  Time-domain Brillouin scattering is an all-optical experimental technique
based on ultrafast lasers applied for generation and detection of coherent
acoustic pulses on time durations of picoseconds and length scales of
nanometers. In transparent materials scattering of the probe laser beam by the
coherent phonons permits imaging of sample inhomogeneity. The transient optical
reflectivity of the sample recorded by the probe beam as the acoustic nanopulse
propagates in space contains information on the acoustical, optical, and
acousto-optical parameters of the material under study. The experimental method
is based on a heterodyning where weak light pulses scattered by the coherent
acoustic phonons interfere at the photodetector with probe light pulses of
significantly higher amplitude reflected from various interfaces of the sample.
The time-domain Brillouin scattering imaging is based on Brillouin scattering
and has the potential to provide all the information that researchers in
material science, physics, chemistry, biology etc., get with classic
frequency-domain Brillouin scattering of light. It can be viewed as a
replacement for Brillouin scattering and Brillouin microscopy in all
investigations where nanoscale spatial resolution is either required or
advantageous. Here we review applications of time-domain Brillouin scattering
for imaging of nanoporous films, ion-implanted semiconductors and dielectrics,
texture in polycrystalline materials and inside vegetable and animal cells, and
for monitoring the transformation of nanosound caused by nonlinearity and
focusing. We also discuss the perspectives and the challenges for the future.
