Showing papers by "Benoit Deveaud published in 2001"

High-temperature ultrafast polariton parametric amplification in semiconductor microcavities.

Abstract: Cavity polaritons, the elementary optical excitations of semiconductor microcavities, may be understood as a superposition of excitons and cavity photons1. Owing to their composite nature, these bosonic particles have a distinct optical response, at the same time very fast and highly nonlinear. Very efficient light amplification due to polariton–polariton parametric scattering has recently been reported in semiconductor microcavities at liquid-helium temperatures2,3,4,5,6,7,8,9,10,11. Here we demonstrate polariton parametric amplification up to 120 K in GaAlAs-based microcavities and up to 220 K in CdTe-based microcavities. We show that the cut-off temperature for the amplification is ultimately determined by the binding energy of the exciton. A 5-µm-thick planar microcavity can amplify a weak light pulse more than 5,000 times. The effective gain coefficient of an equivalent homogeneous medium would be 107 cm-1. The subpicosecond duration and high efficiency of the amplification could be exploited for high-repetition all-optical microscopic switches and amplifiers. 105 polaritons occupy the same quantum state during the amplification, realizing a dynamical condensate of strongly interacting bosons which can be studied at high temperature.

Near-field imaging of one-dimensional excitons delocalized over mesoscopic distances

Abstract: Near-field optical spectroscopy is used to investigate the effects of disorder in the optical processes in semiconductor quantum wires. We observe photoluminescence emissions from extended, delocalized excitons at low temperatures (5 K) and low excitation densities. Combining high spectral and spatial resolution, we isolate homogeneous emission lines from excitons delocalized over distances up to 600 nm in the fundamental state. The energies of the emissions are consistent with different quantum spatial confinements along the win axis. Unlike the photoluminescence originating from localized excitons, these emission lines show a high degree of polarization along the axis of the wire.

Direct observation of longitudinal spatial hole burning in semiconductor optical amplifiers with injection

Abstract: Measurements of spontaneous emission from InGaAsP semiconductor optical amplifiers provide information on both the carrier density and temperature. By spatially resolving the light emitted along the active layer of the device, we find evidence of longitudinal spatial hole burning which results from amplified spontaneous emission in the structure and is modified by the injected optical signal. Under injection, we also observe pronounced asymmetry of the amplified spontaneous emission intensity from the two facets which we relate to the carrier density profile. The experimental results are in good agreement with numerical simulations. An analysis of the measured spectra reveals an unexpected very high temperature (400 K) and its decrease by at least 35 K in the middle of the device when light is injected. (C) 2001 American Institute of Physics.

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

Abstract: 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 well described in the framework of a model of delocalized and thermalized excitons. We infer that the main difference between the behavior of neutral and charged excitons in the time domain stems from the large radiative zone of charged excitons.

DOSSIER SOURCES LASER ET SPECTROSCOPIES FEMTOSECONDES : TENDANCES ACTUELLES TRENDS IN FEMTOSECOND LASERS AND SPECTROSCOPY Femtosecond dynamics and non-linearities of exciton-photon coupling in semiconductor microstructures

Abstract: 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 exciting in the continuum. Most of the experimental findings are well reproduced by a dynamical solution of the Maxwell-Bloch equations for an ensemble of two-level systems. This allows in particular understanding of the occurrence of strong coherent gain in microcavities. An exhaustive description of the experiments is given within the framework of semiconductor Maxwell-Bloch optical equations at the Hartree-Fock level. 2001 Academie des sciences/Editions scientifiques et medicales Elsevier SAS ultrafast dynamics / Rabi flopping / Coulomb correlation / excitons / coherence / non- linearities / pump-probe spectroscopy

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

Abstract: 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 exciting in the continuum. Most of the experimental findings are well reproduced by a dynamical solution of the Maxwell-Bloch equations for an ensemble of two-level systems. This allows in particular understanding of the occurrence of strong coherent gain in microcavitics. An exhaustive description of the experiments is given within the framework of semiconductor Maxwell-Bloch optical equations at the Hartree-Fock level. (C) 2001 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.

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

Abstract: Summary form only given. In the excitonic ac Stark shift in quantum wells the excitonic absorption line is blueshifted by shining on the sample an intense laser pulse, whose frequency is lower than the exciton transition frequency. The effect is bigger when the laser spectrum approaches the resonance with the excitonic absorption. But if the laser and excitonic absorption spectra sensibly overlap, a great amount of electronic population is excited and consequently the Coulomb nonlinearities due to the interaction between excited carries or excitons dramatically increase. One prominent effect under these conditions is the excitation- induced dephasing which can destroy the coherence of the electronic system and therefore the coupling between excitons and light, which is inherently coherent. Some evidence for the resonant Stark effect was already reported, but the direct observation of the spectral splitting was still missing. This observation demonstrates that the coherent exciton light coupling in semiconductors can be strong enough to survive the fast dephasing, and that therefore an efficient ultrafast manipulation of the exciton absorption in the coherent transient is possible. In order to magnify the ac Stark effect and its visibility a special sample was designed: a single In/sub 0.04/Ga/sub 0.96/As quantum well is grown on the top of an AlAs/AlGaAs distributed Bragg reflector; two /spl lambda//2 spacers separate the quantum well from the mirror and from the air. In this configuration the light field incident on the sample and the one reflected by the mirror constructively interfere at the quantum well position, giving a high effective absorption.