Abstract

Using an intense tunable femtosecond laser as an excitation source and associated ultrafast-detection technique, we investigated the changes in optical properties of GaAs multiple-quantum-well structures (MQWS) at low temperatures induced by the presence of electron-hole pairs injected in the form of either bound particles (excitons) or free carriers. This time-resolved study has been performed in several samples with different well and barrier sizes as a function of excitation intensity. It concentrates on two effects that occur under strong electronic excitation: a displacement toward higher energies (blue shift) and bleaching (reduction of oscillator strength) of the exciton resonance. We find that the relative importance of the two effects depends on the exact conditions of excitation and the quantum-well parameters. In the case of free-carrier injection at room temperature, only bleaching occurs (with no shift), while in the case of exciton injection both bleaching and blue shift of the exciton resonance are observed simultaneously. Furthermore, the importance of the exciton blue shift (relative to bleaching) depends markedly on GaAs-well thickness, increasing rapidly for thinner wells.

© 1986 Optical Society of America