W. Schäfer

Magnetically Enhanced Exciton-exciton Correlations In Semiconductors

We present investigations of fs time resolved coherent wave mixing under high magnetic field. Our experiments reveal a new regime at high magnetic field and low excitation density dominated by the Coulomb interaction. This regime is inconsistent with the semiconductor Bloch equations. A model which includes exciton-exciton correlation successfully describes many features of this regime. {copyright} {ital 1997} {ital The American Physical Society}

Dissipative dynamics of an electronic wavepacket in a semiconductor double well potential

The authors present a detailed study of coherent tunneling oscillations of an optically excited electronic wavepacket in a semiconductor double quantum-well structure. The oscillation of the wavepacket between the two wells is traced by time-resolved pump-probe spectroscopy as well as time-resolved degenerate four-wave mixing. The experimental observations are compared with a simple theoretical model. The motion of the wave packet is studied as a function of both an external electric field and the lattice temperature. While the oscillation frequency shows the expected dependence on electric field, it increases with temperature at fixed electric field when perfectly delocalized electronic states exist at low temperature. >

Influence of electron–electron scattering on femtosecond four-wave mixing in semiconductors

The influence of electron–electron scattering processes on the time evolution of four-wave mixing signals for ultrashort excitation in the vicinity of the band edge is investigated. The validity of a Markovian approximation is carefully analyzed. The Markovian relaxation of the one-particle distribution is compared with non-Markovian results obtained from the solution of quantum-kinetic equations for two-point functions. In the non-Markovian case relaxation processes are slowed down.

Transient nonequilibrium and many-body effects in semiconductor microcavity lasers

We analyze the dynamical response of the interacting photon and electron–hole system in short-cavity bulk semiconductor lasers, including many-body effects of the electron–hole plasma and photon system, the interaction of electron–hole pairs and photons, and the selection of photon modes by the cavity. Carrier–carrier and carrier-phonon scattering is treated at the level of a quantum Boltzmann equation. Numerical solutions of the coupled equations for spontaneous and stimulated emission and carrier distributions show the development of lasing out of spontaneous emission and the stabilization of laser action connected with gain saturation and spectral hole burning. The saturation of the output intensity and modifications of the relaxation oscillations are investigated.

Coherent dynamics of exciton wavepackets in semiconductor heterostructures

The coherent dynamics of exciton wavepackets in (GaIn)As/GaAs as well as (GaIn)As/Ga(PAs) multiple quantum well structures is studied by means of transient four-wave mixing (FWM) experiments. The wavepackets are generated by simultaneous excitation of several exciton transitions with laser pulses of about 100 fs duration. The time-integrated FWM signals exhibit a pronounced modulation superimposed on the overall decay which can be attributed to the quantum interference of the different eigenstates. In the time-resolved FWM signals this interference is not present, reflecting the interplay between many-body Coulomb effects and inhomogeneous broadening. This experimental technique is then employed to extract the exciton binding energies in pseudomorphic symmetrically strained (GaIn)As/GaIPAs) with various In contents.

Optical and transport nonlinearities in laser excited semiconductors

In this paper we present the progress that has been made in the theoretical description of laser excited semiconductors resting on the use of nonequilibrium Greens functions. This technique permits consideration of several peculiarities (nonequilibrium character of carrier distributions, memory effects and dephasing of the interband polarization, screening of interactions, temporal and spatial evolution of excitations) simultaneously and on an equal microscopic footing. Applications to spectral hole burning and saturation kinetics under stationary conditions as well as to the propagation of ultrafast pulses are given.

Magnetically enhanced exciton-exciton correlations in semiconductors

Abstract Recent experiments on fs time-resolved coherent wave mixing under a high magnetic field have revealed a new regime at low excitation densities which is not in accordance with the semiconductor Bloch equations. We present a theory of coherently driven biexcitons in a strong magnetic field, which — due to the restriction of excitonic degrees of freedom — results in a strong enhancement of Coulomb correlation effects. The theory describes most of the observed features. Especially, the slow rising time of the time-integrated four-wave mixing signal is shown to result from biexcitonic correlation. The same interaction effects yield a strong dependence of the oscillator strength of the heavy- (hh) and light-hole (lh) excitons on the time delay between the two pulses.

Manybody-effects in nonlinear optics of semiconductors

Many-body Coulomb interactions dominate nonlinear optical properties of semiconductor structures in many respects. Resulting effects show up most distinct in the temporal evolution of transient four-wave mixing signals. The simplest example in systems with homogeneously broadened absorption is the static exchange interaction, which prevents the occurrence of pure free polarization decay and gives rise to an additional interaction induced signal. For inhomogeneous broadening one obtains photon echo-like behaviour. The temporal duration of the signal, however, is not determined by the width of the inhomogeneous broadening, as is predicted by theory if interaction effects are neglected, but is much smaller due to exciton-exciton interaction. Additional to the static interaction effects electron-electron correlations give rise to laser induced renormalization and dephasing processes which depend sensitively on the polarizations of the driving fields. These effects are analyzed on the basis of quantum-kinetic equations, which allow to describe the temporal evolution of the nonlinear polarizations. Numerical results are presented for various excitation conditions and material systems.

On the Lineshape of Time-Resolved Four-Wave Mixing

We study the effect of coherent interaction of polarizations in time-resolved Four-Wave-Mixing. The experimental results on excitons show novel effects. The physical origin of these effects is dicussed using simple models to emphasize their rather general nature.