Physics of Polariton Condensates in GaN-based Planar Microcavities

Since its prediction in 1996 by Imamoğlu and coworkers, the use of a non-equilibrium polariton condensate to produce an intense coherent light source referred to as a polariton laser has attracted a lot of interest in the whole physics community as it should allow the realization of ultralow threshold coherent light-emitting devices due to the release of the Bernard-Duraffourg condition. Excitons-polaritons, admixed particles resulting from the strong coupling between a cavity photon and an exciton, are the eigenmodes of a strongly coupled microcavity and exhibit a very light effective mass at the center of the Brillouin zone (105 times lighter that a free electron) inherited from the cavity photon. In the present work, we are interested in III-nitride based microcavities embedding GaN quantum wells in the active region. Thanks to the stability of the excitons at room temperature in this system and a large oscillator strength, polariton condensation has been observed up to 340K under optical excitation, paving the way toward the realization of the first electrically injected polariton laser. The goal of the present study is to provide a detailed analysis of the system properties accounting for nitride specificities, to describe the mechanisms leading to the formation of polariton condensates and to give the key elements for the optimization of devices relying on polariton nonlinearities. For this purpose, a Fourier-imaging setup allowing for the simultaneous monitoring of real space and far-field energy dispersions was carefully designed to operate in the UV spectral range in order to probe the sample emission at various temperatures. The first main result of this thesis is the establishment of the complete polariton phase diagram of our multiple quantum well-based GaN microcavity, which provides a comprehensive tool to favor or inhibit the condensation threshold by adjusting the microcavity parameters. The condensation is shown to be governed either by the kinetics or by the thermodynamics depending on the strength of the interactions. As polaritons are half-light, half-matter particles, the mechanisms leading to the nonlinear threshold are totally different from those of a conventional semiconductor laser. In particular, the possibility to tune the interactions in the system by changing the photonic fraction of the polaritons or the lattice temperature allows discriminating between different relaxation regimes. Then the spin of the polariton condensate is discussed. It is shown that the dimensionality of the system plays a major role in the polarization state of the emitted light. In particular above threshold, for a bulk microcavity, the polarization is randomly oriented whereas for a GaN multiple quantum well based microcavity, the polarization is pinned by the system anisotropy originating from the static disorder. With increasing pumping power, a depinning of the polarization is observed resulting in a progressive decrease in the polarization degree of the emitted light. These two results are well accounted for by a stochastic model of the condensate formation taking into account the in-plane anisotropy caused by the stationary photonic disorder, the self-induced Larmor precession of the condensate pseudospin and the interplay between energy and polarization relaxation rates. In the last part of this work, the case of nonpolar m-plane GaN based microcavities is addressed. In these structures, the optical axis lies in the plane of the cavity leading to a twofold anisotropy: the birefringence is responsible for the anisotropy of the cavity mode and the distribution of the exciton oscillator strength causes different coupling constants between light and matter along the two orthogonal directions. In such structures different selection rules and optical constants for light polarization perpendicular and parallel to the optical axis can lead to the coexistence of weak and strong coupling regimes with a transition to nonlinear emissions

Reassessment of cell to module gains and losses: Accounting for the current boost specific to cells located on the edges

The power produced by a photovoltaic module is not simply the sum of the powers of its constituents cells. The difference stems from a number of so-called “cell-to-module” (CTM) gain or loss mechanisms. These are getting more and more attention as improvements in cell efficiency are becoming harder to achieve. This work focuses on two CTM mechanisms: the gain due to the recapture of light hitting the apparent backsheet in the “empty” spaces around the cells and the loss from the serial connection of “mismatched” cells i.e. with different maximum power points. In general, for insulation purposes, the spaces on the edges of modules are larger than the spacing between cells. This study reveals that, when reflective backsheets are used, these “edge spaces” provide an additional current boost to the cells placed at the edges that can lead to a 0.5% gain in the output power of modules (with 60 or 72 cells). This location-dependent current boost adds to the usual variations in cell characteristics dictated by the binning size and results in larger “cell-to-cell mismatch losses”. However, the simulations reveal that for short-circuit current bin size smaller than 5%, this additional mismatch loss is lower than 0.05%. All considered, this study demonstrates that the spaces at the edges of PV modules have a significant impact on the cell to module ratios (≈+0.5%abs or ≈16% of the CTM gains) when reflective backsheets are used

Impact of biexcitons on the relaxation mechanisms of polaritons in III-nitride based multiple quantum well microcavities

