Light-driven processes: key players of the functional biodiversity in microalgae

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Optical spectroscopy of GaN/AlN nanostructures embedded in planar microcavities and microdisks

Cette thèse porte sur l'interaction lumière-matière au sein de nanostructures placées dans des cavités optiques à base de semi-conducteurs nitrures. A l'aide d'expériences de micro-photoluminescence dans l'ultra-violet, nous étudions les propriétés optiques de boîtes quantiques GaN/AlN dans des microcavités planaires et celles de puits quantiques GaN/AlN insérés dans des microdisques AlN.Afin d'améliorer la collection du faible signal de photoluminescence de boîtes quantiques uniques, nous utilisons des microcavités planaires pour modifier le diagramme d'émission d'une boîte quantique. Le dessin des microcavités est optimisé grâce à des simulations numériques basées sur la méthode des matrices de transfert en présence d'un émetteur. Nous montrons que, pour une microcavité nitrure à base de miroirs de Bragg AlN/AlGaN, la collection des photons émis par une boîte quantique peut être théoriquement améliorée d'un ordre de grandeur, ce qui est confirmé par nos mesures sur boîtes quantiques uniques, ouvrant ainsi la voie à des études avancées de corrélations de photons dans l'UV.La seconde partie des travaux est dédiée à la réalisation d'un micro-laser opérant dans l'UV profond et à température ambiante. En utilisant des puits quantiques GaN/AlN de 2,8 mono-couches, crûs sur substrat silicium et insérés dans des microdisques AlN, nous observons une émission laser à 275 nm sous pompage optique impulsionnel. Cette démonstration montre le fort potentiel des semi-conducteurs nitrures pour la nano-photonique UV sur silicium.This thesis addresses the light-matter interaction in nitride nanostructures embedded in optical microcavities. By using micro-photoluminescence experiments, we study the optical properties of GaN/AlN quantum dots embedded in planar microcavities and those of GaN/AlN quantum wells in AlN microdisks.By placing quantum dots in planar microcavities, we are able to modify the emission diagram and increase the collection efficiency. The design of the microcavities is optimized by using numerical simulations based on transfer matrix method with an internal emitter. For an AlN microcavity with AlN/AlGaN Bragg mirrors, we show that the collection efficiency could be theoretical increase by one order of magnitude, which is confirmed by our micro-photoluminescence experiments on single quantum dots. This observation opens the way for advanced studies such as photon correlations experiments in the UV range.The second part of our work is devoted to the realization of a micro-laser operating in the deep-UV range at room-temperature. By using thin GaN/AlN quantum wells (2.8 monolayers), grown on silicon substrate and embedded in AlN microdisks, we observe a laser emission at 275 nm under pulsed optical pumping. This demonstration shows the strong potentiality for future developments of nitride-on-silicon nano-photonics

Spectroscopie optique de nanostructures GaN/AlN insérées dans des microcavités planaires et des microdisques

This thesis addresses the light-matter interaction in nitride nanostructures embedded in optical microcavities. By using micro-photoluminescence experiments, we study the optical properties of GaN/AlN quantum dots embedded in planar microcavities and those of GaN/AlN quantum wells in AlN microdisks.By placing quantum dots in planar microcavities, we are able to modify the emission diagram and increase the collection efficiency. The design of the microcavities is optimized by using numerical simulations based on transfer matrix method with an internal emitter. For an AlN microcavity with AlN/AlGaN Bragg mirrors, we show that the collection efficiency could be theoretical increase by one order of magnitude, which is confirmed by our micro-photoluminescence experiments on single quantum dots. This observation opens the way for advanced studies such as photon correlations experiments in the UV range.The second part of our work is devoted to the realization of a micro-laser operating in the deep-UV range at room-temperature. By using thin GaN/AlN quantum wells (2.8 monolayers), grown on silicon substrate and embedded in AlN microdisks, we observe a laser emission at 275 nm under pulsed optical pumping. This demonstration shows the strong potentiality for future developments of nitride-on-silicon nano-photonics.Cette thèse porte sur l'interaction lumière-matière au sein de nanostructures placées dans des cavités optiques à base de semi-conducteurs nitrures. A l'aide d'expériences de micro-photoluminescence dans l'ultra-violet, nous étudions les propriétés optiques de boîtes quantiques GaN/AlN dans des microcavités planaires et celles de puits quantiques GaN/AlN insérés dans des microdisques AlN.Afin d'améliorer la collection du faible signal de photoluminescence de boîtes quantiques uniques, nous utilisons des microcavités planaires pour modifier le diagramme d'émission d'une boîte quantique. Le dessin des microcavités est optimisé grâce à des simulations numériques basées sur la méthode des matrices de transfert en présence d'un émetteur. Nous montrons que, pour une microcavité nitrure à base de miroirs de Bragg AlN/AlGaN, la collection des photons émis par une boîte quantique peut être théoriquement améliorée d'un ordre de grandeur, ce qui est confirmé par nos mesures sur boîtes quantiques uniques, ouvrant ainsi la voie à des études avancées de corrélations de photons dans l'UV.La seconde partie des travaux est dédiée à la réalisation d'un micro-laser opérant dans l'UV profond et à température ambiante. En utilisant des puits quantiques GaN/AlN de 2,8 mono-couches, crûs sur substrat silicium et insérés dans des microdisques AlN, nous observons une émission laser à 275 nm sous pompage optique impulsionnel. Cette démonstration montre le fort potentiel des semi-conducteurs nitrures pour la nano-photonique UV sur silicium

