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Bismuth vanadate has attracted much attention as one of the most promising materials for photoelectrochemical (PEC) water splitting. This Master Thesis is aimed at understanding BiVO4 films growth to use it as photoanodes. We fabricate samples by spin- coating method modifying BiVO4 thickness up to ∼ 40 nm and adding a thin (∼ 5 nm) SnO2 hole blocking layer. Morphological characterization with scanning electron microscopy have confirmed the formation of BiVO4 layers on FTO and have shown an increase on the grain sizes from 64 nm to 98 nm for increasing film thickness. Electrochemical experiments are performed on thin films to characterize its electrical parameters, showing an increase of the photocurrent density with increasing number of layers up to 0.72 mA · cm−2 at 1.23 VRHE for the thicker sample. Moreover, it is demonstrated an optimization of the fill factor for the samples that include a intermediate SnO2 layer, confirmed by Mott-Schottky analysis, obtaining a shift of 80 mV of the flat band potential, improving photogenerated voltage and thus, performance of the photoanode. We have proposed and demonstrated a method to obtain sample thicknesses by a linear relation with the incident photon to current conversion efficiency value at 400 nm.

Singlet fission is a photophysical process which converts an excited singlet state into two independent triplet states. This has attracted significant attention for photovoltaic devices due to the fact that more than one electron can be generated per each absorbed photon, which can boost the quantum efficiency over 100 %. In this project, research on the understanding of singlet fission dynamics of organic thin films has been conducted by using transient absorption spectroscopy as the main characterisation method of the proposed polymer-based films, which are a mixture of TIPS-pentacene and Y6. Results have shown that charge generation is due to the triplet states generated thanks to the excitation at 800 nm of TIPS-pentacene in blends at certain ratios. Moreover, as an extra work, solar cells have been fabricated obtaining a 10 % of quantum efficiency due only to the triplet states generation.

The environmental issues associated with the use of conventional fuels necessitates the utilisation of renewable energy sources, as well as the implementation of energy efficient designs, in order to decrease electricity consumption. Photovoltaic (PV) technology can be employed for both approaches by converting not only natural but, also, artificial light into electricity. Among the different emerging PVs, perovskites achieve the highest power conversion efficiency, providing a widely tuneable bandgap with minimum open circuit losses. Moreover, their fabrication uses readily available materials, and does not necessarily require either the use of high temperature processes or vacuum deposition techniques. In this thesis, we enhance light harvesting in perovskite solar cells, and approach the energy efficiency concept through their optimised fabrication and integration in light selective structures. This is accomplished by the implementation of optical and material strategies applied to specific perovskite solar cell designs. The results prove that such strategies provide enhanced light absorption and optimal PV performance in low temperature devices, and enable the recycling of light into electricity for alternative photonic applications. The approaches presented could be utilised in future procedures to decrease the amount of Pb employed in perovskite solar cells, and to reduce the energy consumption during fabrication and the operation of other optoelectronic devices. The thesis is organised into four chapters. Chapter 1 serves as an introduction, where the current energy situation and PV technology are analysed, together with an insight into light harvesting and energy efficiency in perovskite solar cells. In Chapter 2, we demonstrate the employment of a periodic structure to propagate ergodic light in order to increase light absorption in perovskite solar cells, as would happen by employing randomly textured surfaces. This structure serves as a tool to decrease the Pb content used in perovskite solar cells, since 30% less material can be used to obtain a solar cell with equal performance. Then, in Chapter 3, the same periodic configuration with a thin film structure deposited on its surface is applied as a waveguide, which is also able to transmit polarised light. Moreover, two perovskite solar cells integrated on the sides