We report on the direct observation of biexcitons in a III nitride based multiple quantum well microcavity operating in the strong light-matter coupling regime by means of nonresonant continuous wave and time-resolved photoluminescence at low temperature. First, the biexciton dynamics is investigated for the bare active medium (multiple quantum wells alone) evidencing localization on potential fluctuations due to alloy disorder and thermalization between both localized and free excitonic and biexcitonic populations. Then, the role of biexcitons is considered for the full microcavity: in particular, we observe that for specific detunings the bottom of the lower polariton branch is directly fed by the radiative dissociation of either cavity biexcitons or excitons mediated by one LO-phonon. Accordingly, minimum polariton lasing thresholds are observed, when the bottom of the lower polariton branch corresponds in energy to the exciton or cavity biexciton first LO-phonon replica. This singular observation highlights the role of excitonic molecules in the polariton condensate formation process as being a more efficient relaxation channel when compared to the usually assumed acoustical phonon emission one.This work was supported by the NCCR Quantum Photonics, research instrument of the Swiss National Science Foundation, through Grant No. 129715 and Grant No. 200020-113542, and by the EU-project Clermont4 (Grant No. FP7-235114)

High-temperature Mott transition in wide-band-gap semiconductor quantum wells

The crossover from an exciton gas to an electron-hole plasma is studied in a GaN/(Al,Ga)N single quantum well by means of combined time-resolved and continuous-wave photoluminescence measurements. The two-dimensional Mott transition is found to be of continuous type and to be accompanied by a characteristic modification of the quantum well emission spectrum. Beyond the critical density, the latter is strongly influenced by band-gap renormalization and Fermi filling of continuum states. Owing to the large binding energy of excitons in III-nitride heterostructures, their injection-induced dissociation could be tracked over a wide range of temperatures, i.e., from 4 to 150K. Various criteria defining the Mott transition are examined, which, however, do not lead to any clear trend with rising temperature: the critical carrier density remains invariant around 1012cm−2

Exciton localization on basal stacking faults in a-plane epitaxial lateral overgrown GaN grown by hydride vapor phase epitaxy

We present a detailed study of the luminescence at 3.42 eV usually observed in a-plane epitaxial lateral overgrowth (ELO) GaN grown by hydride vapor phase epitaxy on r-plane sapphire. This band is related to radiative recombination of excitons in a commonly encountered extended defect of a-plane GaN: I-1 basal stacking fault. Cathodoluminescence measurements show that these stacking faults are essentially located in the windows and the N-face wings of the ELO-GaN and that they can appear isolated as well as organized into bundles. Time-integrated and time-resolved photoluminescence, supported by a qualitative model, evidence not only the efficient trapping of free excitons (FXs) by basal plane stacking faults but also some localization inside I-1 stacking faults themselves. Measurements at room temperature show that FXs recombine efficiently with rather long luminescence decay times (360 ps), comparable to those encountered in high-quality GaN epilayers. We discuss the possible role of I-1 stacking faults in the overall recombination mechanism of excitons

Biexcitonic molecules survive excitons at the Mott transition

When the carrier density is increased in a semiconductor, according to the predictions of Sir Nevil Mott, a transition should occur from an insulating state consisting of a gas of excitons to a conductive electron-hole plasma. This crossover, usually referred to as the Mott transition, is driven by the mutual effects of phase-space filling and Coulomb screening because of the presence of other charges nearby. It drastically affects the optical and electrical characteristics of semiconductors and may, for example, drive the transition from a polariton laser to a vertical cavity surface-emitting laser. Usually, the possible existence of excitonic molecules (or biexcitons) is neglected in the understanding of the Mott transition because the biexciton is supposed to be less robust against screening effects. Here, against common beliefs, we observe that the biexciton in a GaN quantum well is more stable towards the Mott transition than the exciton

Intrinsic dynamics of weakly and strongly confined excitons in nonpolar nitride-based heterostructures

Both weakly and strongly confined excitons are studied by time-resolved photoluminescence in a nonpolar nitride-based heterostructure grown by molecular beam epitaxy on the a-facet of a bulk GaN crystal, with an ultralow dislocation density of 2 × 105 cm-2. Strong confinement is obtained in a 4 nm thick Al0.06Ga0.94N/GaN quantum well (QW), whereas weakly confined exciton-polaritons are observed in a 200 nm thick GaN epilayer. Thanks to the low dislocation density, the effective lifetime of strongly confined excitons increases between 10 and 150 K, proving the domination of radiative recombination processes. Above 150 K the QW emission lifetime diminishes, whereas the decay time of excitons in the barriers increases, until both barrier and QW exciton populations become fully thermalized at 300 K. We conclude that the radiative efficiency of our GaN QW at 300 K is limited by nonradiative recombinations in the barriers. The increase of exciton-polariton coherence lengths caused by low dislocation densities allows us to observe and model the quantized emission modes in the 200 nm nonpolar GaN layer. Finally, the low-temperature phonon-assisted relaxation mechanisms of such center-of-mass quantized exciton-polaritons are described

A922 Sequential measurement of 1 hour creatinine clearance (1-CRCL) in critically ill patients at risk of acute kidney injury (AKI)

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