Time-domain Brillouin scattering (TDBS) technique uses ultrafast laser pulses to generate coherent acoustic pulses (CAPs) and monitor their propagation through transparent samples. Local velocity of a CAP along depth of a sample (*V*_A, where A represents longitudinal (L) or transversal (T) sound velocity) is extracted from oscillating intensity of the probe light scattered by the CAP and interfering with that reflected by stationary interfaces [1]. Frequency of the oscillation, *f*_B, is given by the well-known equation used in classical Brillouin light scattering in the backscattering geometry: *f*_B=2 *n* *V*_A/ λ, where λ is the wavelength in vacuum of the probe laser beam and *n* the refractive index of the examined material at this wavelength. Spatial distribution of the *f*_B values can be extracted with micron and submicron resolution in lateral and axial directions, respectively. The latter capability of the TDBS technique permits measurement of single crystal elastic moduli, *C*_ij(*P*), of any transparent solid compressed in a diamond anvil cell (DAC) to Mbar pressures [2] including polycrystalline cubic phases not available at ambient conditions, e.g. high pressure phases [3, 4]. Similarly, shear modulus of an amorphous or of an isotropic polycrystalline solid, *G*( *P*), can be determined if its bulk modulus, *B*( *P*), is known [5]. In the case of elastically anisotropic solids, the TDBS technique provides 3D-images of texture of the polycrystalline samples including orientation of individual crystallites in space [6]. Finally, it permits 3D-monitoring of progress of phase transitions or chemical reactions to pressures accessible using the DAC technique [7]. Combination of the TDBS technique with high-resolution X-ray diffraction in a DAC provides a straight-forward way to an assumption-free high-pressure scale. A novel secondary pressure scale based on the f_B(*P*) dependence of the mixture of methanol and ethanol in the volume ratio 4:1 was already proposed and applied to measure pressure dependence of yield strength, σ_y(*P*), of this amorphous solid [5]. References: 1. Kuriakose M. et al., *Ultrasonics* **69**, 259-267 (2016) 2. Zhang X. et al., *J. Geophys. Res.-Solid Earth* **128**, e2022JB026311 (2023) 3. Raetz S. et al., *Phys. Rev. B* **99**, 224102 (2019) 4. Xu F. et al., *Appl. Phys. Lett.* **125**, 164102 (2024) 5. Hong C. et al., *Phys. Rev. B* **111**, 184110 (2025) 6. Sandeep S. et al., *J. Appl. Phys.* **130**, 053104 (2021) 7. Kuriakose M. et al., *New J. Phys.* **19**, 053026 (2017)

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