Neste trabalho, foram investigados os processos de migração e dissociação do éxciton em filmes ultrafinos de poli(9,9 dioctilfluoreno) (PFO) com espessura menores que o raio típico de migração excitônica (10 nm) próximos de interface semicondutora orgânica e inorgânica. Os filmes de PFO foram produzidos utilizando a técnica de <span style=\'color:black;font-style:italic;language:PT-BR\'>spin-coating<span style=\'color:black;language:PT-BR\'> a partir de soluções de PFO em clorofórmio e em tolueno. Sabe-se que ao se utilizar o clorofórmio como solvente, os filmes de PFO apresentam uma estrutura amorfa. Com isso foi possível obter filmes com espessuras menores que o raio de migração do éxciton com qualidades ideais para o estudo de processos fotofísicos. Esses filmes foram depositados sobre uma camada de nanopartículas de dióxido de titânio (TiO<span style=\'color:black;language:PT-BR\'>2<span style=\'color:black;language:PT-BR\'>) formando assim uma interface orgânica/inorgânica. Técnicas espectroscópicas, tais como, microscopia confocal de fluorescência (LSCM), microscopia de imagem do tempo de vida (FLIM), fotoluminescência de onda contínua e resolvida no tempo bem como espectroscopia de absorção foram utilizadas no presente trabalho. Inicialmente o espectro de fotoluminescência dos filmes de PFO foram caracterizados através da dependência da intensidade da transição puramente eletrônica <span style=\'color:black;font-style:italic;language:PT-BR\'>I<span style=\'color:black;language:PT-BR\'>, largura da linha a meia altura <span style=\'font-family:Symbol;color:black;language:PT-BR\'>G<span style=\'color:black;language:PT-BR\'>o<span style=\'color:black; language:PT-BR\'>, energia da transição puramente eletrônica <span style=\'color:black;font-style:italic;language:PT-BR\'>E<span style=\'color:black;language:PT-BR\'> e parâmetro de Huang-Rhys <span style=\'color:black;font-style:italic;language:PT-BR\'>S<span style=\'color:black;language:PT-BR\'> variando a temperatura. Verificou-se que a intensidade pode ser descrita em termos da ativação térmica da migração do éxciton. Além disso, a temperatura introduz uma desordem térmica que afeta diretamente o tamanho dos segmentos conjugados que é observado em termos dos parâmetros, <span style=\'font-family:Symbol;color:black;language:PT-BR\'>G<span style=\'color:black;language:PT-BR\'>o<span style=\'color:black; language:PT-BR\'>, <span style=\'color:black;font-style:italic; language:PT-BR\'>E<span style=\'color:black;language:PT-BR\'> e <span style=\'color:black;font-style:italic;language:PT-BR\'>S<span style=\'color:black;language:PT-BR\'>. Com os filmes de PFO produzidos com o solvente tolueno foi observado que frações de fase <span style=\'font-family:\"Times New Roman\";color:black;font-style:italic; language:EL\'>&#946;<span style=\'font-family:\"Times New Roman\"; color:black;language:EL\'> <span style=\'color:black;language:PT-BR\'>já são induzidas nesses filmes e que esta fase não está dispersa na matriz amorfa e sim em forma de domínios formados por moléculas na fase <span style=\'font-family:\"Times New Roman\";color:black;font-style:italic; language:EL\'>&#946; <span style=\'color:black;language:PT-BR\'>que correspondente ao ordenamento nos anéis aromáticos do PFO. Também foi observado que nos filmes com espessura menor que 10 nm preparados em clorofórmio, a fase <span style=\'font-family:\"Times New Roman\";color:black;font-style:italic; language:EL\'>&#946;<span style=\'font-family:\"Times New Roman\"; color:black;language:EL\'> <span style=\'color:black;language:PT-BR\'>é induzida e sugerimos que isso ocorre devido a forte interação filme/substrato. Além disso, outros métodos bem conhecidos na literatura, tais como, tratamento a vapor de tolueno e ciclos térmicos de resfriamento/aquecimento foram utilizados para induzir a fase <span style=\'font-family:\"Times New Roman\";color:black;font-style:italic; language:EL\'>&#946;<span style=\'font-family:\"Times New Roman\"; color:black;language:EL\'> <span style=\'color:black;language:PT-BR\'>em filmes de PFO amorfo. Com relação aos filmes contendo a interface TiO<span style=\'color:black;language:PT-BR\'>2<span style=\'color:black;language:PT-BR\'>/PFO, a dissociação do éxciton na interface foi observada através da redução da intensidade da fotoluminescência. Em filmes ultrafinos, a eficiência do processo de dissociação do éxciton na interface é superior a 90%. A partir desses resultados, obteve-se que o raio de migração do éxciton no PFO é de (13 <span style=\'font-family:Symbol;color:black;language:PT-BR\'>±<span style=\'color:black;language:PT-BR\'> 3) nm. Além disso, devido à alta fluência (~ 10<span style=\'color:black;language:PT-BR\'>25<span style=\'color:black;language:PT-BR\'> fótons/cm<span style=\'color:black;language:PT-BR\'>2<span