Publications de Jean-Louis Staehli

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The interplay of disorder effects and exciton-exciton interactions in the four-wave-mixing process in quantum wells is investigated. Experiments are performed employing femtosecond and picosecond pulses to have either good temporal or good spectral resolution. It is shown that the localisation of excitons is of primary importance in the generation of the nonlinear signal. It is shown which states contribute and how to the FWM process.

We present a theoretical model of the time-resolved excitonic photoluminescence in quantum wires. We study the role of disorder and of the exciton-free-carrier interaction on the temperature dependence of the photoluminescence lifetime. We find that at low temperature the exciton population is not thermalized in disordered quantum wires and that the lifetime is mainly determined by the localization length of the exciton. However, for large localization lengths or high enough temperatures the exciton populat

We report on the optical response of a semiconductor microcavity featuring a strong exciton-photon coupling. The weak excitation regime is investigated experimentally by means of time-resolved transmission and four-wave mixing experiments. The influence of electronic disorder on the coherent exciton-photon dynamics is discussed in terms of tl-ie solutions of the Maxwell-Bloch equations. The high excitation regime is investigated by using transient sub-picosecond pump-probe experiments. Optical amplification

The optical gain spectra of strongly excited multiple GaAs-(Ga,Al)As quantum wells have been determined using the variable stripe length method. The gain spectra are obtained by analysing the dependence of the amplified luminescence intensities on stripe length. In the novel fitting procedure we applied it is assumed that the optically generated carriers of the electron-hole plasma are in quasi equilibrium. It is found that this assumption is justified in all the cases considered here where saturation effec

We present femtosecond gain measurements in microcavity-embedded quantum wells. When the excitonic transition is saturated by an intense pump field, the spectrum measured with a weak probe pulse is strongly modulated by pump-probe wave mixing processes. A giant probe amplification occurs within the bandwidth of the empty-cavity mode. The optical feedback provided by the microcavity is responsible for a net gain of the broadband probe pulse. [S0163-1829(99)50424-4].

We report on the experimental observation of excitonic molecules (biexcitons) in high-quality V-shaped quantum wires. By means of spatially resolved near-field photoluminescence spectroscopy, first we unambiguously isolate excitons with a one-dimensional character, and secondly we present the optical transition from 1D biexciton to 1D exciton as a function of excitation power density. The density of these biexcitonic molecules is shown to increase superlinearly compared to the linear variation of exciton de

GaAs/GaAlAs quantum wires grown by modulated flow rate metalorganic chemical vapour deposition were investigated by spatially resolved photoluminescence spectroscopy using a scanning near-field optical microscope. It was found that the wires decompose into a series of regions that emit luminescence of varying intensity. The spectra of these regions feature several narrow emission lines, which means that there is a series of more or less localised exciton states inside each region. It is expected that these

Type-II GaAs/AlAs quantum-well samples grown by low-pressure metal-organic vapour-phase epitaxy consisting of 50 periods of either 2 monolayers (ML), 4 ML, 5 ML, 6 ML, or 7 ML GaAs embedded in 28 ML AlAs were under investigation. Even at room temperature dominant type-I luminescence has been observed. The peak wavelength of this emission ranges from 620 to 440 nm and is determined by the GaAs layer thickness. A comparison of the measured transition energies with model calculations applying an effective mass

Our interest is centred on the very thin layers consisting of only one or a few monolayers of InAs. The optical transition energies, measured by photoluminescence spectroscopy, are compared with theoretical calculations obtained in envelope function approximation and through an empirical tight-binding method. This comparison yields values for the not well-known valence band offset at the InAs/InP interface, and the luminescence lines observed at different energies could be assigned to layers between one and

A theoretical and experimental study of the exciton in ultra-narrow quantum wells (QWs) is performed. A crossover from strong (separate localization of electron and hole levels) to weak confinement (with localization of excitonic center of mass) is predicted to occur for decreasing thickness, and is characterized by a minimum of the oscillator strength per unit area. Cathodoluminescence measurements performed on a series of GaAs/Al0.35Ga0.65As QWs with thicknesses from one to eight monolayers show a minimum

A steady state photoluminescence experiment is reported investigating the lifetime of an electron-hole plasma confined in a quintum well. The results are compared with theoretical evaluations of the pure radiative decay rate and good agreement is found. The effects caused by the filling of electron and hole subbands are also discussed.

