Femtosecond Nonlinear Optical Response of Modulation-Doped Quantum Wells

Modulation-doped quantum wells [1] offer a unique system to study femtosecond carrier interactions. Since the doping impurities are localized in the quantum well barriers, there is no contribution from impurity scattering when exciting carriers in the wells. In optical excitation experiments with semiconductors equal densities of electrons and holes are created. Therefore it is impossible to distinguish the separate effects of electrons and holes. Since in general the electrons and holes have different properties such as mass and phonon interactions we may expect that the scattering rates of electrons will be different than the holes. In addition, the separate contributions of the holes and electrons to the optical saturation signals have not been addressed before. We report new experiments [2] in which the femtosecond nonlinear optical response of modulation-doped quantum wells are studied for the first time and the results are compared to the previous results which have been obtained in undoped quantum wells.