style=\'color:black;language:PT-BR\'>s), efeitos de autoaniquilamento de éxcitons em filmes ultrafinos foram observados nas medidas de tempo de decaimento radiativo. Por fim, em filmes de PFO (contendo a fase <span style=\'font-family:\"Times New Roman\";color:black;font-style:italic; language:EL\'>&#946;<span style=\'font-family:\"Times New Roman\"; color:black;language:EL\'>) <span style=\'color:black;language:PT-BR\'>depositados sobre o TiO<span style=\'color:black;language:PT-BR\'>2<span style=\'color:black;language:PT-BR\'>, somente os éxcitons das regiões amorfas migram até a interface do TiO<span style=\'color:black;language:PT-BR\'>2<span style=\'color:black;language:PT-BR\'> e são dissociados por ela. A energia térmica a temperatura ambiente promove uma maior eficiência da dissociação do éxciton do que em baixas temperaturas (~5 K). No entanto, mesmo em temperatura ambiente, as moléculas de fase <span style=\'font-family:\"Times New Roman\";color:black;font-style:italic; language:EL\'>&#946;<span style=\'font-family:\"Times New Roman\"; color:black;language:EL\'> <span style=\'color:black;language:PT-BR\'>atuam como centros de captura dos éxcitons competindo com o processo de dissociação. A partir desses resultados, obteve-se que o raio de transferência de energia no PFO é igual a (3,5 <span style=\'color:black;text-decoration:underline;language:PT-BR\'>+<span style=\'color:black;language:PT-BR\'> 0,5) nm.<span style=\'color:black; language:PT-BR\'> <![if !vml]> <![endif]> / In this work, we investigated the process of exciton dissociation and migration in ultra-thin films of poly (9,9 dioctilfluoreno) (PFO) with a thickness smaller than the typical radius of excitonic migration (10 nm) and near to organic and inorganic semiconductor interface. The PFO films were produced using the spin-coating technique from a PFO solution in chloroform and toluene. It is known that when using chloroform as solvent, the PFO films have an amorphous structure. Thus, it was possible to obtain films that have thicknesses smaller than the radius of the exciton migration qualities, which is ideal to study photophysical processes. These films were deposited on the titanium dioxide (TiO<span style=\'language:EN\'>2<span style=\'language:EN\'>) nanoparticles layer forming an organic/inorganic interface. Spectroscopic techniques such as fluorescence confocal microscopy (LSCM), fluorescence lifetime imaging microscopy (FLIM), continuous wave photoluminescence and time resolved and absorption spectroscopy were used in this study. First, the photoluminescence spectrum of PFO films were characterized by the dependence of the purely electronic transition intensity <span style=\'font-style:italic;language:EN\'>I<span style=\'language:EN\'>, full width at half maximum <span style=\'font-family:Symbol;language:EN\'>G<span style=\'language: EN\'>o<span style=\'language:EN\'>, purely electronic transition energy <span style=\'font-style:italic;language:EN\'>E<span style=\'language:EN\'> and Huang-Rhys parameter <span style=\'font-style:italic;language:EN\'>S <span style=\'language:EN\'>with the temperature. It was showed that the intensity can be described according to thermal activation of the exciton migration. Furthermore, the temperature introduces a thermal disorder affecting directly the conjugated segment length which is observed according to the parameters, <span style=\'font-family:Symbol;language:EN\'>G<span style=\'language: EN\'>o<span style=\'language:EN\'>, <span style=\'font-style: italic;language:EN\'>E<span style=\'language:EN\'> e <span style=\'font-style:italic;language:EN\'>S<span style=\'language:EN\'>. The PFO films produced with the solvent toluene analysis showed that the fractions of <span style=\'font-family:\"Times New Roman\";font-style:italic;language:EN\'>&#946;<span style=\'language:EN\'> phase are already induced and that this phase is not dispersed in the amorphous matrix. On the order hand, they are shaped domains formed by <span style=\'font-family:\"Times New Roman\";font-style:italic;language:EN\'>&#946;<span style=\'language:EN\'> phase molecules that are related to the PFO aromatic rings organization. It was also observed that the films with thickness smaller than 10 nm prepared using chloroform, the <span style=\'font-family:\"Times New Roman\";font-style:italic;language:EN\'>&#946;<span style=\'language:EN\'> phase is induced, which may be occurring due to the strong interaction observed between film and substrate. Other methods reported in the literature, such as toluene steam treatment and thermal cycles of cooling/heating were used to induce <span style=\'font-family:\"Times New Roman\";font-style:italic;language:EN\'>&#946;<span style=\'language:EN\'> phase in amorphous PFO films as well. The exciton dissociation at the interface in films