# Time-domain Brillouin scattering for the evaluation of materials interface inclination: Application to imaging of crystal degradation upon non-hydrostatic compression

Samuel Raetz, Sathyan Sandeep, Nikolay Chigarev, Nicolas Pajusco, Théo Thréard, Mathieu E Edely, Alain Bulou, Andreas Zerr, Vitalyi Gusev

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Samuel Raetz, Sathyan Sandeep, Nikolay Chigarev, Nicolas Pajusco, Théo Thréard, et al. Time-domain Brillouin scattering for the evaluation of materials interface inclination: Application to imaging of crystal degradation upon non-hydrostatic compression. PHONONS 2023 - XVII INTERNATIONAL CONFERENCE ON PHONON SCATTERING IN CONDENSED MATTER PARIS, Jul 2023, Paris, France. pp.90. hal-04241745

HAL Id: hal-04241745

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Submitted on 13 Oct 2023

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# Time-domain Brillouin scattering for the evaluation of materials interface inclination: Application to imaging of crystal degradation upon non-hydrostatic compression

## Samuel Raetz<fnref n="1" />

Sathyan Sandeep<fnref n="1" />, Nikolay Chigarev<fnref n="1" />, Nicolas Pajusco<fnref n="1" />, Théo Thréard<fnref n="1" />, Mathieu Edely<fnref n="2" />, Alain Bulou<fnref n="2" />, Andreas Zerr<fnref n="3" />, Vitalyi E. Gusev<fnref n="1" />

samuel.raetz@univ-lemans.fr, vitali.gousev@univ-lemans.fr

Laser ultrasonics is a very handy technique to characterize, evaluate and image materials remotely under extreme (harmful) conditions, such as high pressures. Time-domain Brillouin scattering (TDBS) is such a technique that uses ultrafast laser pulses to generate and detect coherent acoustic pulses (CAPs) as they propagate through a transparent material for the probe laser wavelength [1-2]. In TDBS, the probe light scattered by the propagating CAPs interferes with the probe light reflected on a stationary interface, which leads to a signal with Brillouin oscillations (BOs). If a CAP is reflected from and transmitted through a material interface, the TDBS signal can also feature an echo. Both BOs and echo features can be used for profilometry of (buried) interfaces [3-5] by using information from measurements performed at different lateral positions while scanning co-focused pump and probe beams. A recently developed theory of this type of non-collinear sound-light interaction [6] also provides the opportunity of interface profilometry without the drawback of lateral averaging, i.e., from a local estimation of the angle using a single measurement. The TDBS technique was here applied to monitor the degradation of a single crystal made of lithium niobate (LiNbO₃ – LN) upon non-hydrostatic compression in a diamond anvil cell (DAC). The plate-like sample made of a single crystal of LN was prepared from a nominally 20 µm-thick single-crystal disk (x-cut). The nominal geometrical characteristics of the surfaces of the disk provided by the supplier allowed us to estimate the maximum surface inclination to be 0.6° at sub-µm scale and averaged down to 0.02° at µm-scale. One side of this crystal was then coated with a 70 nm-thick Ti layer which served mostly as the optoacoustic generator in our TDBS measurements. The crystal was then loaded in a DAC with water as pressure-transmitting medium and compressed up to 10.4 GPa, where the liquid water transforms to the phase VII of water ice. The 3D TDBS imaging experiments conducted in the DAC reveal modifications of the thickness profile of the crystal with initially plane-parallel surfaces, as well as of one of its face profiles, caused by the non-hydrostatic compression. The map of the echo arrival times reveals at least two orders increase in the inclination angles. Using our developed analytical theory [6], the inclination angle of the Ti-coated surface of the LN relative to the diamond anvil surface has been estimated by fitting the experimental signals. The evaluation performed from each point of measurement allows to locally assess the inclination, which can be revealed either through the modifications of the BOs in the absence of the detectable CAPs, or through the detection of the CAP echoes arrivals at the interface in the absence of BOs, or simultaneously through both, which further complement the usual use of BOs amplitude and frequency variation. The zoology of TDBS signals gathered in this sample is very rich and will necessitate further theoretical investigations to fully explain all the waveforms by taking into account the optical and elastic anisotropy of LN and water ice, for example. Performing TDBS experiments by analyzing probe polarization would also complement our current analysis.

**Acknowledgments:** This research is supported by the Agence Nationale de la Recherche [project ANR-18-CE42-I2T2M].

## References

[1] H. T. Grahn, H. J. Maris, J. Tauc, IEEE J. Quantum. Electron. 25 (1989) 2562-9.

[2] V. E Gusev, P. Ruello, Appl. Phys. Rev. 5 (2018) 031101.

[3] M. Khafizov, et al., Acta Mater. 112 (2016) 209-15.

[4] T. Thréard, et al., Photoacoustics 23 (2021) 100286.

[5] S. La Cavera III, et al. Light: Science & Applications 10 (2021) 91.

[6] S. Raetz, et al., *Time-domain Brillouin scattering for evaluation of materials interface inclination: Theory*, Phonons 2023

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<footnote n="1">¹ Laboratoire d'Acoustique de l'Université du Mans (LAUM), UMR 6613, Institut d'Acoustique-Graduate School (IA-GS), CNRS, Le Mans Université, 72085 Le Mans, France</footnote>

<footnote n="2">² Institut des Molécules et Matériaux du Mans (IMMM), UMR 6283, CNRS, Le Mans Université, 72085 Le Mans, France</footnote>

<footnote n="3">³ Laboratoire de Sciences des Procédés et des Matériaux (LSPM-CNRS UPR-3407), Université Sorbonne Paris Nord (USPN), 93430 Villetaneuse, France</footnote>
PHONONS 2023

XVII INTERNATIONAL CONFERENCE ON PHONON
SCATTERING IN CONDENSED MATTER

PARIS, July 2nd – 7th, 2023

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