Radiative lifetime of excitons in high energy GaN/AlN quantum dots

International audienceSelf-assembled semiconductors QDs are of great interest in fundamental physics as well as inapplied physics. In GaN/AlN heterostructures, the huge band-offset and the strong excitonicbinding lead to an important exciton confinement allowing an emission even at roomtemperature. GaN QDs grown along the (1000) axis are named “polar” due to their stronginternal electic field which lead to the so-called QCSE. The most important modifications due tothis electric field take place in thick QDs emitting around 330nm and more : the luminescence isredshifted and the radiative lifetime is strongly increased [1]. On the other hand, in thin QDs(smaller than 2.3nm), the quantum confinement plays the key role, thus leading to a significantlysmaller homogeneous linewidth and to the resolution of the exciton fine-structure splitting[2]. Concerning the radiative recombination processes, several papers reported measurements at330nm or more either in polar[3] or non-polar[4] nanostructures. Experimental data on excitonicradiative lifetime are also existent in thin QDs but on a limited dynamical range where thepossible influence of QCSE is not discussed [5].In this work, we report time-resolved photoluminescence measurements in GaN/AlN QDsemitting between 280nm and 360nm. For QDs emitting at 300nm we observe a biexponentialdecay dynamics of the photoluminescence signal (Fig.1, left). The fast component (450 ps) isinterpreted as the radiative lifetime of the QDs in the absence of a noticeable redshift of thetransition. Thanks to our better signal-to-noise ratio than in Ref. 5, we are able to extract a slowcomponent (2 ns) which may originate either from feeding from the barrier or other processesinside the QD. Power dependent experiments further confirm this interpretation. We observe thatthe fast component remains the same as a function of excitation power (Fig.1, right) and, moreimportantly, that the intensity ratio keeps the same value. This rules out any contribution fromthe relaxation of multi-carriers complexes such as biexciton or trion in the short time dynamic

Ultrafast dynamics of heme distortion in the O2-sensor of a thermophilic anaerobe bacterium

International audienceHeme-Nitric oxide and Oxygen binding protein domains (H-NOX) are found in signaling pathways of both prokaryotes and eukaryotes and share sequence homology with soluble guanylate cyclase, the mammalian NO receptor. In bacteria, H-NOX is associated with kinase or methyl accepting chemotaxis domains. In the O2-sensor of the strict anaerobe Caldanaerobacter tengcongensis (Ct H-NOX) the heme appears highly distorted after O2 binding, but the role of heme distortion in allosteric transitions was not yet evidenced. Here, we measure the dynamics of the heme distortion triggered by the dissociation of diatomics from Ct H-NOX using transient electronic absorption spectroscopy in the picosecond to millisecond time range. We obtained a spectroscopic signature of the heme flattening upon O2 dissociation. The heme distortion is immediately (<1 ps) released after O2 dissociation to produce a relaxed state. This heme conformational change occurs with different proportions depending on diatomics as follows: CO < NO < O2. Our time-resolved data demonstrate that the primary structural event of allostery is the heme distortion in the Ct H-NOX sensor, contrastingly with hemoglobin and the human NO receptor, in which the primary structural events are respectively the motion of the proximal histidine and the rupture of the iron-histidine bond

Probing the role of Valine 185 of the D1 protein in the Photosystem II oxygen evolution

WOS:000451491000001In Photosystem II (PSII), the Mn4CaO5-cluster of the active site advances through five sequential oxidation states (S-0 to S-4) before water is oxidized and O-2 is generated. The V185 of the D1 protein has been shown to be an important amino acid in PSII function (Dilbeck et al. Biochemistry 52 (2013) 6824-6833). Here, we have studied its role by making a V185T site-directed mutant in the thermophilic cyanobacterium Thermosynechococcus elongatus. The properties of the V185T-PSII have been compared to those of the WT*3-PSII by using EPR spectroscopy, polarography, thermoluminescence and time-resolved UV-visible absorption spectroscopy. It is shown that the V185 and the chloride binding site very likely interact via the H-bond network linking Tyr(z) and the halide. The V185 contributes to the stabilization of S-2 into the low spin (LS), S = 1/2, configuration. Indeed, in the V185T mutant a high proportion of S-2 exhibits a high spin (HS), S = 5/2, configuration. By using bromocresol purple as a dye, a proton release was detected in the S(1)Tyr(Z)center dot -\textgreater S(2)(LS)Tyr(Z) transition in the V185T mutant in contrast to the WT*3-PSII in which there is no proton release in this transition. Instead, in WT*3-PSII, a proton release kinetically much faster than the S(2)(LS)Tyr(z)center dot -\textgreater S(3)Tyr(Z) transition was observed and we propose that it occurs in the S(2)(LS)Tyr(Z)center dot -\textgreater S(2)(HS)Tyr(Z). intermediate step before the S(2)(HS)Tyr(Z)center dot -\textgreater S(3)Tyr(Z) transition occurs. The dramatic slowdown of the S(3)Tyr(Z)center dot -\textgreater S(0)Tyr(Z) transition in the V185T mutant does not originate from a structural modification of the Mn4CaO5 cluster since the spin S = 3 S-3 EPR signal is not modified in the mutant. More probably, it is indicative of the strong implication of V185 in the tuning of an efficient relaxation processes of the H-bond network and/or of the protein

