Lasing in a ZnO waveguide: clear evidence of polaritonic gain obtained by monitoring the continuous exciton screening

The stimulated emission of exciton-polaritons was proposed as a means of lowering the lasing threshold because it does not require the dissociation of excitons to obtain an electron-hole plasma, as in a classical semiconductor laser based on population inversion. In this work we propose a method to prove unambiguously the polaritonic nature of lasing by combining experimental measurements with a model accounting for the permittivity change as a function of the carrier density. To do so we use angle resolved photoluminescence to observe the lasing at cryogenic temperature from a polariton mode in a zinc oxide waveguide structure, and to monitor the continuous shift of the polaritonic dispersion towards a photonic dispersion as the optical intensity of the pump is increased (up to 20 times the one at threshold). This shift is reproduced thanks to a model taking into account the reduction of the oscillator strength and the renormalization of the bandgap due to the screening of the electrostatic interaction between electrons and holes. Furthermore, the measurement of the carriers lifetime at optical intensities in the order of those at which the polariton lasing occurs enables us to estimate the carrier density, confirming it is lower than the corresponding Mott density for zinc oxide reported in the literature

Etude du couplage fort par spectroscopie optique dans des microcavités GaN élaborées sur silicium

This work presents an optical spectroscopy study through reflectivity, photoluminescence and transmission (from 5 K to 300 K) on the light-matter strong coupling regime in bulk and quantum well nitride microcavities grown on silicon substrate. The experiments have been interpreted within the transfer-matrix formalism taking into account both homogeneous and inhomogeneous excitonic broadenings. The influence of the structure (cavity thickness, number of pairs of the Bragg mirror, nature of the mirror, ...) on the strong coupling regime has been studied through experimental results obtained from various cavities. The transfer-matrix model has been compared to the quasiparticle model with experimental results. We show then, this last model works only when the reflectivity is very high, the transfer-matrix model being more accurate because the real structure of the cavity is taken into account. This thesis work ends with the study of double dielectric mirror microcavities. Two different ways to improve the quality factor of the cavity and the excitonic broadening (through the improvement of the active layer quality) are reported. The strong coupling is reported through transmission measurements for the first time.Ce travail présente une étude spectroscopique par réflectivité, photoluminescence et transmission (de 5 K à 300 K) pour la mise en évidence du couplage fort lumière-matière dans des microcavités nitrurées massives et à puits quantiques élaborées sur substrat de silicium. Les expériences sont interprétées grâce à une modélisation utilisant le formalisme des matrices de transfert et prenant en compte les phénomènes d'élargissement homogène et inhomogène des transitions excitoniques. A travers les résultats obtenus sur de nombreuses cavités, l'influence de la géométrie de la microcavité (épaisseur de la cavité, nombre de paires des miroirs de Bragg, nature des miroirs, ...) sur l'obtention du régime de couplage fort a été étudiée. En s'appuyant sur les résultats expérimentaux obtenus, le modèle des matrices de transfert a été comparé à un modèle quasi-particule. Il a été démontré que ce dernier n'est réaliste que dans le cas où la réflectivité des miroirs est très élevée, le modèle des matrices de transfert restant le plus fidèle aux résultats expérimentaux grâce à la prise en compte de la structure réelle de la microcavité. Ce travail de thèse s'achève par l'étude de microcavités à deux miroirs diélectriques. Deux approches différentes visant à améliorer le facteur de qualité de la cavité ainsi que la finesse du mode excitonique (à travers l'amélioration de la qualité cristalline de la couche de GaN) sont présentées et le couplage fort est observé pour la première fois en transmission

Efficient Dispersion of Organic Fluorophores by Size Matching with a Difunctionalized Spacer Interleaved into Layered Double Hydroxide

International audienceStrongly luminescent when diluted in aqueous solution, Sulforhodamine B (SRB) is an organic dye suffering from aggregation-caused quenching in its solid state. Its cointercalation in very small quantities (0.12 mol %) in a layered double hydroxide (LDH) matrix with phenylenedipropionate (PPA) anions, a difunctionalized spacer matching its size, is found to be highly effective, with more than 99% of the SRB molecules taken up. The resulting hybrid material enables luminescence, with an absolute photoluminescence quantum yield (PLQYabs) reaching 39% under UV excitation and even reaching 60% at its maximum. The dispersion of the LDH dye powder in silicone results in homogeneous composite films with loading as high as 40 wt % useable as a masterbatch. The optical properties are found to remain after thermal treatment up to 120 °C, with a decrease in PLQYabs of only 5%, making it a good candidate for the development of rare-earth-free phosphors

Polymeric copper( i )–NHC complexes with bulky bidentate (N^C) ligands: synthesis and solid-state luminescence

International audienceStarting from imidazolium chlorides bearing bulky nitrogen donors, a series of four complexes, mainly [Cu(C^N)Cl]n coordination polymers were obtained directly as luminescent species by simple filtration from the aqueous reaction medium, highlighting a simple, eco-friendly, robust and reproducible synthetic procedure. Additionally, we have shown on the most efficient example that chloride could be exchanged very easily by other halides/pseudohalides (Br−, I−, NCS−, N3−) allowing to slightly modulate the emitted colour while conserving the polymeric structure, except for azide for which a dimer was obtained. The combination of chemical analyses, of photoluminescence studies in the solid state including quantum yield measurement and X-ray diffraction on single crystals and as-synthesized microcrystalline powders highlighted that the polymeric luminescent species was indeed obtained directly by simple filtration and that no major alteration of the structure was observed upon recrystallisation. Samples of all polymeric complexes displayed remarkable stability towards air oxidation remaining unchanged upon storage for several months and partially retaining their photoluminescence properties even after a thermal treatment at 100 °C for 24 h

Encapsulation of InP/ZnS quantum dots into MOF-5 matrices for solid-state luminescence: ship in the bottle and bottle around the ship methodologies.

