  We present a femtosecond time-resolved optical spectroscopy (TRS) as an
experimental tool to probe the changes in the low energy electronic density of
states as a result of short and long range charge density wave order. In these
experiments, a femtosecond laser pump pulse excites electron-hole pairs via an
interband 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 10-100 fs. The presence of an energy gap in the
quasiparticle excitation spectrum inhibits the final relaxation step and
photoexcited carriers accumulate above the gap. The relaxation and
recombination processes of photoexcited quasiparticles are monitored by
measuring the time evolution of the resulting photoinduced absorption. This
way, the studies of carrier relaxation dynamics give direct information of the
temperature-dependent changes in the low energy density of states. Here we
present the application of the femtosecond time-resolved optical spectroscopy
for studying changes in the low energy electronic density of states in low
dimensional charge density wave systems associated with various charge density
wave (CDW) transitions and review some recent experiments on quasi 1D and 2D
CDW compounds.