containing TiO<span style=\'language:EN\'>2<span style=\'language:EN\'>/PFO interface were observed by reduction of photoluminescence intensity; and this same efficiency in ultra-thin films is higher than 90%. From these results, it was reported that the exciton migration radius in PFO is (13 <span style=\'font-family:Symbol;language:EN\'>±<span style=\'language:EN\'> 3) nm. Furthermore, effects of excitons annihilation in ultra-thin films were observed in the decay time radiative measurements due to high fluency (~ 10<span style=\'language:EN\'>25<span style=\'language:EN\'> photons/cm<span style=\'language:EN\'>2<span style=\'language:EN\'>s). Finally, in PFO films (with the <span style=\'font-family:\"Times New Roman\";font-style:italic;language:EN\'>&#946;<span style=\'language:EN\'> phase) deposited on the TiO<span style=\'language:EN\'>2 <span style=\'language:EN\'>just the exciton of amorphous regions migrates to the TiO<span style=\'language:EN\'>2<span style=\'language:EN\'> interface, which dissociates them. The thermal energy at room temperature promotes higher efficiency than the exciton dissociation at low temperatures (~ 5 K). However, even at room temperature, the <span style=\'font-family:\"Times New Roman\";font-style:italic;language:EN\'>&#946;<span style=\'language:EN\'> phase molecules act as the most important molecules to capture exciton, competing to the exciton dissociation process. These results conducted to conclude that the energy transfer radius in PFO is equal to (3.5 <span style=\'text-decoration:underline;language:EN\'>+<span style=\'language: EN\'> 0.5) nm. <![if !vml]> <![endif]>

Le travail de recherche qui est rapporté dans ce manuscrit porte sur les couches minces de pérovskites et leur utilisation dans le cadre de la problématique des microcavités en régime de couplage fort. L’arrangement cristallin des pérovskites forme une structure en multi-puits quantiques dans laquelle les états excitoniques présentent une grande force d’oscillateur et une énergie de liaison importante (quelques 100 meV), en raison des effets de confinements quantique et diélectrique. Un premier axe de ce travail a consisté à collecter des informations sur les propriétés excitoniques de ces matériaux. Sur une pérovskite particulière (PEPI), nous avons notamment effectué des mesures de photoluminescence sous excitation impulsionnelle et des mesures pompe-sonde qui semblent suggérer l’existence, sous forte densité d’excitation, d’un processus de recombinaison Auger des excitons. Un deuxième axe de recherche fut de mettre en cavité des couches de certaines pérovskites. Avec les pérovskites PEPI et PEPC, nous avons montré que la réalisation de microcavités présentant un facteur de qualité de l'ordre de la dizaine suffit à obtenir, à température ambiante, le régime de couplage fort en absorption et en émission avec des dédoublements de Rabi pouvant atteindre 220 meV. Un goulet d’étranglement dans la relaxation des polaritons a été clairement mis en évidence pour la microcavité PEPI. Nous avons d’autre part montré que les pérovskites pouvaient également être associées à des semi-conducteurs inorganiques dans des microcavités dites "hybrides". Selon Agranovich et al., ces dernières pourraient, dans le cadre de la problématique du laser à polaritons, constituer une alternative à l'augmentation du facteur de qualité des microcavités. Dans cette optique, le couple ZnO/MFMPB semble particulièrement prometteur. / The research work which is reported in this manuscript focuses on 2D perovskites and their use to obtain microcavities working in the strong coupling regime. Perovskite structure forms a multi-quantum wells in which the excitonic states have a high oscillator strength and a large binding energy (a few 100 meV) due to quantum and dielectric confinement effects. A first axis of this work was to collect information on the excitonic properties of these materials. On a particular perovskite (PEPI), we performed photoluminescence and pump-probe measurements, which seem to suggest the existence, under high excitation density, a process of Auger recombination of excitons. A second research axis was to put in cavity thin layers of some perovskites. With PEPI and PEPC perovskites, we have shown that the realization of microcavities with a quality factor of the order of ten is sufficient to obtain at room temperature, the strong coupling regime in absorption and emission with Rabi splitting up to 220 meV. A bottleneck effect has been clearly demonstrated for the PEPI microcavity. We have also shown that perovskites could be associated with inorganic semiconductors in “hybrid” microcavities. According Agranovich et al., these microcavities could present polariton lasing with lower quality factors. To this end, the ZnO/MFMPB association seems particularly promising.

