Publications by Jean-Louis Staehli

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A dramatic suppression of the absorption of light by exciton polaritons is observed after resonant excitation of the polaritonic modes in a multiple quantum well Bragg structure. Time-resolved reflectivity measurements confirm that this is due to the formation of a dominant superradiant polariton mode created by the light-induced coupling of excitons in different quantum wells that suppress absorption. The stability of the superradiant mode is demonstrated at high excitation powers and at temperatures of up

We report novel oscillator strength transfer phenomena induced by the exciton-exciton correlation. A high quality semiconductor microcavity is resonantly excited by intense circularly polarised sigma(+) pump pulses (100 fs long) and probed at different delays by weak counter-polarised sigma(-) test pulses. We show that the presence of sigma(+) excitons can completely inhibit the creation of sigma(-) excitons. With increasing sigma(+) exciton density the oscillator strength of the a- transition is transferre

Photoluminescence magnetospectroscopy is employed to examine the low-temperature magnetic phase that is induced by the carrier-mediated ferromagnetic exchange interaction in modulation-doped Cd1-xMnxTe quantum well. Unusual properties of the domain structure are linked to tendency towards spin-density wave formation in this low-dimensional magnetic system. (C) 2000 Elsevier Science B.V. All rights reserved.

We used optical pumping by the Vanderbilt free-electron laser and the technique of internal photoemission to measure with high accuracy the conduction-band discontinuity of semiconductor heterojunction interfaces. The experiment is the first application to our knowledge of a free-electron laser to interface research.

We calculate the shrinkage of the gaps between the two lowest subbands in a quantum well induced by the presence of a photo-excited electron-hole plasma. The results are reported as a function of well width and plasma concentration. The reduction of the higher gaps is found to be almost as strong as that of the fundamental gap. The contribution of the so called 'off-diagonal' terms is shown to be important.

We investigate, experimentally and theoretically, the optical amplification and its saturation in a mirrorless semicondctor quantum well laser. We optically excite a narrow stripe of variable length and measure the profiles of spontaneous and stimulated luminescence spectra. Comparison of theory and experiment reveals that saturation is mainly caused by carrier depopulation through stimulated electron-hole recombination, and by the loss of light travelling in the direction of the stripe.

Transmission spectra of ordered and disordered GaAs/Al0.3Ga0.7As superlattices (SL) were measured at 2 K. The experimental results are compared with numerical simulations obtained by means of a transfer matrix method. The numerical results indicate that a multiple-quantum-well model accounts for the main features of random SL, and that a disorder-induced fine structure is present. The agreement between the measured spectra and the numerical calculations supports the predicted features.

We study the coherent optical response of a very high-finesse microcavity with a single quantum well by means of degenerate subpicosecond pump-probe spectroscopy. The dynamics of the ac Stark effect versus Pump intensity is investigated upon tuning the empty-cavity;mode-below and above the exciton transition energy, at the exciton continuum onset. The evolution from a simple dynamical excitonic shift to a dynamical splitting is clearly observed in both eases. The qualitative, agreement with numerical calcul

The energy of the indirect X exciton in AlxGa1-xAs epitaxial layers (with 0.38 < x < 0.81) has been determined by photoluminescence measurements at a sample temperature of 12 K; from these data, the dependence on the Al concentration x of the energy of the GAMMA-X gap has been obtained: E(g)X(x) = 1.988 + 0.207x + 0.055x2 eV. This relation is characterized by a very small bowing. From our data we deduce also the coordinates of the GAMMA-X crossover point: x(c) = 0.385 +/- 0.016 and E(c) = 2076 +/- 4 meV at

The activities of the recently created Centre de Microscopie [1-3] at the Ecole Polytechnique Federale of Lausanne cover many different surface and interface research areas: we review here the new techniques based on photoemission spectromicroscopy and free electron laser spectroscopy.

A dramatic suppression of the light absorption by exciton-polaritons is observed after resonant excitation of the polaritonic modes in a multiple quantum well Bragg structure. We demonstrate that this is due to the formation of a dominant superradiant polariton mode. The stability of the superradiant mode is demonstrated at high excitation powers and at temperatures of up to 180 K. (C) 1999 Elsevier Science B.V. All rights reserved.

