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International audience; The spatial dynamics of the formation of a polariton condensate under a tightly focused excitation is imaged through 2D near-field and far-field 2D tomography in a ZnO microcavity, up to room temperature. The modelling exhibits the role of the outwards polariton flux caused by the reservoir repulsion, that leads to a 3 to 10 fold increase of the condensation threshold and is imprinted in the shape of the polariton condensate

International audience

A highly sensitive integrated optical biosensor based on the cascade of a Fabry-Perot cavity laser and a microring resonator is investigated theoretically and experimentally. The free spectral ranges of the microring and the laser cavity are designed to be about the same in order to apply intensity interrogation. The variation of the refractive index of the fluidic sample results in a large shift in the envelope function of the total transmission spectrum of the sensor, and consequently, a large change in the transmitted power. The simple light-source-integrated sensing scheme achieved a high sensitivity of 1000 dB/RIU in the preliminary experiment, demonstrating its principle and potential for low-cost practical applications.

An electroplated copper substrate was evaluated for heat dissipation in 1.55-μ.m optically pumped vertical extended cavity surface emitting lasers (OP-VECSELs). It is a cost-effective and flexible solution compared with the previously proposed chemical vapor deposition diamond substrate assembled by metallic bonding. Continuous-wave (CW) lasing operation was demonstrated from a device (with copper electroplated substrate) under optical pumping with pump spot diameter of 100 μm and a maximum output power of 260 mW at 0°C; heatsink temperature was achieved. Room-temperature CW operation with an output power of 75 mW and an external quantum efficiency of 35% was achieved in an optimized plane-concave cavity. The thermal resistance and the maximum output power of VECSEL chips assembled with bonded bulk copper and electroplated copper substrates were measured and compared. A value of ~50 K/W was estimated for both devices, and a similar rollover point was observed, which indicates that the electroplated copper solution leads to similar thermal properties as a bonded bulk copper substrate.

Lasing in a ZnO planar microcavity is reported under optical excitation up to room temperature. The cavity presents a quality factor of 450 and the simultaneous presence of two cavity modes with different interference orders. The transition from the strong coupling to the weak coupling regime is observed at lasing threshold. The gain mechanism below T = 240 K is attributed to excitons, according to the observation of a specific gain transition within the strong coupling regime. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A multiscale approach has been developed in order to simulate the etch process of InP in an inductive coupled plasma (ICP) Cl2/Ar plasma discharge. The model consists of three modules: a global kinetic model of the Cl2/Ar plasma discharge, a sheath model, and a 2-D Monte-Carlo etching model. The densities and the fluxes of all neutral and charged species considered in the reaction scheme as well as the electron temperature are calculated from the global model. The angular and energy distribution functions of ions are computed from the sheath model. The output parameters of both the global kinetic model and the sheath model in terms of particle fluxes, ion angular distribution function and ion energy distribution function are used as input parameters in the 2-D etching model. The latter allows tracking in time the evolution of the etched surface. The ultimate goal of the multiscale approach is to predict the etch rate, the etched surface chemical composition, and the etch profile as a function of the operating conditions (power, pressure, gas flow rates, etc). In this paper, the results from the global model are first compared to the measurements carried out in the ICP etching tool, showing a satisfactory agreement. The etching model is then used to simulate the etching of narrow trench and small-diameter hole in InP. The mechanisms involved in the development of undercut below the mask and in the bowing effect are analyzed. The comparison between simulated and experimental etch profiles evidences the important role of Cl adsorption probability on the development of the undercut, and the significant impact of the redeposition of the etched species on the etch rate variation and on the narrowing in the bottom of the etched hole/trench.

The oxidation of toluene was carried out at room temperature with a Fenton reactant FeII-H2O2 in a diphase medium and a modified Udenfriend FeII-O2 system electrochemically regenerated We remarked that the Udenfriend system is much more selective for oxidation products of the methyl group (benzaldehyde, benzyl alcohol) than for the hydroxylation products of the aromatic cycle (ortho- and para-cresols, methyl-p-quinone). This tendency is the opposite with a Fenton reactant. Furthermore, it is to be noted that the regeneration of the catalyst results in a significant increase in the activity of the two catalytic systems. As for as the Fenton reactant is concerned, the use of a phase transfering agent (tetrabutylammonium bromide) made the reaction possible and improved the activity of the system.