L'interaction entre impulsions lasers ultra-brèves et matériaux diélectriques est un sujet d'étude en constant renouvellement, motivé aussi bien par la naissance d'une multitude d'applications (micro-usinage laser, opérations de la cornée, ...) que par ses aspects fondamentaux (génération d'harmoniques d'ordre élevé, électronique au cycle optique, ...). Pourtant, les mécanismes sous-jacents à cette interaction sont encore mal compris, en particulier d'un point de vue quantitatif. En effet, une bonne partie des phénomènes ont lieu durant le passage de l'impulsion laser, c'est-à-dire pendant quelques dizaines de femtosecondes. De surcroît, les éclairements impliqués dans l'interaction sont élevés : quelques dizaines de TW/cm^2. La compréhension et la quantification des processus élémentaires ayant lieu durant l'interaction requiert donc de concevoir des expériences sur mesure ainsi que d'effectuer un travail approfondi de modélisation. Dans ce manuscrit sont présentées deux techniques expérimentales complémentaires utilisées durant la thèse : l'interférométrie fréquentielle et la spectroscopie d'absorption résolues en temps. Ces deux techniques sont employées pour étudier les processus électroniques d'excitation et de relaxation dans trois matériaux distincts : le quartz, le saphir et l'oxyde de magnésium. La quasi-intégralité des résultats expérimentaux sont analysés à l'aide d'un modèle en équation de taux multiples (MRE) permettant de discriminer l'importance relative de chacun des processus pris en compte. Tout d'abord, une preuve expérimentale directe de l'existence de l'ionisation par impact dans le quartz est présentée. Ces résultats ont été obtenus par l'exposition de l'échantillon à une série de deux impulsions, ce qui permet de moduler indépendamment la densité et la température du plasma. Les résultats expérimentaux sont reproduits avec succès par le modèle MRE intégrant l'ionisation multiphotonique, le chauffage des porteurs photo-excités ainsi que l'ionisation par impact comme processus d'excitation. Nous nous concentrons ensuite sur une seconde série de résultats concernant la relaxation du saphir après excitation induite par laser. Un nouveau mécanisme de relaxation est proposé et testé par la modélisation pour tenter d'expliquer la dynamique de ce processus. Ce mécanisme implique la formation d'excitons auto-piégés puis leur recombinaison. Enfin, le modèle MRE est appliqué à la détection de seuils d'ablation dans les trois solides. Le choix du critère physique déterminant ce seuil, sujet soumis à d'intenses débats dans la littérature, est alors discuté. / The interaction between an ultrashort laser pulse et dielectric materials is a constantly evolving field motivated by the birth of various applications (laser micro-machining, cornea surgery, ...) as well as its fundamental aspects (high-order harmonic generation, electronics at optical-cycle regime, ...). However, the underlying mechanisms of this interaction are still to be understood, especially from a quantitative point of view. Indeed, most phenomena occur during the pulse propagation through the material (i.e. in a few tens of femtoseconds). Moreover, the involved intensities are very high: a few tens of TW/cm^2. These two aspects make it difficult to study this interaction. To push the understanding forward, it is necessary to design new experiments and carry out an extensive work in modelling. Two complementary experimental techniques are presented in this manuscript: time resolved spectral interferometry and absorption spectroscopy. These two techniques are used to study elementary excitation and relaxation electronic processes in three different materials: Quartz, sapphire and magnesium oxide. Almost all experimental results are analyzed using a multiple rate equations (MRE) model allowing us to discriminate the relative importance of each of these processes. Firstly, a direct experimental proof of the existence of impact ionization in quartz is presented. These results have been obtained by exposing the sample to a sequence of two laser pulses, allowing us to tune independently both density and temperature of the plasma. The experimental results are successfully reproduced by the MRE model including multiphoton ionization, inverse bremsstrahlung and impact ionization as excitation processes. We then focus on another series of results on sapphire relaxation after laser-induced excitation. A new relaxation mechanism is proposed and tested by modelling to attempt to explain the dynamic of this process. This mechanism involves the formation of self-trapped excitons and their recombination. Finally, the MRE model is applied to the detection of the laser-induced ablation threshold in all three solids. The choice of the physical criterion defining this threshold, a subject that has caused intense debates over the past years, is discussed.

