  Time-domain Brillouin scattering uses ultrashort laser pulses to generate
coherent acoustic pulses of picoseconds duration in a solid sample and to
follow their propagation in order to image material inhomogeneities with
sub-optical depth resolution. The width of the acoustic pulse limits the
spatial resolution of the technique along the direction of the pulse
propagation to less than several tens of nanometres. Thus, the time-domain
Brillouin scattering outperforms axial resolution of the classical
frequency-domain Brillouin scattering microscopy, which uses continuous lasers
and thermal phonons and which spatial resolution is controlled by light
focusing. The technique benefits from the application of the coherent acoustic
phonons, and its application has exciting perspectives for the nanoscale
imaging in biomedical and material sciences. In this study, we report on the
application of the time-domain Brillouin scattering to the 3D imaging of a
polycrystal of water ice containing two high-pressure phases. The imaging,
accomplished via a simultaneous detection of quasi-longitudinal and quasi-shear
waves, provided the opportunity to identify the phase for individual grains and
evaluate their crystallographic orientation. Monitoring the propagation of the
acoustic waves in two neighbouring grains simultaneously provided an additional
mean for the localisation of the grain boundaries.
