  In this report we review recent experimental results on photoexcited carrier
relaxation dynamics on high temperature superconductors (HTSC) probed by a
femtosecond time-resolved optical spectroscopy, and compare the results with
the data obtained on quasi two dimensional charge density waves. In these
experiments, a femtosecond laser pump pulse excites electron-hole pairs via an
inter-band transition in the material. These hot carriers rapidly release their
energy via electron-electron and electron-phonon collisions reaching states
near the Fermi energy within ~100 fs. If an energy gap is present in the
low-energy density of states (DOS), it inhibits the final relaxation step and
photoexcited carriers accumulate above the gap causing a transient change in
reflectivity arising from excited state absorption. The relaxation and
recombination processes of photoexcited quasiparticles, governed by the
magnitude, anisotropy and the T-dependence of the low energy gap, are monitored
by measuring the resulting photoinduced absorption as a function of time after
the photoexcitation. This way, the studies of carrier relaxation dynamics give
us direct information of the T-dependent changes in the low energy DOS. The
technique is particularly useful to probe the systems with spatial
inhomogeneities, where different local environments give rise to different
relaxation rates. The data on series of HTSC-s show evidence for the
coexistence of two distinct relaxation processes, whose T-dependences seem to
be governed by two different energy scales: a T-independent pseudogap and a
mean-field-like T-dependent gap that opens at Tc. The data suggest the origin
of the two-gap behavior is in the intrinsic microscopic spatial inhomogeneity
of these materials.

Superconductivity;Strongly Correlated Electrons