A study is undertaken of the loss kinetics of H and Cl atoms in an inductively coupled plasma (ICP) reactor used for the etching of III-V semiconductor materials. A time-resolved optical emission spectroscopy technique, also referred to as pulsed induced fluorescence (PIF), has been combined with time-resolved microwave hairpin probe measurements of the electron density in a pulsed Cl2/H2-based discharge for this purpose. The surface loss rate of H, kwH, was measured in H2 plasma and was found to lie in the 125–500 s−1 range (γH surface recombination coefficient of ∼0.006–0.023), depending on the reactor walls conditioning. The PIF technique was then evaluated for the derivation of kwCl, and γCl in Cl2-based plasmas. In contrast to H2 plasma, significant variations in the electron density may occur over the millisecond time scale corresponding to Cl2 dissociation at the rising edge of the plasma pulse. By comparing the temporal evolution of the electron density and the Ar-line intensity curves with 10% of Ar added in the discharge, the authors show that a time-resolved actinometry procedure using Ar as an actinometer is valid at low to moderate ICP powers to estimate the Cl loss rate. They measured a Cl loss rate of ∼125–200 s−1 (0.03≤γCl≤0.06) at 150 W ICP power for a reactor state close to etching conditions. The Cl surface loss rate was also estimated for high ICP power (800 W) following the same procedure, giving a value of ∼130–150 s−1 (γCl∼0.04), which is close to that measured at 150 W ICP power.

Electrically driven CW operation of energy-efficient ultra-compact InP-based nanolasers in a Silicon photonic platform is demonstrated. The nanolaser diode is single mode and emits at 1560 nm with SMSR grater than 20dB. The threshold is low, around 0.1mA at 1 V and the emitted power in the SOI waveguide is of the order of 0.1mW giving a wall-plug efficiency higher than 10 percents.

International audience; The achievement of a 1.51 µm InAs/InP(311)B quantum dot (QD) single-mode Fabry–Perot laser operating under continuous wave at room temperature is reported. A threshold current of 80 mA associated with a 0.12 W/A external efficiency is reported for as-cleaved device at room temperature. The maximum output power, the gain peak wavelength at threshold and the full width at half maximum of the spectral gain are 5 mW, 1512 nm and 60 nm, respectively. Although these performances must be improved in the future, these results constitute to our knowledge the state-of-the-art on InP(3 1 1)B substrate. These results are of great interest since QD-based lasers are expected to play a major role in the next generation telecommunication networks as low-cost devices.

National audience

Two x-ray photoelectron spectroscopy configurations are proposed to analyze the surface chemistry of micron-scale InP ridge structures etched in chlorine-based inductively coupled plasma (ICP). Either a classical or a grazing configuration allows to retrieve information about the surface chemistry of the bottom surface and sidewalls of the etched features. The procedure is used to study the stoichiometry of the etched surface as a function of ridge aspect ratio for Cl2/Ar and Cl2/H2 plasma chemistries. The results show that the bottom surface and the etched sidewalls are P-rich, and indicate that the P-enrichment mechanism is rather chemically driven. Results also evidence that adding H2 to Cl2 does not necessarily leads to a more balanced surface stoichiometry. This is in contrast with recent experimental results obtained with the HBr ICP chemistry for which fairly stoichiometric surfaces have been obtained.