recycle the non-transmitted light into electricity, increasing the energy efficiency of the optical process, with further application in liquid crystal displays (LCDs). Finally, in Chapter 4, we demonstrate the suitable application of a nanoparticle bilayer made of one layer of SnO2 and another of TiO2 as n-type materials in perovskite solar cells. These types of devices, based on low temperature processes, are proven to perform better than those containing one type of nanoparticles, especially in semi-transparent devices. In such devices we achieved an enhancement in performance of up to 30% for solar cells based on extremely thin active layers. Los problemas medioambientales asociados al uso de combustibles convencionales requieren del uso de fuentes de energía renovables, así como de la implementación de diseños eficientemente energéticos para reducir el consumo de energía. La tecnología fotovoltaica puede emplearse para cubrir ambas estrategias convirtiendo no sólo la luz natural, sino también la artificial, en electricidad. De entre las diferentes tecnologías fotovoltaicas emergentes, las perovskitas alcanzan la más alta eficiencia en conversión de potencia, al mismo tiempo que proporcionan una banda de energía prohibida ampliamente ajustable con pérdidas mínimas de tensión de circuito abierto. Además, su fabricación usa materiales abundantemente disponibles, y no requiere necesariamente de procesos a alta temperatura ni de técnicas de deposición en vacío. En esta tesis, mejoramos la colección de luz en celdas de perovskitas, a la vez que abordamos el concepto de eficiencia energética a través de una fabricación optimizada y su integración en estructuras selectivas de luz. Esto es conseguido gracias a la implementación de estrategias ópticas y materiales aplicadas a diseños específicos de celdas solares de perovskita. Los resultados demuestran que tales estrategias proporcionan una colección de luz y un rendimiento fotovoltaico mayor aplicable a dispositivos fabricados a baja temperatura, y permiten el reciclaje de luz en electricidad para aplicaciones fotónicas alternativas. Las técnicas presentadas podrían ser utilizadas en procedimientos futuros para disminuir la cantidad de Pb empleado en celdas solares de perovskita, y para reducir el consumo de energía durante su fabricación y el funcionamiento de otros dispositivos optoelectrónicos. La tesis está organizada en cuatro capítulos. El Capítulo 1 sirve como una introducción, donde la actual situación energética y la tecnología fotovoltaica son analizadas junto a una descripción de la recolección de luz y la eficiencia energética en celdas solares de perovskita. En el Capítulo 2, demostramos el uso de una estructura periódica para propagar luz ergódicamente y así aumentar la absorción de luz en las celdas solares de perovskita, de manera equivalente a lo que se obtendría usando superficies aleatoriamente texturizadas. Esta estructura sirve como herramienta para reducir el contenido de Pb empleado en celdas solares de perovskita, ya que se puede utilizar 30% menos de material para obtener una celda solar con un rendimiento equivalente. En el Capítulo 3, la misma configuración periódica con una estructura de capa fina depositada en su superficie es empleada como guía de luz, la cual es, además, capaz de transmitir luz polarizada. Además, dos celdas de perovskita integradas en sus laterales reciclan la luz no transmitida en electricidad, incrementando la eficiencia energética del proceso óptico, lo cual podría tener futura aplicación en pantallas de cristal líquido. Finalmente, en el Capítulo 4, demostramos la aplicación de una bicapa de nanopartículas hecha de una capa de SnO2 y otra de TiO2 como materiales de tipo n en celdas solares perovskita. Este tipo de dispositivos, basados en procesos a baja temperatura, funcionan mejor que los que integran un único tipo de nanopartículas, especialmente en dispositivos semitransparentes. En tales dispositivos conseguimos un funcionamiento hasta 30% mejor para celdas solares basadas en capas activas extremadamente finas. Fotònica

