Bien que la méthode des liaisons fortes avec son modèle sp3d5s représente l'état de l'art des calculs des propriétés électroniques et optiques des nanostructures à base de semiconducteurs, elle souffre d'un grave défaut : la forme spatiale des fonctions de base est inconnue, ce qui empêche le calcul des effets de corrélations entre quasi-particules. Nous avons proposé dans ce travail une méthode d'interpolation des fonction d'ondes mono-électroniques qui complète sur ce point la théorie des liaisons fortes. Cette méthode consiste à partir d'une base d'orbitales atomiques (dans la pratique, des orbitales de Slater) dotée de paramètres d'écrantage ajustables et de mettre en oeuvre une procédure d'orthogonalisation de Löwdin pour obtenir une base de projection de l'Hamiltonien des liaisons fortes. Ensuite, nous avons utilisé une procédure d'optimisation pour ajuster les paramètres d'écrantage de la base de départ de telle sorte que les propriétés optiques calculées à partir de l'Hamiltonien des liaisons fortes et celles calculées à partir des fonctions d'onde coïcident. Les résultats de cette une approche auto-cohérente ont été comparés à ceux des calculs ab initio et de la méthode des pseudopotentiels empiriques. Un premier test de la qualité des fonctions d'onde obtenues, ainsi qu'une application de la méthode, ont été réalisés en calculant les interactions de Coulomb directe et d'échange entre des paires électron-trou dans le GaAs massif. Nous avons ainsi obtenu la courbe de dispersion de la structure fine des excitons, en traitant sur un pied d'égalité tous les ingrédients du problème : détails de la dispersion monoélectronique tels que le warping de la bande de valence et les splittings de spin des électrons et des trous, interaction de Coulomb directe et interactions d'échange à courte et longue portée. Nos résultats sont en très bon accord avec les valeurs expérimentales de l'énergie de liaison et du splitting longitudinal-transverse.

We demonstrate electromagnetically-induced anomalous quantum dynamics of an exciton in a photonic band gap (PBG) - quantum well (QW) hetero-structure. Within the engineered electromagnetic vacuum of the PBG material, the exciton can propagate through the QW by the emission and re-absorption of virtual photons in addition to the conventional electronic hopping mechanism. When the exciton wavevector and recombination energy coincide nearly with a photonic band edge, the exciton kinetic energy is lowered by 1-10meV through coherent radiative hopping. This capture of the exciton by the photonic band edge is accompanied by strong electromagnetic dressing in which the exciton's renormalized effective mass is 4-5 orders of magnitude smaller than in the absence of the PBG environment. This dressed exciton exhibits a long radiative lifetime characteristic of a photon-atom bound state and is robust to phonon-assisted, re-combinative decay. By inheriting properties of the PBG electromagnetic vacuum, the bound electron-hole pair becomes a stable, ultra-mobile quantum excitation. Unlike traditional exciton-polariton modes created by placing a QW in a one-dimensional optical cavity, our PBG-QW excitons exhibit strong coupling to optical modes and retain a long lifetime. This is crucial for unambiguous observation of quantum coherence effects such as Bose-Einstein condensation. We present a model for the equilibrium quantum statistics of a condensate of repulsively interacting bosons in a two-dimensional trap. Particle correlations in the ground state are treated exactly, whereas interactions with excited particles are treated in a generalized Bogoliubov mean-field theory. This leads to a fundamental physical picture for condensation of interacting bosons through an anharmonic oscillator ground state coupled to excited Bogoliubov quasiparticles in which the quantum number statistics of condensate particles emerges self-consistently. Our anharmonic oscillator model for the exciton ground state manifold goes beyond the conceptual framework of traditional Bogoliubov theory. Below the Bose-Einstein condensation temperature, our model exhibits a crossover from particle bunching to Poissonian statistics and finally antibunching as temperature is lowered or as the trapping area is decreased. When applied to Bose condensation of long-lived dressed excitons in a photonic band gap material, our model suggests that this system may serve as a novel tunable source for non-classical states of light.