<p dir="ltr">Long-range exciton transport, when coupled with reduced exciton-exciton annihilation (EEA), is pivotal for the enhanced performance of organic photovoltaics and the efficiency of natural light-harvesting systems. This thesis explores strategies to optimize exciton transport and EEA rates in molecular materials by manipulating the quantum nature of excitons, particularly exciton delocalization. In addition, we also aim to understand factors limiting the transport of delocalized excitons within molecular materials. To this end, self-assembled perylene diimide (PDI) molecular aggregates are ideal candidates for this study due to their conducive properties for engineering exciton delocalization. <b>Chapter 1 </b>establishes a fundamental understanding of exciton delocalization, outlining strategies to tune this phenomenon within PDI aggregates and presenting the open questions this thesis addresses. <b>Chapter 2 </b>details the synthesis of PDI aggregates and delineates the spectroscopic techniques used for characterization, including steady-state absorption and emission, transient photoluminescence (PL), and transient absorption spectroscopy. It also describes the microscopy methods implemented to visualize exciton transport, such as transient PL microscopy and transient absorption microscopy (TAM). <b>Chapter 3 </b>introduces the thesis's primary theme: the suppression of exciton-exciton annihilation (EEA) in molecular aggregates through quantum interference. This chapter demonstrates that the spatial phase relationship of delocalized excitons is crucial in EEA, with band bottom excitons in H aggregates exhibiting an oscillating spatial phase relationship displaying a coherent suppression of EEA. <b>Chapter 4 </b>discusses how coupling to static and dynamic disorder affects coherent exciton propagation. High spatial and temporal resolution TAM experiments, along with temperature-dependent studies, help disentangle the contributions of static and dynamic disorder to exciton transport. <b>Chapter 5 </b>delves into the concept of band shape engineering, whereby the microscopic electronic couplings within PDI aggregates are fine-tuned by altering the packing motifs to regulate exciton transport. Through low-temperature TAM experiments, this chapter illustrates how the interplay between long-range Coulombic and short-range charge transfer electronic couplings can determine exciton bandwidth and influence the coherent propagation of excitons. <b>Chapter 6 </b>provides a summary of the work and discusses future directions, paving the way for continued exploration in the field of exciton transport and interaction in molecular aggregates.</p>

The field of organic electronics has received considerable attention over the past several years due to the promise of novel electronic materials that are cheap, flexible and light weight. While some devices based on organic materials have already emerged on the market (e.g. organic light emitting diodes), a deeper understanding of the excited states within the condensed phase is necessary both to improve current commercial products and to develop new materials for applications that are currently in the commercial pipeline (e.g. organic photovoltaics, wearable displays, and field effect transistors). To this end, a model for pi-conjugated molecular aggregates and crystals is developed and analyzed. The model considers two types of electronic excitations, namely Frenkel and charge-transfer excitons, both of which play a prominent role in determining the nature of the excited states within tightly-packed organic systems. The former consist of an electron-hole pair bound to the same molecule while in the later the electron and hole are located on different molecules. The model also considers the important nuclear reorganization that occurs when the system switches between electronic states. This is achieved using a Holstein-style Hamiltonian that includes linear vibronic coupling of the electronic states to the nuclear motion associated with the high frequency vinyl-stretching and ring-breathing modes. Analysis of the model reveals spectroscopic signatures of charge-transfer mediated J- and H-aggregation in systems where the photophysical properties are determined primarily by charge-transfer interactions. Importantly, such signatures are found to be sensitive to the relative phase of the intermolecular electron and hole transfer integrals, and the relative energy of the Frenkel and charge-transfer states. When the charge-transfer integrals are in phase and