National audience; ZnO est un matériau semiconducteur présentant des excitons forts, en terme de force d’oscillateur et d’énergie de liaison. Il est donc particulièrement intéressant pour la génération et le contrôle de condensats de polaritons, jusqu’à température ambiante.La réalisation de microcavités ZnO planaires de grand facteur de qualité a longtemps constitué un défi important. Nous avons réalisé dès 2011 le premier laser à polaritons ZnO (à T=120K et un désaccord exciton-photon nul) [1], dans une microcavité hybride (DBR AlN/AlGaN, ZnO réalisé par épitaxie MBE, DBR SiO2/SiN). Son facteur de qualité, Q=450, était cependant trop faible pour permettre un fonctionnement du laser à polaritons à température ambiante. Dans une approche radicalement différente, une microcavité a ensuite été réalisée en insérant une couche active ZnO massive de grande qualité cristalline entre deux miroirs diélectriques SiO2/HfO2 ; le facteur de qualité a alors atteint Q>2000, pour un dédoublement de Rabi de 250 meV. Cette cavité a permis d’observer la condensation des polaritons sur une plage de composition exciton-photon et de température sans précédent, jusqu’à T=300K [2]. Le diagramme de phase complet du laser à polaritons a été mesuré, montrant que le seuil de condensation à 300K est seulement 6 fois supérieur à celui observé à 8K. Ce résultat est en bon accord qualitatif avec des simulations de la relaxation des excitons et des polaritons dans un modèle cinétique. L’accordabilité de cette microcavité constitue un progrès important en comparaison des travaux antérieurs [3,4].La formation et la propagation du condensat de polaritons sont étudiées dans la microcavité de Q>2000 par imagerie 2D de l’émission via un spectromètre imageur, dans l’espace réel comme dans l’espace de Fourier des impulsions. Le laser d’excitation est focalisé sur quelques microns, et les contributions du réservoir d’excitons et des différentes branches polaritoniques sont mesurées (figure 1). Les excitons diffusent sur quelques microns, alors que le condensat de polaritons se propage sur des distances atteignant 10 à 20 µm. Les profils spatiaux du condensat sont comparés aux résultats d’un modèle de formation et de propagation en symétrie cylindrique, permettant d’identifier les différents régimes de fonctionnement : génération, répulsion du condensat par interaction avec le réservoir d’excitons, amplification du condensat lors de la propagation. Ces résultats seront comparés avec les démonstrations récentes dans les microcavités GaAs.La microcavité précédente présente un gradient important d’épaisseur, qui rend difficile la réalisation de condensats de forme variée. Des cavités équivalentes ont alors été réalisées sur des DBR AlN/AlGaN sur mesas Si, dans lesquels le laser à polaritons fonctionne aussi jusqu’à température ambiante [5]. Elles ont permis de travailler sous faisceau laser d’excitation défocalisé ou façonné. Le rôle de la localisation des polaritons sur la distribution spatiale du condensat sera discuté.

International audience; A low power Hall Effect Thruster (HET), based on a permanent magnet circuit, was investigated in the GREMIlaboratory facility. The thruster operated in the working range between 50 and 300 W and the previouslymeasured thrust is between 4 and 16 mN for an anodic efficiency respectively between 15% and 27%. Thepulsed character of the thruster current is an important feature of this HET. The ion current’s bursts are recordedat 30 and 70 cm from the exit plane in the thruster plume and are time-resolved, which lead to a preliminaryanalysis of the time of flight (TOF) phenomena. This paper presents a detailed study of these bursts of ioncurrent in the plume. The total ion current is shown to be a superposition of 2 distinct contributions of chargedspecies.In complement, a controlled single current interruption in stable anodic current condition leads to exactlythe same features than in oscillating mode. This crucial verification garantees the validity of the time of flightorigine of the two distinct contributions.Then, the slower one is the more intense and is proportional to the ion Xe+ current whereas the fasterone could be attributed either to doubly-charged Xe++ or to superfast Xe+. The work presents a way todetermine unambiguously the nature of the fast contribution by recording the Retardated Potential Analyser (RPA) signals at various repelling grid potentials with respect to time. The energy distribution of the 2 wellseparatedcontributions are reconstructed and confirms the contribution of doubly-charged xenon ions (Xe++)in the plume. This way of RPA collecting data and interpretation presents the main advantage to be an easy wayfor the identification of the nature of the charged species in the plume.

