Stephan W. Koch

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.

Ultrafast ac Stark effect switching of the active photonic band gap from Bragg-periodic semiconductor quantum wells

The ultrafast suppression and recovery of an active photonic band-gap structure constructed from the periodic complex susceptibility of quantum well excitons is demonstrated. For resonant pumping, the corresponding superradiant mode is slaved by the external field, and the structure forms a mirror that can be switched on and off at a bandwidth limited only by the width of the pump-pulse and the photonic band gap. Absorption and creation of free carriers is suppressed by the accelerated decay of the superradiant mode of the light-coupled quantum wells.

Influence of the valence-band offset on gain and absorption in GaNAs/GaAs quantum well lasers

The dependence of the gain and absorption in GaNAs/GaAs quantum well lasers on the valence-band offset is investigated. The calculated absorption strength, gain amplitudes, and gain bandwidth are found to depend crucially on the value of this offset. The shift of the peak gain transition energy with carrier density is shown to depend strongly on the magnitude of the offset, providing what should be a useful means to determine the offset experimentally.

Clamping of the linewidth enhancement factor in narrow quantum-well semiconductor lasers

The linewidth enhancement factor in single quantum-well GRINSCH semiconductor lasers is investigated theoretically and experimentally. For thin wells a small linewidth enhancement factor is obtained which clamps with increasing carrier density, in contrast to the monotonous increase observed for thicker wells. Microscopic many-body calculations reproduce the experimental observations attributing the clamping to a subtle interplay between excitation dependent gain shifts and carrier population distributions.

Theory of nonlinear optical absorption in coupled-band quantum wells with many-body effects

Nonlinear optical absorptionspectra and refractive index changes are computed for coupled‐band semiconductorquantum wells by numerically solving the interband polarization equation. The theory combines band‐structure engineering with many‐body techniques and is applied to lattice‐matched GaAs‐AlGaAs and strained InGaAs‐GaAs systems with carrier densities ranging from the excitonic to the gain regimes. Good agreement with recent experimental results is found.

Subpicosecond switching in a current injected GaAs/AlGaAs multiple‐quantum‐well nonlinear directional coupler

We have demonstrated ultrafast switching behavior in a current injected GaAs/AlGaAs multiple‐quantum‐well nonlinear directional coupler at room temperature. The results show low crossover pulse energy (10 pJ) and full recovery within 1 ps.

Pair-Correlation Functions in Incoherent Electron–Hole Plasmas

A density-matrix many-body theory for the description of the interacting electron-hole system in direct-gap semiconductors is presented. The Coulomb interaction between all electrons, the coupling to a quantized light field, and a reservoir of phonons are included. The theory is evaluated to compute the electron-hole and electron-electron correlation functions after initialization of the system as uncorrelated plasma. The dynamical development of pair correlations is discussed.

Dephasing in the gain region of II–VI semiconductor nanocrystals

We investigate the dephasing times in highly excited CdSe and CdS0.3Se0.7 nanocrystals by spectral hole burning throughout the gain region. The energy dependence of the dephasing time T2 is compared between quantum dots in the strong-confinement regime and bulklike microcrystals. T2 in strongly confined quantum dots remains rather constant, whereas the bulklike sample shows a continuous increase of T2 toward the transparency point. This observation is attributed to the different gain mechanisms in the strong and the weak quantum-confinement regimes.

Quantum confinement and strain effects on the lateral mode stability of an unstable resonator semiconductor laser

The gain medium effects on the lateral mode stability of an unstable resonator semiconductor laser are investigated. A physical optics laser model based on a many‐body semiclassical laser theory of the gain medium is used. The consistent treatment of bulk, quantum well, and strained quantum well structures shows that quantum confinement or strain can result in single lateral mode operation over significantly wider ranges of unstable resonator configurations and gain medium excitation.

Microscopic modeling of GalnNAs semiconductor lasers

We calculate microscopically the gain and absorption, linewidth enhancement factor and carrier capture times for a GaInNAs/GaAs quantum-well laser operating in the 1.3 micrometers wavelength regime. The results are compared to those for an InGaAsP/InP and an InGaAlAs/InP structure with similar fundamental transition energies. The much higher confinement for carriers in the GaInNAs quantum well is shown to lead to larger gain bandwidths and, for low to moderate carrier densities, to lower linewidth enhancement factors than for the later two material systems. On the other hand, the high depth of the wells leads to longer carrier capture times in GaInNAs/GaAs.