We demonstrate electromagnetically-induced anomalous quantum dynamics of an exciton in a photonic band gap (PBG) - quantum well (QW) hetero-structure. Within the engineered electromagnetic vacuum of the PBG material, the exciton can propagate through the QW by the emission and re-absorption of virtual photons in addition to the conventional electronic hopping mechanism. When the exciton wavevector and recombination energy coincide nearly with a photonic band edge, the exciton kinetic energy is lowered by 1-10meV through coherent radiative hopping. This capture of the exciton by the photonic band edge is accompanied by strong electromagnetic dressing in which the exciton's renormalized effective mass is 4-5 orders of magnitude smaller than in the absence of the PBG environment. This dressed exciton exhibits a long radiative lifetime characteristic of a photon-atom bound state and is robust to phonon-assisted, re-combinative decay. By inheriting properties of the PBG electromagnetic vacuum, the bound electron-hole pair becomes a stable, ultra-mobile quantum excitation. Unlike traditional exciton-polariton modes created by placing a QW in a one-dimensional optical cavity, our PBG-QW excitons exhibit strong coupling to optical modes and retain a long lifetime. This is crucial for unambiguous observation of quantum coherence effects such as Bose-Einstein condensation. We present a model for the equilibrium quantum statistics of a condensate of repulsively interacting bosons in a two-dimensional trap. Particle correlations in the ground state are treated exactly, whereas interactions with excited particles are treated in a generalized Bogoliubov mean-field theory. This leads to a fundamental physical picture for condensation of interacting bosons through an anharmonic oscillator ground state coupled to excited Bogoliubov quasiparticles in which the quantum number statistics of condensate particles emerges self-consistently. Our anharmonic oscillator model for the exciton ground state manifold goes beyond the conceptual framework of traditional Bogoliubov theory. Below the Bose-Einstein condensation temperature, our model exhibits a crossover from particle bunching to Poissonian statistics and finally antibunching as temperature is lowered or as the trapping area is decreased. When applied to Bose condensation of long-lived dressed excitons in a photonic band gap material, our model suggests that this system may serve as a novel tunable source for non-classical states of light.

GaN quantum dots grown by molecular beam epitaxy are examined by micro-photoluminescence. The exciton and biexciton emission are identified successfully by power-dependence measurement. With two different samples, it can be deduced that the linewidth of the peaks is narrower in the thicker deposited layer of GaN. The size of the GaN quantum dots is responsible for the binding energy of biexciton (EbXX); EbXX decreases with increasing size of GaN quantum dots. Under polarization studies, polar plot shows that emission is strongly linear polarized. In particular, the orientation of polarization vector is not related to any specific crystallography orientation. The polarization splitting of fine-structure is not able to resolve due to limited resolution of the system. The emission peaks can be detected up to 80 K. The curves of transition energy with respect to temperature are S-shaped. Strain effect and screening of electric field account for  blueshift of transition energy, whereas Varshni equation stands for redshifting. Both blueshifting and redshifting are compensated at temperature ranging from 4 K to 40 K.

Molecular photoabsorption enhancement under ambient solar radiations can improve efficiency substantially in renewable energy production. Here, we explore the theoretical basis and experimental evidences that nanostructured molecular assemblies exhibit an unprecedented property of synergistic photon absorption enhancement. The molecular mechanism of this enhancement phenomenon originates from the combined effect of the photon-molecule interaction and the electronic energy transfer between two adjacent molecular assemblies. For a natural system, the synergistic photon absorption enhancement factor of green algae (Chlorella vulgaris) in vivo at 632.8 nm was determined to be 103. This enhanced photon absorption process in nanostructured molecular assemblies opens a doorway to create entangled double excitons by incoherent solar radiations.

