Enhanced coherent control of carrier and spin density in a zinc-blende semiconductor by cascaded second-harmonic generation

Abstract

Phase- and polarization-dependent optical processes involving pulses with frequencies ω and 2ω can be used to independently control electron and spin density in zinc-blende semiconductors such as GaAs. One such process is quantum interference control (QUIC) where interference between transition amplitudes associated with one- and two-photon absorption alters the carrier∕spin generation rate. A second process, which has been acknowledged but not utilized, is cascaded second-harmonic (CASH) generation in which phase-dependent upconversion∕downconversion between the two pulses modulates the 2ω pulse intensity and∕or polarization and hence modulates the carrier or spin generation rate by single-photon absorption at 2ω. Here we report the use of (110)-oriented GaAs∕AlGaAs quantum wells with a 500-nmAlGaAs buffer layer to enhance CASH and to allow independent control of spin and carrier densities. Experiments conducted with 100-fs pulses at 775 and 1550nm or at 715 and 1430nm, with different polarization states and with different sample orientations, show how QUIC and CASH processes vary with excitation frequency and demonstrate the dominant role played by CASH. We point the way to achieving nearly 100% control through CASH.