The utilization of InP-based quantum dots (QDs) as alternative luminescent nanoparticles to cadmium-based QDs, known for their toxicity, is actively pursued. However, leveraging their luminescent attributes for solid-state applications presents challenges due to the sensitivity of InP QDs to oxidation and aggregation-caused quenching. Hence an appealing strategy is to protect and disperse InP QDs within hybrid materials. Metal-organic frameworks (MOFs) offer a promising solution as readily available crystalline porous materials. Among these, MOF-5, composed of {Zn4O}6+ nodes and terephthalate struts, can be synthesized under mild conditions (at room temperature and basic pH), making it compatible with InP QDs. In the present work, luminescent InP QDs are successfully incorporated within MOF-5 through two distinct methods. Firstly, employing the bottle around the ship (BAS) approach, wherein the MOF was synthesized around2 the QDs. Secondly, utilizing the ship in the bottle (SIB) strategy, the QDs were embedded via capillarity into a specially engineered, more porous variant of MOF-5. Comparative analysis of the BAS and SIB approaches, evaluating factors such as operational simplicity, photoluminescence properties, and the resistance of the final materials to leaching are carried out. This comparative study provides insights into the efficacy of these strategies for the integration of InP QDs within MOF-5 for potential solid-state applications in materials chemistry

Carboxylate BODIPY integrated in MOF-5: easy preparation and solid-state luminescence

International audienceBoron-dipyrromethenes (BODIPYs) are remarkable organic fluorophores widely used in solution. However, like many other organic phosphors, BODIPYs suffer from aggregation-caused quenching inhibiting the solid-state luminescence, which limits their potential for various applications. Metal–organic frameworks (MOFs), a well-known class of hybrid materials consisting of organic ligands bridging metal clusters, are considered herein as solid-state templates allowing restoration of the luminescence of BODIPY in the solid state. By a simple room-temperature synthetic procedure, we were able to integrate carboxylate functionalized BODIPYs into a MOF-5 structure (composed of {Zn4O}6+ clusters and terephthalate ligands). This allowed limiting solid-state aggregation phenomena. The structure and the morphology of different MOF@BODIPY samples have been characterized by XRD and/or microscopy. The luminescence properties have been characterized in the solid state and compared to BODIPY in solution. The MOF@BODIPY samples offer high external photoluminescence quantum yields of up to 27% in the solid state. Emission and excitation spectra were similar to those of BODIPY in solution. The amount of BODIPY integrated into the structures has been confirmed by UV-vis spectroscopy and an optimal loading has been determined. The thermo-stability and UV ageing have been preliminarily tested

Enhancement of light extraction in Y3Al5O12:Tb3+ thin films through nanopatterning

International audienceLight extraction is a key parameter to improve the performances of optical devices. Nanopatterned Y3Al5O12:Tb3+ luminescent coatings usable in such devices have been elaborated and have shown enhanced emission efficiency compared to their unpatterned counterparts. These nanostructured films were obtained by using the colloidal lithography combined with the Langmuir-Blodgett technique. It is the first time to our knowledge that this patterning technique is directly applied on YAG matrix. Resulting nanostructuring is a hexagonal network capable of modifying the light travelling path within the substrate. Conventional and angular-resolved photoluminescence were investigated on both unpatterned and patterned samples. Due to the nanostructuring, the extraction efficiency is improved by 26% and 131% depending on the crystallinity of the sample. Noticeably, nanostructuring is found to have an influence on the angular distribution of photoluminescence whose intensity has been evaluated to its maximum normal to the film surface

The low temperature limit of the excitonic Mott density in GaN: an experimental reassessment

International audienceThe research on GaN lasers aims for a continuous reduction of the lasing threshold. An approach to achieve it consists in exploiting stimulated polariton scattering. This mechanism, and the associated polariton lasers, requires an in-depth knowledge of the GaN excitonic properties, as polaritons result from the coupling of excitons with photons. Under high excitation intensities, exciton states no longer exist due to the Coulomb screening by free carriers; this phenomenon occurs at the so-called Mott density. The aim of this work is to study the bleaching of excitons under a quasi-continuous optical excitation in a bulk GaN sample of high quality through power dependent micro-photoluminescence and time-resolved experiments at 5 K. Time-resolved photoluminescence allows to measure the carrier lifetime as a function of excitation intensity, which is required for a reliable evaluation of the injected carrier density. The vanishing of excitonic lines together with the red-shift of the main emission evidences the occurrence of the Mott transition for a carrier concentration of (6 ± 3) × 10 16 cm −3. This value is more than an order of magnitude smaller than previous determinations published in the literature and is in accordance with many-body calculations

Reliability study under thermal and photonic stresses of sulforhodamine B (SRB) confined in layered double hydroxide (LDH)

International audienceWhen sulforhodamine B (SRB) is entrapped in a tightly packed hybrid material composed of dodecylsulfate anions interleaved in layered double hydroxide, the organic dye is considered as a possible red-emitting phosphor for white light-emitting diodes (WLEDs). To confirm such promising potential, a reliability study is here undertaken and the materials and their associated silicone films are subjected to different thermal and photonic stresses. Optical properties, photoluminescence quantum yields and emission spectra are recorded after photo-aging studies. Interestingly, the composite silicone film is found to be completely stable under blue LED excitation, while in the absence of the hybrid LDH the emission of SRB decays rapidly, thus underlining the protective role of the LDH hybrid cargo. At this stage, these results confirm the potential offered by a system consisting of such an emitter film and a blue LED. This system also opens up new possibilities for interesting organic dyes that are sensitive to photonic and/or thermal stresses