Dynamics of semiconductor microcavities using ultrashort pulse lasers.

Abstract

Using femtosecond optical spectroscopy, we perform an extensive study of dynamics of an AlAs/AlGaAs/GaAs-based semiconductor quantum microcavity, which exhibit cavity-polariton behavior owing to strong coupling between the exciton and cavity modes. We explore the dynamics of time-domain vacuum Rabi oscillations, the spatial coherence transfer of cavity-polariton states, and cavity-polariton dynamics in the nonlinear regime. In the time domain, when the microcavity is impulsively excited by a short coherent optical pulse, we observe the vacuum Rabi oscillations in the radiation, corresponding to the cavity-polariton mode splitting of the microcavity. Interferometric pump-probe measurements clearly show the coherent evolution of the cavity-polaritons. At high intensity, the normal mode splitting collapses due to bleaching of the excitonic oscillator strength. The dynamics of the splitting reveal the momentum relaxation of the cavity-polaritons due to inhomogeneous broadening, and provides evidence for delayed exciton-exciton scattering due to vacuum Rabi oscillation. When the cavity is excited coherently at an oblique angle, we also observe coherent radiation in the normal direction of the substrate, with a nearly fixed delay of 450 fs. This radiation is coherent with the excitation pulse and dependent on excitation density. Specifically, the initially-excited spatial cavity-polariton state is coherently transferred to the other spatial state selected by the cavity mode. The excitation dependence suggests this coherence transfer is associated with exciton scatterings. A density-matrix analysis for two cavity-polariton systems shows our model is in good qualitative agreement with the experiment. In order to perform femtosecond semiconductor spectroscopic experiments, it was necessary to develop the ultrafast laser source. We describe a cw-argon-laser-pumped Ti:sapphire laser system and a real-time femtosecond-optical-pulse analyzer, for femtosecond spectroscopy. The laser system includes a novel 60-fs self-mode-locked Ti:sapphire oscillator and a 250-kHz 3.1-$\mu$J 85-fs Ti:sapphire chirped-pulse regenerative amplifier. For optimization of such a high-repetition-rate amplifier, we demonstrate pulse stretching and compression using high-efficiency holographic transmission gratings with third-order-dispersion compensation. The real-time femtosecond-laser-pulse analyzer provides simple but reliable analysis of femtosecond pulses for characterization of the phase and intensity using a spectrally and temporally resolved upconversion technique (STRUT). Characterization of 2-nJ 60-fs Ti:sapphire oscillator pulses with a quantitative dispersion measurement is achieved with 0.5-second data acquisition time and 0.2-second computational time.