Galina Khitrova

We investigate the interaction of strong single-cycle THz pulses with a coherent excitonic population, observing the transition from Rabi flopping of the 1s-2p transition to multi THz-photon ionization with increasing field strength.

Interactions of strong few-cycle THz pulses with the induced optical polarization in a QW microcavity reveal that the exciton-polariton modes and the 2p-exciton state form a unique Λ-type three-level system of the coupled light-matter modes.

Interaction of strong THz pulses with a QW microcavity reveals that exciton-polariton modes and 2p-exciton state form a λ-type three-level system. THz pulses resonantly drive the transitions from the exciton-polariton states to the 2p-exciton state.

The adiabatic driving of the resonant electron dynamics in a one-dimensional resonant photonic band gap is proposed as an optical mechanism for nonlinear ultrafast switching. Pulsed excitation inside the photonic gap results in an ultrafast suppression and recovery of the gap. This behavior results from the adiabatic carrier dynamics due to rapid radiative damping inside the band gap.

Under some conditions photoluminescence at the exciton resonance is emitted predominantly by an electron-hole plasma, under other conditions by excitons with exciton formation most efficient at intermediate densities and low temperatures.

The normal modes in a nonperturbatively coupled quantum-well semiconductor microcavity are linear superpositions of the QW exciton and cavity mode when the QW exciton transition is resonant with the cavity mode. When the lower normal mode is excited by a phase-locked pair of optical pulses, the nonlinear response of a probe pulse tuned to the upper mode is controlled. Thus the normal modes are coupled in their nonlinear optical response due to the nonlinearity of the exciton underlying the two normal modes. The cavity enhancement of the excitonic nonlinearity gives rise to a large signal; modulating the relative phase of the excitation pulses produces a differential reflectivity of up to 10%. Besides the coherent control of normal modes which is explainable with in the frame of semiclassical models, we observe a purely quantum mechanical phenomenon in our system. The quantum correlations between the field and carrier density lead to intraband coherences which live much longer than the interband dephasing time.

Data are presented which show eight excitonic density (Rabi) oscillations in an In_0.1Ga_0.9As/GaAs multiple quantum well at 5 K. Our time resolved, two-color pump-probe experimental technique for observing these oscillations is described. The quantum well sample geometry and linear spectrum are shown along with data characterizing the pump and probe pulses. Experimental data is shown to be in excellent agreement with our theoretical calculation of exciton density versus time, verifying the important affect of the Coulomb interaction between carriers in renormalizing the Rabi frequency. The theoretical calculations include the two-fold degenerate light-hole, heavy-hole, and valence bands in the Hartree-Fock form of the semiconductor Block equations, with all variables selected to conform to the experiment.

The possibility of enhancing the luminescence efficiency of Er ions embedded in a semiconductor was investigated by growing about forty erbium-doped lnGaAs/GaAs and GaAs/AlGaAs multiple quantum well samples by molecular beam epitaxy. The idea was to enhance the semiconductor-to- erbium transfer by tuning the quantum-well transition energy to equal one of the erbium ion transitions. Although photoluminescence was seen up to room temperature, strong diffusion of erbium and interdiffusion of Ga and Al ions degraded the quantum wells and formed Er traps that differ by positions of fine structure lines, photoluminescence lifetimes, and temperature dependences.