Optical properties of small GaN/Al0.5Ga0.5N quantum dots grown on (11-22) GaN templates

GaN-Al0.5NGa0.5N quantum dots deposited on (11-22) planes have been grown by combining Molecular Beam Epitaxy and Metal Organic Vapour Phase Epitaxy. This combination is interesting for realization of ultraviolet operation light emitting diodes, lasers andsingle photon sources,… (1,3) The growth of dots was achieved by MBE using ammonia as nitrogen precursor and growth interruption in ammonia less conditions to trigger corrugation of GaN and dot formation (4). The (11-22) GaN oriented peudosubstrate was realized by MOVPE starting from a M-plane oriented sapphire substrate. The orientation of the growth plane dictates in-plane anisotropies which are effectively found leading to a transition from isolated dots to nanochains - oriented along the direction as evidenced from Atomic Force Microscopy features or optical properties: polarization rates and temperature dependent measurements of the radiative recombination process for instance(5). We here restrict to small size isolated quantum dots and present innovative optical properties among which are micro-photoluminescence data versus pump power, polarization of the emitted photons at different temperatures. We also analyse the photoluminescence decay times and model our finding in the context of the effective mass approximation. The crystal field splitting is measured in Al 0.5NGa 0.5N lattice-matched to (11-2) oriented GaN by polarized microphotoluminecence under high photo- injection conditions

Biexcitons in semiconducting carbon nanotubes

International audienceSingle-walled carbon nanotubes (SWNT) are one-dimensional nanostructures where the Coulomb interactions between charge carriers are strongly enhanced compared to systems of higher dimensionality. This results in an electron-hole bound state -the so-called exciton- with a binding energy of the order of one third of the bandgap, which controls the SWNT's optical properties. As a matter of fact, the exciton-exciton interactions are particularly efficient and drive the exciton recombination and dephasing dynamics. The investigation of excitonic complexes in semiconducting SWNTs is currently a topic of intense debate. The biexciton and the trion are expected to have a binding energy of about a hundred meVs. Whereas the trion has been recently observed, first in doped nanotubes and then by means of all-optical generation, there is no experimental evidence for the biexciton. Here we present the first observation of the biexciton in semiconducting single-wall carbon nanotubes using nonlinear optical spectroscopy. Our experiments consist in a spectrally resolved pump-probe technique in SWNTs embedded in a gelatine at cryogenic temperature. Our measurements of the differential transmission spectrum reveal the universal asymmetric line shape of the Fano resonance intrinsic to the biexciton transition

Biexciton in carbon nanotubes

National audienceThe questions of the existence and stability of multi-excitonic states, in which the one-dimensional character of carbon nanotubes implies strong Coulomb interactions, are currently the subject of intense debates. These states and their coupling with excitonic states play a fundamental role within the exploitation of excitonic properties. Recently, experimental studies reported the observation of the trion (exciton-carrier bound state). In order to study the biexciton, we have performed nonlinear spectral-hole burning experiments at a low temperature, bringing the first evidence for biexcitonic induced-absorption

A nitride-on-Silicon microdisk laser emitting at 275 nm and room-temperature

International audienceThe development of semiconductor lasers in the deep ultra-violet (UV) spectral range is attracting a strong interest, related to their multiple applications for optical storage, biochemistry or optical interconnects. UV-emitting ridge lasers usually embed nitride heterostructures grown on complex buffer layers or expensive substrates – an approach that cannot be extended to nano-photonics and microlasers. We demonstrate here the first deep UV microlaser by combining binary GaN/AlN thin quantum wells (QWs) grown on a silicon substrate and high quality factor microdisk resonators. Those microdisk lasers operate at 275nm at room temperature under optical pumping.The nitride heterostructures grown on silicon present a strong interest for nanophotonic devices emitting in the blue and UV range. The etching selectivity of the silicon substrate allows to realize free membrane photonic structures with high quality factors (Q) [1-3]. In the present microdisk resonators, the electromagnetic modes, the so-called Whispering-Gallery Modes (WGMs), present a low modal volume and Q factors of 6000 [4]. The difficulty in extending their lasing operation in the UV range mainly lies in the control of the active layer. Low defect density quantum wells grown on thick buffer layers or nitride substrates are usually employed for ridge lasers. Here we show that binary GaN/AlN ultra-thin quantum wells directly grown on a silicon substrate can maintain a large emission efficiency and lead to lasing at room temperature (Figure 1). This active layer can form free-standing membranes and is further compatible with future developments of nitride nanophotonic platforms on silicon