Tunable all-optical delay via nonlinear optical processes in semiconductor quantum wells

The dramatic experimental demonstration of slow light in atomic vapors via electromagnetically induced transparency (EIT) has stimulated considerable interest in the dynamic control of the group velocity of light and in the development of tunable all optical delays. This dissertation presents experimental studies of all-optical tunable delays via nonlinear optical processes in semiconductor quantum wells (QWs). Two different approaches have been pursued. The first employs EIT arising from electron spin coherence in semiconductors and the second is based on efficient carrier induced exciton dephasing in QWs.; The EIT-based approach takes advantage of the spin-orbit coupling in the valence band to couple two electron spin states to a common light-hole valance band state in a GaAs QW waveguide. Induced transparency due to electron spin coherence in optical absorption has been demonstrated by investigating both the polarization and magnetic field dependence of the induced transparency. Signature of Rabi oscillations for excitonic transitions has also been observed. The relatively small transparency, however, has limited the fractional delay that can be achieved. Nevertheless, our studies have shown a novel approach for inducing and manipulating electron spin coherence in a semiconductor with neither external nor effective internal magnetic fields.; We have overcome the shortcomings of the EIT-based approach by exploiting unique incoherent nonlinear optical processes in semiconductors. In this approach, a control laser beam injects free carriers above the band gap of a GaAs QW. Strong Coulomb interactions between excitons and free carriers lead to highly efficient broadening and bleaching of the exciton absorption resonance, effectively modifying the group velocity of a signal pulse propagating near the exciton resonance. Fractional delay exceeding 200% has been obtained for an 8 ps optical pulse tuned near the heavy-hole exciton resonance, representing an improvement of more than one order of magnitude in terms of both fractional delay and signal bandwidth for tunable optical delays in semiconductors. In addition, pulse reshaping and pulse breakup observed near the exciton absorption line center also motivate further investigation of coherent propagation effects such as self-induced transparency in semiconductors.