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The time-resolved secondary emission of resonantly created excitons in GaAs quantum wells is studied using femtosecond up-conversion spectroscopy. The behaviour of the rise and decay of the secondary emission and reflectivity in quantum welts is strongly dependent upon the disorder at the interfaces, the exciton density and the temperature. In the case of low densities and temperatures the emission is independent of the exciton density and rises quadratically in time, in excellent agreement with recent theo

We study the ultrafast properties of secondary radiation of semiconductor quantum wells under resonant excitation. We show that the exciton density dependence allows one to identify the origin of secondary radiation. At high exciton densities, the emission is due to incoherent luminescence with a rise time determined by exciton-exciton scattering. For low densities, when the distance between excitons is much larger than their diameter, the temporal shape is independent of density and rises quadratically, in

Time-resolved up-conversion measurements of secondary emission from multiple quantum wells under resonant femtosecond excitation are reported for GaAs multiple quantum wells with qualitatively different interface disorder. The transient resonant Rayleigh scattering presents very peculiar features such as a long-lived signal and a nonmonotonic behaviour, which cannot be explained by a classical model. A microscopic model is proposed, where the quantum mechanical equation for the microscopic polarization is s

We have performed spectral interferometry with Rayleigh scattered light coming from a single speckle and emitted after resonant creation of excitons in GaAs quantum wells:: The experiment confirms the coherence of the secondary emission at early times, regardless of sample:parameters. but underlines the stochastic nature of a single speckle. The scattered electric field is calculated, correctly accounting for the center-of-mass quantization in the disordered potential. We find that the amplitudes of the ele

We have studied the luminescence of narrow quantum wires at photoexcitation densities of up to similar to 3 x 10(6) cm(-1). We show that even at these densities, which are well above the expected Mott density of 8 x 10(5) cm(-1), excitonic recombination dominates over other recombination channels in stark contrast with the behavior of quantum wells and bulk structures at equivalent densities. As we observe no significant shift in the peak energy with density, an upper limit to the band gap renormalization c

We have studied experimentally the radiative lifetimes of negatively charged excitons (X-) and neutral excitons (X) in GaAs and CdTe quantum wells. Despite the different optical properties (e.g. trion binding energies) and the different quantum well structures of the studied samples, we find quite similar X- lifetimes in the range of 45 to 60 ps. On the other side, the X lifetimes are very different and influenced by the material. For relatively low electron concentrations, the X- lifetime is found to be co

The initial dynamics of resonantly excited secondary emission from quantum wells are dominated by Rayleigh scattering due to excitons localized in the disordered quantum well potential. We show experimentally and theoretically how the temporal shape of the early response is related to details of the two-dimensional disorder potential, and we determine the mean amplitude of the fluctuations and the correlation length. Spectral interferometry performed with a. single speckle allows to investigate the phase of

Time-resolved measurements of the resonant Rayleigh scattering from quantum well excitons are shown to provide information on the energy-level statistics of the localized exciton states. The signal transients are reproduced by a microscopic quantum model of the exciton two-dimensional motion in presence of spatially correlated disorder. This model allows quantitative determination of the average energy separation between the localized states. Here this quantity turns out to be only a few times smaller than

We present a comparative study of time-integrated four-wave-mixing and femtosecond emission under resonant excitation on excitons in weakly disordered GaAs quantum wells. At highest exciton densities when dephasing dominates the spectral width (homogeneous broadening), we find that the rise time of the incoherent luminescence signal is given by T-2/2. At lowest densities, optical coherence times approach the exciton radiative lifetime (15 to 20 ps). This confirms our previous result that coherent resonant R

Wide quantum wells, where the energy spacing between the first two energy levels is lower than the energy of the polar optical phonon, could represent the ideal condition to obtain a population inversion and the production of infrared lasers. On the other hand in these quantum wells the efficiency of carrier-carrier scattering is increasing with the width of the well and can produce a relatively quick relaxation of carriers in the first subband. In this work we present a detailed and systematic joint theore

The longitudinal spatial hole burning (LSHB) in gain-clamped semiconductor optical amplifiers (GCSOA's) is investigated by means of a numerical model, which is based on position-dependent rate equations for the carrier density and the propagation equations for the optical power, The simulation results show that the carrier densities are nonuniformly distributed within the active layer of GCSOA's, The nonuniformity can be large, especially for high currents and optical signal powers near the saturation. It i

Using reflectivity and picosecond time-resolved photoluminescence, we have studied the intrinsic optical properties of negatively charged excitons in modulation doped CdTe quantum wells. In emission, we observe simultaneously a low energy exponential tail in the charged exciton spectral line and a linear increase of its radiative lifetime with temperature. In absorption, we find a consistent decrease of the charged exciton oscillator strength with temperature. For low electron concentrations these observati

The coexistence of localized and free holes in modulation doped InGaAs/InP quantum wells, which show Fermi edge singularities in the cw photoluminescence and excitation spectra, has been determined by picosecond time-resolved luminescence spectroscopy. A long-lived peak at the tow-energy side of the emission spectrum is observed at 5 K and weak excitation. The peak, which vanishes on either increasing the temperature at similar to 30 K or increasing the excitation density, is attributed to recombination of

Pump and probe measurements with femtosecond resolution are performed on a gain-clamped semiconductor optical amplifier, a structure that contains a lasing mode. The corresponding relaxation oscillations are observed in the temporal gain recovery of the amplifier. The current dependence of the oscillations is well reproduced by a small-signal analysis of the laser rate equations. The gain recovery can be very fast, we measure damping rates of up to 40 GHz.

