FEMTOSECOND REFRACTIVE-INDEX SPECTRAL HOLE BURNING IN INTRINSIC AND DOPED GAAS

The temporal and spectral dependence of carrier-induced changes of the refractive index in intrinsic, n-type and p-type GaAs samples is measured using femtosecond pump and continuum-probe techniques. We observe, for the first time, a refractive-index spectral hole burning arising from a nonthermal carrier distribution generated around the initial excited states. Such spectral hole burning is not present within our time resolution in the n-type sample when the injected carrier density is low. The extremely fast initial scattering time in the presence of cold electrons is attributed to relaxation through the emission of phonon-plasmon coupled modes.