The present developments of Hall thrusters for satellite control and space mission technologies represent a new step towards their routine use in place of conventional thermal thrusters. In spite of their long R and D history, the complex physics of the E ? B discharge at work in these structures has prevented, up to now, the availability of predictive simulations. The electron transport in the accelerating layers of these thrusters is one of the remaining challenges in this direction. From the experimental point of view, any diagnostics of electron transport and electric field in this critical layer would be welcome for comparison with code predictions. Appropriate diagnostics are difficult, due to the very aggressive local plasma conditions. This paper presents the first step in the development of a new tool for characterization of the plasma electric field in the very near exhaust thruster plume and comparison with simulation code predictions.The main idea is to use very short bursts of electrons, probing local electron dynamics in this critical plume area. Such bursts can be obtained through photoelectric emission induced by a UV pulsed laser beam on a convenient target. A specific study, devoted to the characterization of the electron burst emission, is presented in the first section of the paper; the implementation and testing of the injection of electrons in the critical layer of Hall thruster plasma is described in the second section. The design and testing of a fast and sensitive system for characterizing the transport of injected bursts will be the next step of this program. It requires a preliminary evaluation of electron trajectories which was achieved by using simulation code. Simulation data are presented in the last section of the paper, with the full diagnostic design to be tested in the near future, when runs will be available in the renewed PIVOINE facility. The same electron burst injection could also be a valuable input in the present discussion on the physics of the 5?10?MHz instability observed in almost all Hall thrusters.

conférence invité sans acte; International audience

Poster; We investigate the momentum distribution of weakly interacting 1D Bose gases at thermal equilibrium both experimentally and theoretically. Momentum distribution of single 1D Bose gases is measured using a focusing technique, whose resolution we improve via a levitation scheme. The momentum distribution compares very well with quantum Monte Carlo calculations for the Lieb-Liniger model at finite temperature, allowing for an accurate thermometry of the gas that agrees with (and improves upon) the thermometry based on {\it in situ} density fluctuation measurements. The quasi-condensation crossover is investigated via two different experimental parameter sets, corresponding to the two different sides of the crossover. Classical field theory is expected to correctly describe the quasi-condensation crossover of weakly interacting gases. We derive the condition of validity of the classical field theory, and find that, in typical experiments, interactions are too strong for this theory to be accurate. This is confirmed by a comparison between the classical field predictions and the numerically exact quantum Monte Carlo calculations.

Ten years of R&D on Hall plasma thrusters (HPTs) in former USSR led to successful tests in space as early as 1972. Nevertheless, they remain actively investigated within the perspective of a routine use in space technologies. This long story underlines the contrast between a rather simple concept and a complex physical background, up to now not fully understood in a quantitative way. This lack of knowledge limits the development of fully predictive simulation codes. Industrial developments are, nevertheless, under way, with new goals in terms of thruster performances. Illustrations of such developments are given, but this paper is mainly focused on a review of recent results and open questions underlined through a coordinated research programme.

International audience; Fiber-textured polycrystalline silicon thin films have demonstrated their interest as seed layers for the epitaxial growth of high-quality materials on substrates such as glass or plastics. In the present work, we report a comprehensive study of the aluminum-induced crystallization (AIC) of isolated Si domains. Our study not only demonstrates the fabrication by AIC of shape- and size-controlled Si monocrystals at the nanoscale but also allows for the prediction of minimal annealing conditions for complete crystallization of such structures. These ultrathin [111]-oriented monocrystals are promising candidates as selected-area epitaxy substrates with both an easy control on the morphology of these structures and a low-temperature fabrication process.

The polariton condensation phase diagram is compared in GaN and ZnO microcavities grown on mesa-patterned silicon substrate. Owing to a common platform, these microcavities share similar photonic properties with large quality factors and low photonic disorder, which makes it possible to determine the optimal spot diameter and to realize a thorough phase diagram study. Both systems have been investigated under the same experimental conditions. The experimental results and the subsequent analysis reveal clearly that longitudinal optical phonons have no influence in the thermodynamic region of the condensation phase diagram, while they allow a strong (slight) decrease of the polariton lasing threshold in the trade-off zone (kinetic region). Phase diagrams are compared with numerical simulations using Boltzmann equations, and are in satisfactory agreement. A lower polariton lasing threshold has been measured at low temperature in the ZnO microcavity, as is expected due to a larger Rabi splitting. This study highlights polariton relaxation mechanisms and their importance in polariton lasing.

Light-matter coupling in a cavity results in a ladder of states with splittings determined by the coupling strength. We observe the higher ladder rungs in a semiconductor microcavity using multiquantum coherent optical photocurrent spectroscopy.