the energy of the charge-transfer state is higher than the Frenkel state, the system exhibits J-aggregate characteristics including a positive band curvature, a red shifted main absorption peak, and an increase in the ratio of the first two vibronic peaks relative to the monomer. On the other hand, when the charge-transfer integrals are out of phase and the energy of the charge-transfer state is higher than the Frenkel state, the system exhibits H-aggregate characteristics including a negative band curvature, a blue shifted main absorption peak, and a decrease in the ratio of the first two vibronic peaks relative to the monomer. Notably, these signatures are consistent with those exhibited by Coulombically coupled J- and H-aggregates. Additional signatures of charge-transfer J- and H-aggregation are also discovered, the most notable of which is the appearance of a second absorption band when the charge-transfer integrals are in phase and the charge-transfer and Frenkel excitons are near resonance. In such instances, the peak-to-peak spacing is found to be proportional to the sum of the electron and hole transfer integrals. Further analysis of the charge-transfer interactions within the context of an effective Frenkel exciton coupling reveals that the charge-transfer interactions interfere directly with the intermolecular Coulombic coupling. The interference can be either constructive or destructive resulting in either enhanced or suppressed J- or H- aggregate behavior relative to what is expected based on Coulombic coupling alone. Such interferences result in four new aggregate types, namely HH-, HJ-, JH-, and JJ-aggregates, where the first letter indicates the nature of the Coulombic coupling and the second indicates the nature of the charge-transfer coupling. Vibronic signatures of such aggregates are developed and provide a means by which to rapidly screen materials for certain electronic characteristics. Notably, a large total (Coulombic plus charge-transfer) exciton coupling is associated with an absorption spectrum in which the ratio of the first two vibronic peaks deviates significantly from that of the unaggregated monomer. Hence, strongly coupled, high exciton mobility aggregates can be readily distinguished from low mobility aggregates by the ratio of their first two vibronic peaks. Analysis of the spatial dependence of the intermolecular interactions reveals that all four aggregate types (HH-, HJ-, JH-, JJ-) can be achieved by enforcing the appropriate crystalline packing arrangement. Such tunability is possible due of the different length scales over which the natures of the two coupling sources interconvert from J-like to H-like; whereas the nature of the Coulombic coupling is known to be sensitive to displacements on the order of half the molecular length, the nature of the charge-transfer mediated exciton coupling is sensitive to geometric displacements of approximately a carbon-carbon bond length. It is proposed that such sensitivity should allow for fine tuning of the total excitonic coupling via modifications in the packing structure, as determined, for example, by the side chains. Several examples of the different aggregate types are provided throughout this dissertation as the model is used to probe the excited state character of several relevant conjugated organic systems. Such examples include pentacene and 7,8,15,16-tetraazaterrylene (TAT) along with several derivatives from the perylene family. / Chemistry

Made available in DSpace on 2014-12-17T15:15:02Z (GMT). No. of bitstreams: 1 FabioFM.pdf: 1459636 bytes, checksum: 4f2bb557c6a5bddb80e46f325bd5b4dd (MD5) Previous issue date: 2004-12-03 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / In this work we study the spectrum (bulk and surface modes) of exciton-polaritons in infinite and semi-infinite binary superlattices (such as, ???ABABA???), where the semiconductor medium (A), whose dielectric function depends on the frequency and the wavevector, alternating with a standard dielectric medium B. Here the medium A will be modeled by a nitride III-V semiconductor whose main characteristic is a wide-direct energy gap Eg. In particular, we consider the numerical values of gallium nitride (GaN) with a crystal structure wurtzite type. The transfer-matrix formalism is used to find the exciton-polariton dispersion relation. The results are obtained for both s (TE mode: transverse electric) and p (TM mode: transverse magnetic) polarizations, using three diferent kind of additional boundary conditions (ABC1, 2 e 3) besides the