Hélène Ulmer-tuffigo - Academia.edu (original) (raw)

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Papers by Hélène Ulmer-tuffigo

Ce travail apporte une contribution a l'etude des proprietes optiques des puits quantiques et... more Ce travail apporte une contribution a l'etude des proprietes optiques des puits quantiques et superreseaux de semiconducteurs. Le systeme que nous avons etudie, cdte/cdznte, reunit les caracteristiques suivantes: il s'agit d'un systeme contraint possedant un faible decalage de bande de valence; de plus, l'interaction coulombienne de la paire electron-trou formant un exciton est importante, du meme ordre que les energies de confinement des porteurs. Nous avons etudie a l'aide de differentes techniques de spectroscopie optique une serie de puits quantiques simples cdte/cdznte, d'epaisseur variant entre 1000 et 20 angstroms. Les spectres des puits les plus larges sont caracterises par la presence de nombreuses raies fines correspondant aux etats quantifies du mouvement du centre de masse de l'exciton dans la couche de cdte. Ce nouveau type de quantification se transforme en la description plus habituelle de l'exciton bidimensionnel forme a partir d'u...

Summary form only given. Final state stimulation is the origin of amplification in optical lasers... more Summary form only given. Final state stimulation is the origin of amplification in optical lasers and atom lasers. Matter-wave amplification based on an incoherent gain medium has yet to be demonstrated, but is very attractive, particularly for the excitonic system, because electrically injected incoherent excitons can amplify probe excitons without requiring a coherent pump laser. In previous work, we have shown final state stimulation using the exciton-exciton scattering of polaritons in a GaAs based microcavity quantum well; the gain was negligibly small due to the small exciton-exciton scattering rate. However, the rate of exciton-exciton scattering at the exciton saturation density is 9 times larger in CdTe microcavity exciton-polaritons compared to those in GaAs.

We describe experiments demonstrating an exciton-polariton laser and amplifier based on an incohe... more We describe experiments demonstrating an exciton-polariton laser and amplifier based on an incoherent exciton-polariton reservoir in CdTe microcavity quantum wells. The gain mechanism is real excited excitonexciton scattering, in which excitons created at large in-plane wave vectors are thermalized and accumulate at the bottleneck lower polariton states at smaller in-plane wave vectors. Because the exciton-exciton scattering rate for CdTe at the saturation density is higher than that for GaAs, the threshold for spontaneous polariton lasing is more easily reached in the case of CdTe with respect to GaAs. We demonstrate a high-gain amplification of bottleneck lower polaritons close to the lasing threshold. By performing a pulsed pump and probe experiment, we observe unambiguous evidence of real excited exciton-exciton scattering gain in the form of exp(const N exc 2), where N exc is the exciton-polariton reservoir population. This result is in sharp contrast to the recently demonstrated parametric polariton amplifier based on virtual coherent four wave mixing, in which gain is proportional to exp(const N exc). ͓P.G.

Superlattices and Microstructures, 1997

We demonstrate the effectiveness of the giant Zeeman effect in II-VI semimagnetic semiconductors ... more We demonstrate the effectiveness of the giant Zeeman effect in II-VI semimagnetic semiconductors to tune the exciton resonance of quantum wells onto the Fabry-Pérot resonance of a microcavity. A large oscillator strength of 3 × 10 13 cm −2 per quantum well is deduced from the measured 10.6 meV vacuum Rabi splitting.

Superlattices and Microstructures, 1998

Atomic layer epitaxy (ALE) is investigated for the growth of CdTe/MnTe superlattices. A systemati... more Atomic layer epitaxy (ALE) is investigated for the growth of CdTe/MnTe superlattices. A systematic structural characterization (X-ray diffraction, transmission electron microscopy), together with a magneto-optical study (reflectivity and photoluminescence), demonstrate that: (i) for MnTe ALE, all deposited Mn atoms are incorporated, so that no autoregulated growth mode can be obtained, in contrast with CdTe ALE, (ii) atomic layer epitaxy allows well-controlled CdTe/MnTe superlattices to be achieved but does not prevent the exchange between Cd and Mn atoms which occurs at the interfaces between CdTe and MnTe, as observed in CdTe/MnTe superlattices grown by conventional molecular beam epitaxy.

