| Self-organized quantum-dot (QD) active regions are being researched intensively for laser and amplifier devices because of their low threshold current density, reduced temperature sensitivity, and high differential gain. We have previously used femtosecond time-resolved spectroscopy to study carrier capture and intradot carrier relaxation in the low-density (absorption) regime, and have observed the effects of the phonon bottleneck and intradot electron-hole scattering. In order to understand the behavior of QD lasers, we made population inversion in QD by optical pumping in GaAs barriers.; In this thesis, we use three-pulse femtosecond white light spectroscopy to study gain recovery dynamics and spectral hole-burning in both the excited and ground states, which directly shows the carrier relaxation between the discrete energy levels in the QD's. The gain recovery due both to carrier relaxation from the confined n = 2 state and to carrier capture from the barrier region are observable and distinguishabel in our spectrally and temporally resolved experiments. Transparency conditions are also measured in the ground and the first excited states at different temperature and wavelengths. These data shows that a large density of states in continuum region as well as discrete energy gap in QD's influences the thermal carrier distributions in the QD's. Level degeneracy and the thermal equilibration process are experimentally observed. By investigating the temporal and spectral behavior after resonant pumping in ground state, a non-degenerate biexciton, composed of a ground and an excited states, is observed and a binding energy of 15 meV is found which is consistent with theoretical calculations. Ultrafast carrier excitations from ground state to excited state and continuum region are observed at room temperature. At high temperature (≥125 K), ultrafast spectral broadening inside the inhomogeneous line generates the fast temporal decays of peak amplitude in ground state. Possible explanations for this broadending and excitations are discussed. |
