Nils C. Nielsen

Coherent nonlinear pulse propagation on a free-exciton resonance in a semiconductor

Publisher Summary This chapter discusses the coherent nonlinear pulse propagation. It identifies coherent exciton light coupling over a broad intensity range and permits comparison with numerical calculations based on the semiconductor Maxwell–Bloch equations. At low light intensities, polariton propagation beats owing to the interference between excited states on both polariton branches. In an intermediate intensity regime, the temporal polariton beating is suppressed in consequence of exciton–exciton interaction. At the highest light intensities, self-induced transmission and multiple pulse breakup are identified as a signature for carrier density Rabi flopping. Exciton–phonon scattering is shown to gradually eliminate coherent nonlinear propagation effects due to enhanced dephasing of the excitonic polarization. The experiments can be described theoretically using the semiconductor Maxwell–Bloch equations, which accomplish the transition from linear to nonlinear optics by taking into account many-body interactions consisting of mean-field and correlation effects. The chapter, in addition, discusses the intensity to pulse area relation, pulse delays, and effective propagation velocities in dependence on the pulse intensity yield quantitative agreement between the experiment and the semiconductor Maxwell–Bloch theory.

Transition between different coherent light-matter interaction regimes analyzed by phase-resolved pulse propagation

We present phase-resolved pulse propagation measurements that allow us to fully describe the transition between several light-matter interaction regimes. The complete range from linear excitation to the breakdown of the photonic bandgap on to self-induced transmission and self-phase modulation is studied on a high-quality multiple-quantum-well Bragg structure. An improved fast-scanning cross-correlation frequency-resolved optical gating setup is applied to retrieve the pulse phase with an excellent signal-to-noise ratio. Calculations using the semiconductor Maxwell-Bloch equations show qualitative agreement with the experimental findings.

Temporal and spatial pulse compression in a nonlinear defocusing material

We investigate the spatiotemporal characteristics of subpicosecond pulse propagation in the nonlinear defocusing regime below the band edge of bulk GaAs. We observe temporal and spatial pulse compression and instabilities.

Nonlinear light pulse propagation in bragg-periodic multiple semiconductor quantum well samples: ultrafast switching of a resonant photonic band gap

We investigate theoretically the ultrafast nonlinear suppression of the resonant photonic band gap by strong laser pulses in semiconductor multiple quantum wells. We achieve good agreement with our measurements on reflection samples

Temporal phase evolution during excitonic rabi flopping in semiconductors

Theoretically and experimentally, we investigate temporal phase evolution during Rabi flopping on the A-exciton resonance in CdSe using a novel fast-scanning XFROG method and observe phase changes smaller than pi/2 compared to the slightly chirped input pulse

Femtosecond pulse breakup and compression in resonant multiple quantum wells

We study the propagation of subpicosecond pulses resonant to the hhls exciton in Bragg-periodic and detuned multiple quantum wells. We show pulse breakup depending on the radiative interwell coupling, excitation-induced dephasing and nonlinear pulse compression.

Influence of exciton-phonon scattering on self-induced transmission in semiconductors

Summary form only given. High-intensity pulse propagation of femtosecond laser pulses in optically thick semiconductor crystals can lead to self-induced transmission when the laser is tuned to a free exciton resonance. This effect means that at intensities on the order of MW/cm/sup 2/, pulses in the subpicosecond range lead to Rabi flopping of the carrier density. This causes the pulses to break up temporally after long-distance coherent propagation. The effect depends strongly on the coherence of the incident light field with the polarization of the exciton transition created in the semiconductor. Despite the expected excitation-induced dephasing, a large amount of coherent nonlinear transmission and a high contrast ratio of the Rabi flops are found at sample temperatures /spl les/10 K. In order to study the dynamics of the coherently driven polarization and the dependence on external dephasing, we introduced phonon-scattering by raising the temperature of our samples. The experiments were conducted with 800 fs pulses of an optical parametric amplifier in epitaxially grown CdSe. Our experiments clearly demonstrate that the coherent coupling of the free carrier polarization to the driving field of a high-intensity subpicosecond pulse with an area larger than /spl pi/ effectively prevents the expected dephasing by carrier-carrier scattering. Opening an additional dephasing channel by introducing phonon-scattering gradually weakens the coupling and suppresses characteristic features of self-induced transmission.