Structure-property relationships for two-photon absorption (2PA) in branched organic chromophores is a topic of current interest, as is the design of chromophores with advantageous properties for two-photon laser scanning microscopy (2PLSM). The main goals of this dissertation were to study and explain the one-photon absorption (1PA) and 2PA properties of cruciform chromophores based on 1,4-distyryl-2,5-bis(phenylethynyl)benzene with varying electron donor (D) and acceptor (A) groups, and to characterize the 2PLSM-relevant response of some of these chromophores and a set of dipolar chromophores to binding with zinc ions. The compounds were studied by 1PA, fluorescence and 2PA spectroscopy. A ππ* exciton model was developed to explain the spectral properties of the 1,4-distyryl-2,5-bis(phenylethynyl)benzene cruciform with no D or A groups or with four identical D groups at the termini of the linear arms of the chromophore. This model indicated that there is some coupling and mixing of the lowest excited states e of the linear arms, leading to splitting of the 1PA spectrum of the cruciform. There was little coupling or mixing of the higher excited states e′ accessed in 2PA, leading to a two-band 2PA spectrum for the chromophore, in contrast to cruciform compounds in the literature with identical conjugated arms, which have one visible 2PA band. For cruciforms with D groups on the styryl arm and A character on the terminal phenyls of the phenylethynyl arms (D/A cruciforms), the ππ* exciton model was complemented with a charge-transfer (CT) exciton model describing interactions of charge-transfer pathways between the D and A groups. This model explained the broadness of the 1PA band of D/A cruciforms as well as the two 2PA bands observed for these chromophores. The fluorescence and 2PA spectral responses to binding of Zn²⁺ ions to the D or A groups of some cruciform compounds were also assessed, to provide insight into the design of new analyte-sensing cruciforms for 2PLSM that take advantage of enhancement or reduction of D/A character upon analyte binding. It was found that canceling charge donation from the D groups in differing D/A cruciforms resulted in fluorescence and 2PA spectra nearly indistinguishable from each other, suggesting that turn-off of D groups is not an optimal modality of 2PLSM analyte sensing in cruciforms. Binding Zn²⁺ to A groups was shown to result in an increase in the D/A character of the cruciform, with fluorescence peak energies that changed depending on the location of the A group. It is suggested that the use of non-binding donors and analyte-binding A groups in differing patterns on the arms could be a valuable design motif to achieve 2PLSM sensor compounds based on this cruciform structure. The 2PA spectra of a set of dipolar Zn²⁺ sensing dyes designed for ratiometric imaging in 2PLSM were also studied. These dyes had moderate 2PA strength, with redshifts of fluorescence 2PA spectra on Zn²⁺ binding. The isosbestic point of 2PA of most chromophores was within the range of 2PLSM excitation sources commonly used, rendering these dyes good candidates for use in ratiometric sensing in 2PLSM.