We have grown high-electron mobility transistor structures in the Al0.3Ga0.7As/In0.25Ga0.75As/GaAs material system with In0.25Ga0.75As channel thicknesses in the range 40-200 Angstrom. We have monitored the onset of channel relaxation using Hall mobility measurements, polychromatic cathodoluminescence mapping, time-resolved photoluminescence, transmission electron microscopy, low-frequency noise,and deep-level transient spectroscopy measurements. It appears that the first relaxation symptom, the Stransky-Kr

We have investigated the intersubband scattering of electrons in GaAs quantum wells using luminescence up-conversion with 100-fs resolution. The decay time of the n=2 electron-to-heavy-hole transition (e,hh)(2) depends both on the excess energy of the charge carriers and on the excitation density. A Monte Carlo simulation allows us to reproduce the experimental data with high accuracy. The intrinsic LO-phonon scattering rate is found to be 2.0x10(12) s(-1) for 80-meV subband separation. We show the wave-vec

A dynamical theory of multiphoton transitions in semiconductors is developed using a density matrix approach. It is shown that, in dipole approximation, these transitions require a mixed parity of the basis stales and that band-diagonal coupling's of the electromagnetic field to Bloch electrons have to be included. The general theory is outlined using the simple, but solvable model of a diatomic tight-binding chain. Possible extensions and applications of the approach are discussed.

Pump and Probe measurements with femtosecond pulses are performed on a Gain-Clamped Semiconductor Optical Amplifier (a special laser structure). The relaxation oscillations of the carriers density are observed directly in the time domain, Reasonable values of the differential gain and the nonlinear gain factors are extracted from the current dependence of the oscillations.

It is demonstrated that during the first few picoseconds after resonant femtosecond excitation the secondary radiation from a strongly coupled semiconductor microcavity containing one quantum well is dominated by Rayleigh scattered light. At early times after excitation (10 ps) the lower polariton branch provides the most intense emission which is due to incoherent luminescence.

Using time-resolved photoluminescence, we have investigated the radiative behavior of neutral and negatively charged excitons in CdTe-based quantum wells. We find that the photoluminescence of negatively charged excitons can be well reproduced by a model of delocalized and thermalized three-particle complexes. A large radiative zone in k-space results from the transfer of the charged exciton momentum to the remaining electron (recoil effect). On the other hand, the photoluminescence decay of excitons is not

Stefan Haacke

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Femtosecond dynamics of secondary radiation formation from quantum well excitons

Resonant femtosecond emission from quantum well excitons: The role of Rayleigh scattering and luminescence

Resonant Rayleigh scattering on quantum well excitons: Theory and experiment

Interferometric analysis of resonant Rayleigh scattering from two-dimensional excitons

Coulomb correlation and band gap renormalization at high carrier densities in quantum wires

Radiative lifetime of negative trions in GaAs and CdTe quantum wells

Excitonic dephasing effects studied by time- and phase-resolved secondary emission

Optical signatures of energy-level statistics in a disordered quantum system

Relevance of dephasing processes for the ultrafast rise of emission from resonantly created excitons in quantum wells

Carrier-carrier interaction and fast intersubband scattering in wide GaAs quantum wells

Longitudinal spatial hole burning and associated nonlinear gain in gain-clamped semiconductor optical amplifiers

Radiative behavior of negatively charged excitons in CdTe-based quantum wells: A spectral and temporal analysis

Role of hole localization in the optical singularities of a two-dimensional electron gas studied by time-resolved photoluminescence

Time-resolved relaxation oscillations in gain-clamped semiconductor optical amplifiers by pump and probe measurements

Optimum channel thickness of Al0.3Ga0.7As/In0.25Ga0.75As/GaAs heterostructures for electron transport applications

Femtosecond luminescence measurements of the intersubband scattering rate in AlxGa1-xAs/GaAs quantum wells under selective excitation

Dynamical density matrix theory of multiphoton transitions in semiconductors

Direct observation in the temporal domain of relaxation oscillations in a semiconductor laser

Resonant Rayleigh scattering versus incoherent luminescence in semiconductor microcavities

Negatively charged excitons in CdTe-based quantum wells: A time-resolved study