We investigate a semiconductor microcavity containing a quantum well, a sample of exceptional quality. We observe a four-wave mixing response which resembles that of an inhomogeneously broadened exciton system, while the linear transmission measurements are affected by the homogeneous part of the broadening and have a completely different decay time. We show that the four-wave mixing response cannot be calculated using the excitonic parameters (linewidth and relaxation time) extracted from the linear transm

A novel method of controlling light absorption by exciton-polaritons is presented. The coherent light-induced coupling of excitons in multiple quantum well Bragg structures is exploited to sup press dramatically the absorption. Our results show that excitonic light absorption is not an intrinsic property of excitons but rather an extrinsic effect that occurs when the excitons dephase.

We report the first experimental evidence of the existence of extended states in one-dimensional random potentials. Our results are obtained by investigating photoconductivity and optical absorption of GaAs/(AlGa)As disordered superlattices. The experimental results agree with the numerical and analytical predictions, thus indicating the possibility to design narrow bands of extended states at selected energies in disordered superlattices. (C) 1997 Elsevier Science Ltd.

The full experimental evidence of fine structure in disordered superlattices is reported. The experimental results agree with the numerical predictions, thus indicating that this effect is related to residual coupling between adjacent identical wells in the case of Anderson localization. This feature indicates an intrinsic difference between disorder-induced and electric-field induced localization, where no spectral fine structure can be observed in the strong localization limit. Copyright (C) 1996 Elsevier

Ultrafast pump-probe measurements evidence the dynamical Stark splitting in the exciton absorption spectrum of a single semiconductor quantum well. The intensity of the Stark sidebands in the spectrum is comparable to the linear absorption and their separation scales as the pump field amplitude. In the time domain the Rabi oscillations are observed. The semiconductor Bloch equations solved in the Hartree-Fock approximation demonstrate that the coherent exciton-light coupling dominates over the many-body dep

We engineer a semiconductor microcavity through an appropriate choice of geometry and materials in order to maximise the exciton-photon coupling. The polaritonic nonlinearities in such devices survive at high temperatures and high excitation densities. We measure parametric polariton amplification up to 220 K in CdTe microcavities and up to 120 K in GaAs ones. The amplification is also outstanding, reaching as much as 5000 at low temperature. The experimental observations suggest that the operating temperat

The influence of biexciton formation on the four-wave-mixing response of multiple quantum wells having a strong inhomogeneous broadening of the exciton transitions has been investigated. The experimental results are consistent with a model which distinguishes between crystal regions where the exciton energy is almost constant over the spatial extension of the biexciton state, and regions where the fluctuations of the exciton energy are comparable to or larger than the biexciton binding energy.

Pump-probe measurements in a microcavity containing a quantum well show that a population of circularly polarized (sigma(+)) excitons can completely inhibit the transition to sigma(-) one-exciton states by transferring the oscillator strength to the biexcitonic resonance. With increasing pump intensity the linear exciton-polariton doublet evolves into a triplet polariton structure and finally into a shifted biexciton-polariton doublet. A theoretical model of interacting excitons demonstrates that the crosso

Jean-Louis Staehli

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Suppression of exciton-polariton light absorption in multiple quantum well Bragg structures

Crossover from exciton to biexciton cavity polaritons

Ordered magnetic phase in Cd1-xMnxTe/Cd1-y-zMgyZnzTe : N heterostructures: magnetooptical studies

Interface Measurements of Heterojunction Band Lineups with the Vanderbilt Free-Electron Laser

Subband Renormalization in the Presence of a Dense Electron-Hole Plasma in a Quantum-Well

Photon and Carrier Density Profiles in a Semiconductor Optical Amplifier

Absorption spectra of GaAs/Al-x Ga1-xAs random superlattices at 2 K

Direct observation of an ac Stark splitting in semiconductor microcavities excited above the continuum onset

Indirect-Energy-Gap Dependence on Al Concentration in Alxga1-Xas Alloys

New Frontiers in Surface and Interface Instrumentation - Spectromicroscopy, Ultrabright Synchrotron-Radiation, Free-Electron Lasers

Suppression of exciton-polariton light absorption in multiple quantum well Bragg structures

The importance of disorder in very high quality semiconductor microcavities

Is light absorption by excitons in quantum wells an intrinsic process?

Extended states in one-dimensional random potential

Anderson and Stark localization in superlattices: Residual coupling and fine structure

Direct observation of the excitonic ac Stark splitting in a quantum well

Towards a room temperature polariton amplifier

Site-selective four-wave-mixing experiments in GaAs/(GaAl)As multiple quantum wells

Crossover from exciton to biexciton polaritons in semiconductor microcavities

Polariton quantum boxes in semiconductor microcavities