High Density Femtosecond Excitation of Nonthermal Carrier Distributions in Intrinsic and Modulation Doped GaAs Quantum Wells.

We investigate the dynamics of non-thermal carrier distributions in undoped and modulation doped GaAs Quantum Well Structures (layer thickness ~ 100A) with near bandgap-resonant intense femtosecond light pulses. We are able to study selectively the carrier-carrier scattering process up to very high densities and we carry out the first optical investigation of non-thermal carrier generation in the presence of a thermalized Fermi-sea of electrons. In undoped quantum wells we find a reduction of the thermalization time from about 100 fs to about 30fs as the photocarrier density increases from N sub (eh) ~2X10 sup (10) cm sup (-2) to ~ 10 sup (12) cm sup (-2). The thermalization time is found to be sensitive to background doping with excess electrons in modulation doped samples.