Figure 4. Revealing sub-μm-scale texture in a polycrystalline H₂O ice aggregate at megabar pressures via two-dimensional imaging by the time-domain Brillouin scattering technique with an in-depth spatial resolution of ~0.26 μm.

(a) and (b): Two-dimensional depth profiles of the Brillouin frequency in H₂O ice at 84 GPa, obtained by displacing the sample relative to the pump and probe co-focused laser beams in a lateral direction, i.e., parallel to the ice/diamond interface. The profiles in the figure are measured at six particular positions shifted with respect to each other in the same direction by 1 μm. The size of the moving Hanning time window for the Fourier transform is 17 ps FWHM, providing the in-depth spatial resolution of 0.26 μm. While the 3D representation in (a) makes the spatial order of the measurements clear, the view in (b) of the same profiles provides a better understanding of both in-depth and lateral structuring. The colour code for the measured velocity profiles is the same in both images. The temporal images are presented starting at the 100 ps delay time to avoid the region of pulse propagation in the vicinity of the Fe photo-generator, where the results are commonly less precise because of non-perfect elimination of the high-amplitude thermo-reflectance contribution from the time-resolved reflectivity data. (c): Superposition of the short-scale texturing revealed in Fig. 4(a, b) with the coarse scale texturing revealed in Fig. 3(b). The variations of the Brillouin frequency are extracted from the TDBS signal accumulated at a lateral coordinate of 1 μm in Figs. 3(b) and 4(a, b). The maximum changes of the Brillouin frequency relative to the average level are approximately ±26%. (d): Time-frequency spectra of the selected signal obtained with different durations of the moving Hanning window.