This thesis deals with exciton relaxation processes in thin polycrystalline films and matrix-isolated molecules of the perylene derivatives PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) and MePTCDI (N,N'-dimethylperylene-3,4,9,10-dicarboximide). Using femtosecond pump-probe spectroscopy, transient absorption spectra, excitonic relaxation in the lowest excited state subsequent to excitation, and exciton-exciton interaction and annihilation at high excitation densities have been addressed. Transient absorption spectroscopy in the range 1.2eV-2.6eV has been applied to thin polycrystalline films of PTCDA and MePTCDI and to solid solutions of PTCDA and MePTCDI molecules (monomers) in a SiO2 matrix. We are able to ascribe the respective signal contributions to ground state bleaching, stimulated emission, and excited state absorption. Both systems exhibit broad excited-state absorption features below 2.0eV, with dominant peaks between 1.8eV and 2.0eV. The monomer spectra can be consistently explained by the results of quantum-chemical calculations on single molecules, and the respective experimental polarization anisotropies for the two major transitions agree with the calculated polarizations. Dimer calculations allow to qualitatively understand the trends visible in the experimental results from monomers to thin films. The broad excited state absorption band between 1.8eV and 2.0eV allows to probe the population dynamics in the first excited state of thin films. We show that excitons created at the Gamma point relax towards the border of the Brillouin zone on a 100fs time scale in both systems. Excitonic relaxation is accelerated by increase of temperature and/or excitation density, which is attributed to stimulated phonon emission during relaxation in k-space. Lower and upper limits of the intraband relaxation time constants are 25fs (resolution limit) and 250fs (100fs) for PTCDA (MePTCDI). These values agree with the upper limit for the intraband relaxation time of 10ps, evaluated from time-resolved luminescence measurements. While the luminescence anisotropy is in full accordance with the predictions made by a luminescence anisotropy model being consistent with the exciton model of Davydov-split states, the pump-probe anisotropy calls for an explanation beyond the models presently available. At excitation densities 10^(19)cm^(-3), the major de-excitation mechanism for the relaxed excitons is exciton-exciton annihilation, resulting in a strongly reduced exciton life time. Three different models for the microscopic behavior have been tested: a diffusion-limited annihilation model in both three and one dimensions (with diffusion constant D as fit parameter) as well as a long-range single-step Förster-type annihilation model (with Förster radius RF as fit parameter). For PTCDA, the latter two, being structurally equivalent, allow to fit a set of multiexponential decay curves for multiple initial exciton densities with high precision. In contrast, the three-dimensional diffusion-limited model is clearly inferior. For all three models, we extract annihilation rates, diffusion constants and diffusion lengths (or Förster radii), for both room and liquid helium temperature. Temperature dependence and orders of magnitude of the obtained parameters D or RF correspond to the expectations. For MePTCDI, the 1D and the Förster model are in good agreement for a smaller interval of excitation densities. For a initial exciton densities higher than 5 x 10^(19)cm^(-3), the 3D model performs significantly better than the other two.

Cette thèse est consacrée à l'étude expérimentale des propriétés électroniques des nanotubes de carbone par des techniques de spectroscopie de photoluminescence.<br /> Le travail a d'abord consisté en la préparation et la caractérisation d'échantillons de nanotubes de carbone isolés les uns des autres dans une suspension de surfactant. Ils présentent alors de la luminescence. Intégrés ensuite dans un gel, ils sont adaptés aux températures entre 10 et 300 K. <br /> Le premier volet d'expériences a concerné la spectroscopie de luminescence en régime stationnaire. Les mesures sur des ensembles macroscopiques permettent d'identifier les classes de chiralité présentes et mettent en évidence divers phénomènes de couplage des nanotubes entre eux et à leur environnement. Grâce à un montage de microscopie confocale, on a en outre étudié la luminescence d'un nanotube unique. On s'affranchit ainsi de l'inhomogénéité de l'échantillon, comme le montrent les faibles largeurs de raies (moins de 1 meV à 10 K) et les phénomènes de diffusion spectrale et de clignotement observés à l'échelle de la seconde.<br /> Le second volet d'expériences est consacré à la spectroscopie de photoluminescence résolue en temps à l'échelle picoseconde, sur des ensembles de nanotubes. On mesure la dynamique de recombinaison des excitations élémentaires (excitons), sur 3 ordres de grandeur de variation, dans les chiralités (9,4) et (10,2). L'évolution du temps de vie (300 ps à 10 K, 50 ps à 300 K) et de l'intensité de la luminescence (présentant un maximum à 50 K) donne des informations sur la structure des états excitoniques. On estime en particulier qu'il existe un niveau noir 4 meV en dessous de l'état luminescent.

