”

“…where d is the thickness of the sample and t is the travel time of the compressional wave through the sample as determined from the picosecond acoustics mentioned above. Previous works have measured the sample thickness before compression or after recovery to ambient conditions 17,27,28,34,35 . They then indirectly estimated the thickness in a compressed state, assuming that the thickness changes according to the equation of state of the sample 17 .…”

“…In 2016, Zinin et alreported skimming wave velocities of pure Fe up to 22 GPa and 2520 K, but the temperature gradient in the sample due to the single-sided laser heating is a concern53 . Recently, Boccato et al reported a travel time delay for Mo at 34 GPa by heating the sample to 2400 K34 , but the laser heating method may have a large temperature uncertainty. The present study updates the highest temperature of the studies in which the picosecond acoustic method has been used to determine the longitudinal wave velocities at high temperatures.…”

Sound Velocity Anisotropy and Single‐Crystal Elastic Moduli of MgO to 43 GPa

“…In contrast, Brillouin light scattering (BLS) is a versatile technique permitting measurement of sound velocities in transparent solids to much higher pressures: With adapted diamond anvil cell (DAC), BLS has been used to measure sound velocities in MgO single crystals up to 55 GPa (Fan et al., 2019; Sinogeikin & Bass, 2000; Zha et al., 2000). Recently, another promising technique, namely time‐domain Brillouin scattering (TDBS) coupled with a DAC, was used to study C ij ( P ) of compressed transparent solids (Boccato et al., 2022; Gusev & Ruello, 2018; Kuriakose, Raetz, et al., 2017; Nikitin et al., 2015; Raetz et al., 2019). TDBS, also called picosecond laser ultrasonics (Thomsen et al., 1986; Zhao et al., 2019), can be considered as a combination of the advantages of the two abovementioned techniques: (a) it measures directly and in real time velocities of the stimulated coherent acoustic pulses, CAPs, which guarantees the signal quality (as in conventional ultrasonic technique), (b) it is an all‐optical method which allows measurements at pressures achievable in a DAC (as is the case for the BLS).…”