A. Knorr

Amplification, absorption, and lossless propagation of femtosecond pulses in semiconductor amplifiers.

Light propagation in semiconductor amplifiers is investigated numerically for femtosecond pulses whose frequency spectra are fully inside the gain region. Whereas amplification is observed for low-intensity pulses, lossless propagation and even pulse absorption are found for large input intensities. These novel features arise from the competition between amplifying and absorbing states in the semiconductor continuum.

Photoluminescence and terahertz emission from femtosecond laser-induced plasma channels.

Luminescence as a mechanism for terahertz emission from femtosecond laser-induced plasmas is studied. By using a fully microscopic theory, Coulomb scattering between electrons and ions is shown to lead to luminescence even for a spatially homogeneous plasma. The spectral features introduced by the rod geometry of laser-induced plasma channels in air are discussed on the basis of a generalized mode-function analysis.

Many-Body Theory of Coherent Optical Effects in Semiconductors

An overview of the physics and the microscopic theory of coherent light-matter interaction in semiconductors and semiconductor heterostructures is presented. After an introduction of the basic concepts, a series of ultrafast coherent phenomena is reviewed, including the optical Stark effect, photon echoes, Rabi oscillations, quantum beats, multi-wave mixing, and pulse propagation effects.

Optical switching and propagation effects in nonlinear GaAs MQW waveguides

We summarize our ultrafast switching results in GaAs multiple quantum well directional couplers and report on coherent pulse propagation in single strip-loaded GaAs multiple quantum well waveguides. The transmitted pulse shape is measured by sum frequency generation cross-correlation and compared with calculations based on the coupled semiconductor Bloch and Maxwells equations.

Femtosecond pulse compression and adiabatic following in semiconductor amplifiers

pulse and is not in equilibrium with the atomic lattice because heat exchange is slow (time scale: several psec) and can be neglected here. The electron-hole plasma causes a strong repulsive interaction between the atoms, which induces an instability of the diamond lattice. In Fig. 1 we present our results for the cohesive energy per atom Eb of Si as a function of the amplitudes of the transverse acoustic (6,) and longitudinal optical (6,) phonons at the L-point. In this case 15% of the electrons have been excited from the valence band into the conduction band. Thus the diamond structure (6, = 6, = 0 and Eb = 0) becomes an unstable saddle point of the cohesive energy surface. A deep valley opens, leading down to very large negative energies of El = -0.45 eV. The average time-dependent displacement d( t ) of the atoms from their initial positions are shown in Fig. 2. These results are obtained from a numerical integration of the equation of motion and using the cohesive energy as a potential. Within 120 fsec the atomic displacement increases to about 1 A, which is roughly half the bond length in Si. This has strong effects on optical properties because the gap between the valenceand the conduction band breaks down. In the same time, the kinetic energy of the atoms has become larger than 0.4 eV, which is far above the melting temperature of Si. Thereafter the crystal melts very rapidly. These results also apply to GaAs. The larger mass of the atoms increases the time required for the instability to about 200 fsec. Note that this is in ood agreement with recent experiment

Light pulse propagation and charge carrier scattering in semiconductor amplifiers

-5 done on Si and GaAs. This theory also might be applied to laser-induced surface reconstruction, laser damage of ionic solids, and desorption of adsorbates.

Theoretical study of resonant ultrashort-pulse propagation in semiconductors

We present theoretical results on light pulse propagation in inverted semiconductors and semiconductor laser diodes. The theory is based on the semiconductor Maxwell Bloch equations and includes incoherent phenomena due to charge-carrier scattering based on the solution of the appropriate Boltzman equation.