We report on the enhancement of the photoluminescence (PL) intensity from a nanostructured organic material composed of nanosized aggregates of poly(para-phenylene) (PPP) in a polystyrene insulating matrix. In addition, the peak of the PL emission spectrum from these nanostructured organic semiconductors is blue-shifted from that of unstructured PPP. Furthermore, these PL characteristics correlate with the presence of an unusual sub-gap absorption feature that is present only in the nanostructured PPP. Thes

We report the direct observation of one-dimensional (ID) excitonic molecules (biexcitons) in high quality disorder free semiconductor quantum wires. By means of spatially resolved near-field photoluminescence spectroscopy, we detected delocalized 1D excitons through the spatial extension of their homogeneous emission. 1D biexciton transitions are evidenced by their correlation to the 1D excitons as well as their power dependence. The photoluminescence intensity from biexcitons is shown to increase quadratic

Differences and analogies between disorder-induced localization and electric-field-induced localization are discussed. Calculations using a tight binding model and transfer matrix method for disordered superlattices are reported. Our results indicate the existence of residual structures in the energy spectrum of the disordered superlattices, which are absent in the Stark localization case. The residual structure in the Anderson localization case can be interpreted as arising from the presence of the neighbo

We present second-harmonic generation experiments in p-doped asymmetrically stepped quantum wells using the emission of a free-electron laser in the wavelength interval between 13 and 18 mu m. The wells are designed such that the three fewest valence-subbands are about equally spaced. We measured the angular and frequency dependence of the second-harmonic signal excited by intense laser irradiation chosen to be nearly resonant with the transitions between the three lowest valence subbands. We see evidence o

We discuss the growth of lattice matched GaInAs, GaInAsP and GaInAsP/GaInAs multiple quantum well structures for high speed laser applications. Optical cavities, including five 90 angstrom GaInAs quantum wells with GaInAsP barriers and waveguides showed a 4 K photoluminescence line width of 8.7 meV. Transmission electron microscopy and X-ray diffraction confirm the good control over the growth of these structures.

We report femtosecond pump and probe experiments in a semiconductor microcavity containing quantum wells. At high pump fields, the exciton-polariton Rabi doublet changes into a triplet structure. The triplet splitting increases as the square root of the input intensity. The transmitted probe intensity is modulated with the corresponding frequency Versus pump-probe delay. The experimental results are discussed in terms of Mollow spectra of a saturated transition of a two-level system.

We report on a distance control system for low-temperature scanning near-field optical microscopy, based on quartz tuning fork as shear force sensor. By means of a particular tuning fork optical fiber configuration, the sensor is electrically dithered by an applied alternate voltage, without any supplementary driving piezo, as done so far. The sensitivity in the approach direction is 0.2 nm, and quality factors up to 2850 have been reached. No electronic components are needed close to the sensor, allowing t

The coherent optical response of high quality microcavities containing quantum wells is investigated. The dynamics of the ac Stark effect is investigated by degenerate pump-probe experiments, by exciting the quantum well excitons at various intensities and detunings. The evolution from a simple dynamical excitonic Stark shift to a splitting is dearly observed, even when the excitation is degenerate with the continuum states. The experimental results agree well with the predictions of a theoretical model bas

We succeeded in observing the two-photon absorption of germanium for both the direct and the indirect gaps, using the Vanderbilt free-electron laser. The two-photon absorption for the indirect gap was found to be 3 orders of magnitude smaller than the direct gap. The indirect-gap threshold position indicates that the two-photon absorption is assisted by an LO phonon, providing a long-delayed positive test of the Bassani-Hassan predictions.

Jean-Louis Staehli

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Influence of Disorder and Exciton-Exciton Interactions on 4-Wave-Mixing in Quantum-Wells

Modeling the photoluminescence lifetime in realistic quantum wires

Exciton-photon dynamics in weakly and strongly excited semiconductor microcavities

Optical Gain Measurements in Multiple Quantum-Wells at 2-K

Strong coherent gain from semiconductor microcavities in the regime of excitonic saturation

Probing excitonic nonlinearities in quantum wires

Level repulsion of localised excitons observed in near-field photoluminescence spectra

Ultrathin GaAs layers embedded in AlAs: a perspective for intense short wavelength emission

Optical study on ultrathin InAs/InP single quantum wells

Minimum of oscillator strength of excitons in ultra-narrow GaAs/AlGaAs quantum wells: Theory and experiment

Electron-Hole Plasma Decay Time in Gaas/(Ga, Al)as Quantum-Wells

Blue Light-Emission from a Nanostructured Organic Polymer Semiconductor

One-dimensional biexcitons in a single quantum wire

Anderson and Stark localization in GaAs/(AlGa)As disordered superlattices

Second-harmonic generation involving valence-subband transitions in p-doped GaAs/AlxGa1-xAs quantum wells

Growth of Gainas(P) and Gainasp/Gainas Mqw Structures by Cbe

Strongly driven semiconductor microcavities: From the polariton doublet to an AC Stark triplet

Ultra stable tuning fork sensor for low-temperature near-field spectroscopy

The microcavity AC Stark triplet: Excitation in the exciton continuum

Free-Electron Laser Studies of Direct and Indirect 2-Photon Absorption in Germanium