The formation and propagation of a polariton condensate under tightly focused excitation is investigated in a ZnO microcavity both experimentally and theoretically. 2D near-field and far-field images of the condensate are measured under quasi-continuous non-resonant excitation. The corresponding spatial profiles are compared to a model based on the Gross-Pitaevskii equation under cylindrical geometry. This work allows to connect the experiments performed with a small excitation laser spot and the previous kinetic models of condensation in a 2D infinite microcavity, and to determine the relevant parameters of both the interaction and the relaxation between the reservoir and the condensate. Two main parameters are identified: the exciton-photon detuning through the polariton effective mass and the temperature, which determines the efficiency of the relaxation from the reservoir to the condensate., Comment: 13 pages, 3 tables and 9 figures

International audience; The amplitude and phase noises of a dual-frequency vertical-external-cavity surface-emitting laser (DF-VECSEL) operating at telecom wavelength are theoretically and experimentally investigated in detail. In particular, the spectral behavior of the correlation between the intensity noises of the two modes of the DF-VECSEL is measured. Moreover, the correlation between the phase noise of the radio-frequency beat note generated by optical mixing of the two laser modes with the intensity noises of the two modes is investigated. All these spectral behaviors of noise correlations are analyzed for two different values of the nonlinear coupling between the laser modes. We find that to describe the spectral behavior of noise correlations between the laser modes, it is of utmost importance to have precise knowledge about the spectral behavior of the pump noise, which is the dominant source of noise in the frequency range of interest (10 kHz to 35 MHz). Moreover, it is found that the noise correlation also depends on how the spatially separated laser modes of the DF-VECSEL intercept the noise from a multimode fiber-coupled laser diode used for pumping both the laser modes. To this aim, a specific experiment is reported which aims at measuring the correlations between different spatial regions of the pump beam. The experimental results are in excellent agreement with a theoretical model based on modified rate equations

In the last twelve years the research on dusty plasmas was at first required to understand the dust-induced problems in low-pressure plasma tools and surface processing technologies. Meanwhile, the research on physics and chemistry of dusty plasmas led to the discovery of a fascinating domain. The scientific knowledge acquired on dusty plasmas leads to new ideas for applications and the aim of this contribution is to present examples of such connections. An exhaustive review of the matter would be clearly impossible and only three aspects are emphasized. The first concerns plasmas where nanosized particles are grown and their use for deposition of thin films with new properties. The second concerns situations where a significant amount of dust is stored in a discharge, inducing high electron energies, with strong impact on plasma chemistries. Finally, the last one will concern the well-known confinement of dust particles by electrostatic sheaths and some examples of its actual or potential applications.

International audience

We describe a compact, ultralow noise, and high-power semiconductor laser implemented in a high performances wideband externally modulated optical link. The laser is based on a vertical external cavity surface emitting laser (VECSEL) designed for high-power and low-noise operation. Thanks to a specific design of the gain chip, the half-VCSEL, based on a metamorphic Bragg mirror and a report on a copper substrate, an optical power of 110 mW is obtained at 1.55 μm in the single frequency regime. For low-noise operation, the laser cavity is designed for free-relaxation-oscillations operation, i.e., in the so-called class-A regime. The Class-A VECSEL exhibits a relative intensity noise below −170 dB/Hz over a wide frequency bandwidth, from 300 MHz to 40 GHz, except at the laser free spectral range (20.4 GHz). In the low-frequency range, the laser noise, mainly due to transfer of pump noise to laser noise, goes from −110 dB/Hz at 1 kHz down to −158 dB/Hz at 10 MHz. Two externally modulated optical links are implemented and compared in terms of the RF gain and the noise figure. The first optical link is based on the ultralow noise class-A VECSEL and the second one is based on a low noise class-B DFB laser. Thanks to the outstanding noise properties of the designed VECSEL, the VECSEL-based optical link outperforms the DFB-based one, in particular for frequencies larger than 20 GHz.

International audience; Five-period AlN/BAlN heterostructure containing boron as high as 11% has been successfully grown by MOVPE. Good periodicity of two alternative layers has been observed by both SIMS profile and Z-contrast HAADF-STEM images. The BAlN layers demonstrate columnar polycrystalline feature. The BAlN layers exhibit an emission peak by CL and absorption edge in transmission spectra at around 260 nm. The results enable the development of BAlGaN based multi-layered heterostructure for UV and deep UV applications.