This dissertation describes the assessment and use of carbon optically transparent electrodes (C-OTEs) based on pyrolyzed photoresist films (PPFs) as a platform for spectroelectrochemical investigations. C-OTEs are examined for use in UV-Vis spectroelectrochemistry and electrogenerated chemiluminescence and compared to non-transparent glassy carbon (GC) and the conventional transparent electrode indium tin oxide (ITO). Chapter 1 provides a general overview of transparent electrodes, carbon electrodes, and spectroelectrochemistry. Chapter 2 details a UV-Vis spectroelectrochemical investigation of electrogenerated graphitic oxides (EGO) on the surface of the C-OTE in the presence of KCl. X-ray photoelectron spectroscopy and time of flight secondary ion mass spectroscopy are used to determine EGO composition. Several supporting electrolytes are investigated to determine the mechanism of EGO formation. Chapter 3 details experiments to electrochemically access the exciton emission from self-assembled double-walled tubular J-aggregates via electrogenerated chemiluminescence (ECL). Optimization of ECL intensity with respect to the coreactant concentration and the supporting electrolyte pH is performed on opaque glassy carbon electrodes. ECL and fluorescence spectra are compared, and C-OTEs are utilized to determine the source of disagreement between the spectra. Chapter 4 describes the preparation and characterization (i.e. transparency, thickness, sheet resistance, rms roughness, and electroactive surface area) of C-OTEs and explores C-OTEs for general use in ECL under a variety of conditions. Simultaneous cyclic voltammograms and ECL transients are obtained for three thicknesses of PPFs and compared to non-transparent GC and the conventional transparent electrode ITO in both front face and transmission electrode cell geometries. Despite positive potential shifts in oxidation and ECL peaks, attributed to the internal resistance of the PPFs that result from their nanoscale thickness, the PPFs display similar ECL activity to GC, including the low oxidation potential observed for amine coreactants on hydrophobic electrodes. Overall, C-OTEs are promising electrodes for spectroelectrochemical applications because they yield higher ECL than ITO in both oxidative-reductive and reductive-oxidative ECL modes, are more stable in alkaline solutions, display a wide potential window of stability, and have tunable transparency for more efficient detection of light in the transmission cell geometry. Future directions for this research are discussed in Chapter 5, which outlines several approaches to designing and improving spectroelectrochemical sensors. / text

Advances in nanofabrication have bolstered the development of new optical devices with potential uses ranging from conventional optoelectronics, such as lasers and solar cells, to novel devices, like single photon or entangled photon sources. Quantum encryption of optical communications, in particular, requires devices that couple efficiently to an optical fiber and emit, on demand, indistinguishable photons. With these goals in mind, ultrafast spectroscopy is used to study the electron dynamics in epitaxially grown InAs/GaAs quantum dots (QDs). Quantifying the behavior of these systems is critical to the development of more efficient devices. Studies of two newly developed InGaAs QD structures, quantum dot clusters (QDCs) and QDs embedded in photonic wires, are presented herein. GaAs photonic wires with diameters in the range of 200 to 250 nm support only the fundamental HE11 guided mode. To fully quantify these new systems, the emission dynamics of QDs contained within wires in a large range of diameters are studied. Time correlated single photon counting measurements of the ground state exciton lifetimes are in very good agreement with predicted theoretical values for the spontaneous emission rates. For diameters smaller than 200 nm, QD emission into the HE11 mode is strongly inhibited and non-radiative processes dominate the decay rate. The best small diameter wires exhibit inhibition factors as high as 16, on par with the current state of the art for photonic crystals. The QDCs are the product of a hybrid growth technique that combines droplet heteroepitaxy with standard Stranski-Krastanov growth to create many different geometries of QDs. The work presented in this dissertation concentrates specifically on hexa-QDCs consisting of six InAs QDs around a GaAs nanomound. The first ever spectral and temporal properties of QDs within individual hexa-QDCs are presented. The QDs exhibit narrow exciton resonances with good temperature stability, indicating that excitons are well confined within individual QDs. A distinct biexponential decay is observed even at the single QD level. This behavior suggests that non-radiative decay mechanisms and exciton occupation of dark states play a significant role in the recombination dynamics in the QDCs. / text