Solid-State Electronics, 1996

We present magneto-optical results obtained on MBE-grown CdTejCdMgTe heterostructures embedding u... more We present magneto-optical results obtained on MBE-grown CdTejCdMgTe heterostructures embedding ultra-thin magnetic MnTe insertions. These submonolayer insertions modify considerably the spectra at zero field, as predicted by a calculation based on the envelope function approximation: moreover. the transitions observed exhibit very large splittings under magnetic field that can be the indication of the modification of the spin state for very thin MnTe layers due to the lack of nearest neighbours, or on various disordering processes.

Materials Science and Engineering: B, 2001

Single Si/SiO 2 quantum wells were fabricated using silicon on insulator (SOI) substrates, the up... more Single Si/SiO 2 quantum wells were fabricated using silicon on insulator (SOI) substrates, the upper barrier being a thin thermally oxidized Si layer. Samples were Er and O implanted at low energy in order to center their concentration profile in the well, and then annealed. A bulk Si sample implanted under the identical conditions was used as a reference. We present low temperature photoluminescence results using UV excitation in order to create photogenerated carriers mainly in the Si well. We show that the 1.54-mm photoluminescence is much more intense for Er in a well than for Er in implanted bulk silicon. We show that infrared emission occurs through an energy transfer mechanism from Si to Er centers. Confinement prevents the escape of photogenerated carriers towards the substrate and promotes energy transfer towards optical active erbium center.

Journal of Crystal Growth, 1998

The occurrence of a stimulated emission line exactly superimposed on the lower branch of a II-VI ... more The occurrence of a stimulated emission line exactly superimposed on the lower branch of a II-VI microcavity polariton shows that laser emission can be obtained from a strongly coupled exciton-photon system. We present spectral, time and power data as experimental evidence for this.

Journal of Crystal Growth, 1996

We present experimental results on the optical properties of CdZnTe and CdMnTe based microcavitie... more We present experimental results on the optical properties of CdZnTe and CdMnTe based microcavities. The CdZnTe microcavity shows a well defined resonance at 1.06 μm, whereas the CdMnTe cavity shows two resonances in the near-infrared region. By taking advantage of the specificity of the semimagnetic semiconductor CdMnTe, we are able to tune one resonance by 24 Å using a magnetic field of 8 T at helium temperature.

Applied Surface Science, 1998

This paper is devoted to the description of the interface between MnTe and CdTe, and to its magne... more This paper is devoted to the description of the interface between MnTe and CdTe, and to its magneto-optical properties. Indeed, the epitaxy of an heterojunction with these two II-VI semiconductors tends to form a few monolayers of Cd~ _xMn,Te, which is a well known semimagnetic semiconductor very sensitive to a magnetic field [1]. Thus, thanks to the enhanced giant Zeeman splitting of the excitonic line observed in MnTe/CdTe heterostructures under a magnetic field, we have a very powerful tool for probing the quality of this interface at the atomic scale. Systematic studies can be performed in order to analyse the influence of different growth conditions. It is shown that the magnetic properties of MnTe/CdTe interfaces grown at 280°C are mainly due to an exchange between Mn and Cd atoms that occurs, during the growth, between the last incorporated monolayer and the one being grown. Thanks to a modelisation of the kinetics effects in the Mn segregation process, we can compute the Mn concentration profile monolayer by monolayer. We are then able to 'quantify' the experimentally observed influence of the substrate temperature, of the impinging fluxes, of the growth method, and finally of the cations involved at the interfaces, on the Mn segregation process. By optimising all these parameters, we show how to control the abruptness of the MnTe/CdTe interfaces.

Physical Review Letters, 1994

We have found strong evidence for tunneling of excitons as an entity in CdTe/CdMnTe and CdTe/CdZn... more We have found strong evidence for tunneling of excitons as an entity in CdTe/CdMnTe and CdTe/CdZnTe asymmetric double quantum wells (QWs). An external magnetic field changes the coupling between the two QWs by allowing resonances to occur between excitonic states. The tunneling dynamics are investigated by time-resolved and steady-state photoluminescence spectroscopies under magnetic field. Very efficient tunneling is found when the transfer of a spatially direct exciton is possible, either with the emission of LO phonons or via the resonance with the 2s state of the low energy QW.