International audience

International audience; ZnO is a wide bandgap semiconductor with strong excitonic properties, in particular a large oscillator strength and a large exciton binding energy. It therefore raises a strong interest for the generation and control of polariton condensates up to room temperature.The realization of planar ZnO microcavities with good quality factors has long been a strong challenge. We first demonstrated in 2011 a ZnO polariton laser operating at zero exciton-photon detuning and a temperature of 120 K [1], based on a hybrid microcavity (AlN/AlGaN DBR, MBE-grown ZnO, SiO2/SiN DBR) with a quality factor Q=450. This value was still too small to allow the observation of polariton lasing up to room temperature. A different approach was implemented based on a ZnO active layer of high crystalline quality, embedded between two dielectric DBR; such a fully-hybrid microcavity exhibits a high quality factor (Q>2000) and a large Rabi splitting (250 meV). We have observed the condensation of polaritons in this bulk ZnO microcavity over an unprecedented range of exciton-photon compositions and of temperatures, up to room temperature [2]. The complete phase diagram of the ZnO polariton laser has been measured, showing that its threshold is only 6 times larger at 300 K than at 8 K. It is in a good qualitative agreement with the simulations of exciton and polariton relaxation in a kinetic model. This tunability represents an important progress compared to our previous demonstration [1], as well as to other recent reports [3]. It also confers a strong advantage to ZnO microcavities compared to GaN [4], since strong excitonic condensates can here be investigated.The cavity with Q>2000 however presents a strong gradient of the cavity thickness hindering the generation of extended condensates. Equivalent cavities without such a gradient have therefore been developed in the last years, based on high quality AlN/AlGaN DBRs on mesas. Our most recent investigations are dedicated to the role of the shape of the exciting laser spot on the condensate generation. Condensate propagation is observed under tightly focused excitation, driven by the interaction with the reservoir as in GaAs microcavities, whereas localization of the polariton condensates is observed for larger excitation areas.

The incorporation of metal atoms into silicon nanowires during metal-particle-assisted growth is a critical issue for various nanowire-based applications. Here we have been able to access directly the incorporation and redistribution of metal atoms into silicon nanowires produced by two different processes at growth rates ranging from 3 to 40 nm s(-1), by using laser-assisted atom probe tomography and scanning transmission electron microscopy. We find that the concentration of metal impurities in crystalline silicon nanowires increases with the growth rate and can reach a level of two orders of magnitude higher than that in their equilibrium solubility. Moreover, we demonstrate that the impurities are first incorporated into nanowire volume and then segregate at defects such as the twin planes. A dimer-atom-insertion kinetic model is proposed to account for the impurity incorporation into nanowires.

International audience; We create polariton clouds and polariton condensates to characterize their propagation at different temperatures. Our experimental results show that in a thickness gradient microcavity, polariton propagation is mainly ballistic and does not depend on the microcavity gradient. Above threshold, we compare the experimental results to a theoretical model [1] to understand the peculiar k-space images of the polariton condensate. Our results indicate that these images are due to the repulsion of the condensate from the center of excitation by the exciton reservoir created by a small excitation spot (2~3 µm).

Fabrication of high Q-factor InP-based Photonic Crystal nanobeam cavities is demonstrated by improving ICP etching. We measure Q-factors as high as ~130,000 for passive cavities by integrating them on to SOI wire waveguides. Nanolasers exhibiting low lasing threshold are then obtained using this technology.

We present results on InAs/InP quantum dash based broad area and ridge-waveguide lasers with emission wavelength up to 2 um. In addition a simulation-based design for laterally coupled DFB/DBR lasers is discussed, and the optimization of the processing steps for the realization of this design is presented. The design is fully compatible with readily available processing techniques and enables sufficiently high coupling coefficients for the realization of a tunable laterally coupled DBR (LC-DBR) laser, without the need of employing any regrowth steps over corrugated substrates, used for conventional DBR lasers.

The design of an ion extraction system often requires the help of computer codes. This article describes an experimental setup based on a helicon source equipped with a three-grid extraction system and that is used as an ion source. A numerical approach based on a particles in cells (PIC) method is introduced. The results obtained with the ion source for different gases are compared with the simulation and are used in order to validate the computer code. The numerical code is the first component of an ion source design package that deals with grid erosion and ion beam current distribution on the target.