standard Maxwell's boundary conditions. Moreover, we investigate the behavior of the exciton-polariton modes for diferent ratios of the thickness of the two alternating materials forming the superlattice. The spectrums shows a confinement of the exciton-polariton modes due to the geometry of the superlattice. The method of Attenuated Total Reflection (ATR) and Raman scattering are the most adequate for probing this excitations / Neste trabalho estudamos o espectro (modos de volume e de superf?cie) dos polaritons de exciton em uma super-rede bin?ria infinita e semi-infinita (tal como, ???ABABA???), onde um meio semicondutor (A), cuja fun??o diel?trica depende da frequ?ncia e do vetor de onda, alterna-se com um diel?trico comum (B). Aqui, o meio A ser? modelado por um semicondutor da fam?lia dos nitretos (semicondutor III-V) que tem como caracter?stica principal um gap de energia (Eg) direto e largo. Em particular, consideramos os valores num?ricos para o nitreto de g?lio (GaN) com uma estrutura cristalina tipo wurtzite. A t?cnica da matriz de transfer?ncia ? utilizada para encontrarmos a rela??o de dispers?o do polariton de exciton. Os resultados s?o obtidos para os modos de polariza??o s (ou modo TE: transversal el?trico) e p (ou modo TM: transversal magn?tico), usando tr?s diferentes condi??es de contorno adicionais (ABC1, 2 e 3), mais as condi??es de contorno padr?es de Maxwell. Al?m disso, investigamos o comportamento dos modos do polariton de exciton para diferentes raz?es entre as espessuras das camadas dos dois materiais que comp?em a super-rede Os espectros encontrados evidenciam um comportamento de confinamento dos polaritons de exciton devido ?s geometrias empregadas. As t?cnicas experimentais ATR ("Attenuated Total Reflection") e o espalhamento Raman s?o as mais adequadas para a caracteriza??o dessas excita??es

In this thesis ultrafast exciton dynamics of several colloidal quantum dots have been studied using visible transient absorption spectroscopy. The resultant transient decays and differential transmission spectra were analysed to determine the ultrafast relaxation channels, multiple exciton generation (MEG) efficiency and multi-exciton interactions in the observed materials. All QDs were preliminarily optically characterized using steady state absorption and photoluminescence spectroscopies. In addition, a high repetition infrared femtosecond pump probe experiment was designed and built to detect the picosecond intraband carrier relaxations in quantum dots. Picosecond carrier dynamics of type-II ZnTe/ZnSe and of CuInSe2 and CuInS2 type-I quantum dots were investigated. The common feature of these materials is that they are eco-friendly materials, being alternatives to the toxic Cd- and Pb- based materials. It was found that surface trapping occurred in both cases for electrons in the hot states, and in the minimum of the conduction band for ZnTe/ZnSe core/shell materials. Trion formation was observed in ZnTe/ZnSe core/shell dots at high power and unstirred conditions. The hot and cold electron trapping processes in type-II dots and CuInS2 and CuInSe2 dots shifted, distorted and moderately cancelled the bleach features. In addition, intra-gap hole trapping was observed in CuInS2 and CuInSe2 dots which results in a long decay feature in the recorded transients. MEG competes with Auger cooling, surface mediated relaxation and phonon emission. To enhance the MEG quantum yield, the rival mechanisms were suppressed in well-engineered CdSe/CdTe/CdS and CdTe/CdSe/CdS core/shell/shell and CdTe/CdS core/shell type-II quantum dots. The MEG slope efficiency and threshold for a range of different core size and shell thickness were found to be (142±9)%/Eg and (2.59±0.16)Eg, respectively. The observed threshold was consistent with the literature, whereas, the obtained slope efficiency was about three times higher than the previously reported values. The biexciton interaction energy of the dots stated in the previous paragraph was also studied. To date, time-resolved photoluminescence (TRPL) has been employed to study exciton interactions in type-II quantum dots and large repulsive biexciton interaction energy values between 50-100 meV have been reported. However, unlike the TRPL method, the TA experiment ensures that only two excitons remain in the band edge of the dot. Using this method, large attractive biexciton interaction energies up to ~-60 meV was observed. These results have promising implications regarding enhancing the MEG quantum yield.

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