  We have studied the sub-picosecond quasiparticle dynamics in the perovskite
manganite La0.7Ca0.3MnO3 and the layered manganite La1.4Sr1.6Mn2O7 using
ultrafast optical spectroscopy. We found that for T > TC, initial relaxation
proceeds on the time scale of several hundred femtoseconds and corresponds to
the redressing of a photoexcited electron to its polaronic ground state. The
temperature and dimensionality dependence of this polaron redressing time
provides insight into the relationship between polaronic motion and spin
dynamics on a sub-picosecond time scale. We also observe a crossover to a more
conventional electron-phonon relaxation in the ferromagnetic metallic phase
below Tc.
