Publications de Jean-Louis Staehli

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Results are presented of both linear optical transmission and femtosecond four-wave mixing measurements, on a II-VI semiconductor microcavity into which three quantum wells have been inserted. The excitons (Xs) confined in the wells are strongly coupled to the photons, and at resonance the transmission spectra reveal two polariton peaks of less than 2 meV width, and a Rabi splitting of 11.5 meV, demonstrating the high quality of the sample. The nonlinear emission shows strong and well-resolved beats at the

Picosecond transient reflectometry and dynamic grating techniques have been applied to investigate the perpendicular and parallel transport of optically excited carriers in strained-layer Si0.83Ge0.17/Si superlattices. We present results of the carrier ambipolar diffusivity and effective lifetime measurements in the layered structure and substrate within the 10(17)-10(20) cm(-3) density range. The combined experimental data are discussed in terms of parallel and perpendicular diffusion of carriers, interfac

An investigation of the parametric amplification and its coherent control in a semiconductor micro-cavity is presented. The time and angle resolved pump and probe experiments show that several ps after pumping parametric scattering is still phase coherent. The experimental data are in qualitative agreement with the numerical data obtained from a relatively simple theoretical model based on three polarisation components, pump, probe, and idler. This model also predicts that under particular circumstances, th

Free electron lasers (FEL's) are not new instruments: for a long time, they have produced high-intensity photon beams, primarily in the infrared. They can be, therefore, excellent complementary facilities to synchrotron radiation, whose primary spectral domains are the ultraviolet and the xrays. The use of free electron lasers for experiments, however, has been quite limited; this situation is rapidly changing, with programs being put in operation at Santa Barbara, at Vanderbilt University in Nashville, at

Photoluminescence spectra of GaSe at moderately high densities of electron-hole pairs are reported for lattice temperatures between 2 to 200 K. We observed, at energies below the exciton recombination line, an additional narrow emission which has been attributed to exciton-free carrier scattering. Its narrowness at low temperatures is due to the selection rules for optical interband transitions in GaSe. Using numerical calculations based on second order perturbation theory, we fit quite well the experimenta

We present low temperature photocurrent (PC) spectra of ordered and disordered GaAs/AlGaAs superlattices (SLs). The investigated disordered SLs were designed to provide both localized and extended states. Our results show a dramatic difference between the PC of ordered and disordered SLs. In the ordered case, the PC is observed to decrease abruptly on increasing the applied voltage V, as expected. In contrast, the total PC signal in the disordered SLs increases in steps as a function of the electric field.

We present a novel semiconductor structure in which 0D polaritons coexist with 2D microcavity polaritons. Spatial trapping of the 2D microcavity polaritons results from the confinement of their photonic part in a potential well, consisting of an adjustable ...

In this letter the liquid phase epitaxial growth of In0.71Ga0.29As0.68P0.32 (lambda(g) = 1.32 mum) single quantum well structures lattice matched to (001) InP substrates is reported. The electrical and optical confinement was formed either by InP or In0.88Ga0.12As0.26 p0.74 (lambda(g) = 1.05 mum). Low-temperature photoluminescence was employed to prove the samples capable of displaying quantum size effects. Energy upshifts up to 125 meV were measured for InP-clad quantum wells of about 50 angstrom thickness

We investigate the dependence of four-wave mixing response on the photon energy close to the fundamental exciton (X) resonance in GaAs quantum wells. We find that cross-polarised incident fields give rise to a non-linear signal which decays faster at energies below the X line centre than above. We show that this behaviour cannot be assigned to biexciton transitions alone but rather suggests that the delocalised X states above the mobility edge are excited off-resonantly by the laser light having slightly lo

We report on a transparent analysis of the luminescence spectra of dense electron-hole plasmas confined in GaAs/(Ga,Al)As quantum wells. We fit the spectra using the fact that the total number of particle states does not depend on collision broadening. Further, the effective dispersion of the valence and conduction subbands and the dependence of the optical matrix elements on wavevector is accounted for. We find (a) that the lowest subbands shift almost rigidly with changing plasma density, and (b) that the

Differential photoluminescence excitation (DPLE) spectroscopy is shown to be an efficient way to measure gain and absorption spectra of excited semiconductors. Very small excited volumes can be probed, and thus DPLE spectroscopy is ideal to probe nanostructures. This technique is applied to investigate the width of the gap between the second electron and hole subbands of a quantum Well versus carrier density, in a sample which contains only a few wellS. It is found that the reduction of this gap is a smooth

In a pump-probe experiment, we have been able to control, with phase-locked probe pulses, the ultrafast nonlinear optical emission of a semiconductor microcavity, arising from polariton parametric amplification. This evidences the coherence of the polariton population near k = 0, even for delays much longer than the pulse width. The control of a large population at k = 0 is possible although the probe pulses are much weaker than the large polarization they control. With rising pump power the dynamics of the

