  Carrier and phonon dynamics in Bi2Se3 crystals are studied by a spatially
resolved ultrafast pump-probe technique. Pronounced oscillations in
differential reflection are observed with two distinct frequencies, and are
attributed to coherent optical and acoustic phonons, respectively. The rising
time of the signal indicates that the thermalization and energy relaxation of
hot carriers are both sub-ps in this material. We found that the thermalization
and relaxation time decreases with the carrier density. The expansion of the
differential reflection profile allows us to estimate an ambipolar carrier
diffusion coefficient on the order of 500 square centimeters per second. A
long-term slow expansion of the profile shows a thermal diffusion coefficient
of 1.2 square centimeters per second.