In highly excited semiconductor systems near the transition from direct to indirect band structure (cross-over) the energetic distance between different conduction band minima is a nonmonotonous function of the excitation intensity. A theoretical explanation of this experimental result is given. Our investigations are based on the evaluation of photoluminescence spectra using a careful lineshape analysis. The nonmonotony is a consequence of the unequal shrinkage of the direct and indirect gaps as a function

We study the subpicosecond gain dynamics in a high-finesse semiconductor microcavity containing quantum wells by means of degenerate pump-probe spectroscopy. Tuning the optical excitation in resonance with both the exciton and the cavity modes, at high pump fields the probe pulses are strongly amplified near the energy of the empty-cavity mode. At zero probe delay, integration of the probe spectrum yields a net gain of several percents. The coherent nature of the gain is demonstrated by its oscillatory beha

We report transient sub-picosecond pump and probe experiments performed on a semiconductor microcavity into which quantum wells have been inserted. The cavity is featuring resonance and strong coupling with the exciton. It is strongly and resonantly excited. Optical amplification of the probe beam (up to a factor of 5) appears, a blueshift of the polaritons occurs once the excitation reaches the exciton saturation density. The intensity of the probe, reflected or transmitted by the sample, oscillates while

In this work we present the coherent control of polariton parametric scattering with weak phase locked probe pulses. It is demonstrated that the signal emission arising from the parametric scattering is completely coherent for moderate excitation power, whereas at high pump density the dynamics appears less coherent due to the depletion of the pump polariton reservoir. It is furthermore demonstrated that the phase of the probe pulses controls the emission from signal, pump, and idler at the same time.

We have studied the femtosecond dynamics of excitonic resonances in quantum well microcavities under strong excitation. Very strong non-linearities are observed, which bear clear resemblance to the non-linearities of an atomic two-level system. The fact that the excitonic system undergoes Rabi flopping and AC Stark splitting is clearly evidenced in a number of cases. Excitation induced dephasing shows an effect much stronger than the light dressing and prevents the observation of the Rabi flopping only when

The temperature and excitation intensity dependencies of the photoluminescence (PL) spectra of GaAs/Al0.3Ga0.7As superlattices (SL's) having randomly distributed well widths are studied. Our results indicate that the electronic properties of disordered SL's are similar to those of other disordered semiconductors: at low energies the electronic states are essentially localized, while at higher energies they are extended. Further, the PL spectra feature a disorder-induced fine structure, and they shift to the

We investigate the mechanisms leading to the optical non-linearities observed on a strongly excited semiconductor microcavity containing quantum wells. This study of a coupled exciton-cavity system is possible owing to the availability of a sample of exceptionally high quality, permitting to exclude effects of disorder and inhomogeneous broadening. Sub-picosecond pump and probe experiments are performed, in the regime of an incoherent exciton gas of high density. For such a gas the dephasing due to collisio

We investigate photoluminescence excitation (PLE) and selective photoluminescence (SPL) of ordered and disordered superlattices, showing that the coexistence of localized and extended states in disordered superlattices produces a dramatic dependence of the PLE spectra on the detection energy. We demonstrate that the PLE of disordered superlattices gives ambiguous information on joint density of states, and we use SPL in order to show that this effect is a consequence of the localization and of the lack of t

Jean-Louis Staehli

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Coherent exciton-photon dynamics in high-quality II-VI semiconductor microcavities

Lateral and cross-well transport of highly and moderately excited carriers in Si1-xGex/Si superlattices

Quantum kinetics of parametric polariton scattering in microcavities

First experimental programs with the free electron laser

Luminescence from Exciton-Free Carrier Collisions in GaSe

Low temperature photocurrent spectra of ordered and disordered superlattices

Zero dimensional exciton-polaritons

Characterization of lattice-matched single In1-xGaxAsyP1-y quantum wells grown by conventional liquid phase epitaxy

4-Wave-Mixing Signals from Extended Exciton-States above the Mobility Edge in Gaas (Ga,Al)as Multiple-Quantum Wells

Spontaneous Emission of Highly Excited Gaas/(Ga,Al)as Quantum-Wells

Differential Photoluminescence Excitation Spectroscopy - a Novel and Sensitive Tool to Probe Gain and Absorption in Semiconductor Nanostructures

Coherent control of polariton parametric scattering in semiconductor microcavities

Optical-Properties and Switching Behavior of Excited Iii-V Semiconductors near the Direct Indirect Cross-Over

Net coherent optical gain in strongly pumped semiconductor microcavities

Optical stark effect and coherent gain of excitons in a semiconductor microcavity

Coherence properties of polaritons in semiconductor microcavities

Femtosecond dynamics and non-linearities of exciton-photon coupling in semiconductor microstructures

Temperature and excitation intensity dependencies of the photoluminescence spectra of GaAs/(AlGa)As disordered superlattices

Intrinsic non-linearities in exciton-cavity-coupled systems

Photoluminescence excitation spectra of disordered superlattices