Solution-processed thin film solar cells emerged as very promising photovoltaic technologies suitable for a low cost roll-to-roll upscale production. Such thin film character also ensures lightweight and flexibility for the solar cell modules, making them ideal for a wide variety of applications where silicon panels cannot be used. In addition to the above-mentioned advantages, common in all solution-processed thin film technologies, polymer solar cells (PSCs) have a unique semitransparency, which makes them very useful for solar window applications and very competitive in building integrated photovoltaics. In recent years, a remarkable progress has been achieved in the field of PSCs. The power conversion efficiency of PSCs has already surpassed the 11% barrier. However, to be able to eventually compete with other solution-processed thin film technologies, such device efficiency must be further improved. Given the low charge carrier mobility in commonly used organic p-conjugated semiconductors, the tradeoff between optical absorption and charge collection, limits the thickness of the majority of photoactive layers currently being used to approximately 100 nm. To overcome the limited light absorption in such thin active layers, an adequate optical management becomes very important. Ideally, a light absorption or short-circuit current enhancement should be achieved without affecting the other photovoltaic parameters, such as the photovoltaic device open circuit voltage and fill factor. In this thesis, we implement a one-dimensional new optical planar cavity that exhibits a resonant character at two different nonharmonic frequencies of each other, which we named two-resonance tapping cavity (TRTC). With such TRTC we demonstrate that one may reach an optimal broadband light trapping in thin film cells, largely improving the photocurrent of the solar without sacrificing the device electrical properties. A limited stability is another obstacle that may prevent any industrial application of the PSC technology. In accordance, in parallel to a device efficiency increase one must address the problem of a short operational device lifetime. In the current thesis, we performed several experiments, which lead us to understand the physics behind the rapid destruction of the active layer nanomorphology under illumination. In addition, we propose and implement a new procedure based on the formation of a highly ordered PCBM phase to circumvent such degradation path, and achieve high-performance PSCs with long lifetimes. The current thesis has been divided into five chapters. Chapter 1 briefly reviews some photon management approaches and some basics of degradation mechanisms in PSCs. Chapter 2 describes the TRTC concept and the experimental implementation of PTB7-Th:PC71BM cells integrated in such TRTC. In Chapter 3 we describe the use of the TRTC concept presented in Chapter 2 to achieve an optimal balance between open circuit voltage and photocurrent in flexible PBDBT:ITIC cells. In Chapter 4 we demonstrate an approach to increase the operational lifetimes of PSCs based on a UV treatment to actively remove chemisorbed oxygen on the ZnO interlayer. Finally, in Chapter 5, an in-depth study of the fast burn-in loss for PTB7-Th:PC71BM cells is given, and a new explanation to the such degradation path is proposed. In addition, we propose and implement an approach to circumvent such degradation and achieve long lifetime high efficiency solar cells., Las celdas solares de capa fina procesadas por métodos en solución surgieron como una interesante tecnología fotovoltaica para una producción roll-to-roll a bajo coste. El carácter de película delgada también asegura el bajo peso y la flexibilidad de los módulos solares, haciéndolos ideales para una gran variedad de aplicaciones donde los paneles de silicio no pueden ser usados. Además de las ventajas mencionadas, compartidas por todas las tecnologías de capa fina procesadas en solución, las celdas solares de polímero (PSCs) presentan propiedades únicas de semi-transparencia, lo que las hace útiles para aplicaciones en ventanas solares y muy competitivas como elementos integrados en edificios. En los últimos años, se ha logrado un progreso notable en el campo de las PSCs. La eficiencia de conversión de energía de las PSCs ha superado ya la barrera del 11%. Sin embargo, para poder competir finalmente con otras tecnologías de capa delgada procesadas en solución, la eficiencia del dispositivo debe mejorarse aún más. Debido a la baja movilidad de los portadores de carga para los típicos semiconductores orgánicos p-conjugados, el compromiso entre absorción óptica y recolección de carga limita el espesor de la mayoría de las capas fotoactivas que se utilizan actualmente a aproximadamente 100 nm. Para mejorar la limitada absorción de luz en dichas capas activas, el uso de estrategias ópticas adecuadas es muy relevante. Idealmente, se debería lograr un aumento de la absorción de la luz o de la corriente a cortocircuito sin afectar a los demás parámetros fotovoltaicos, tales como el voltaje a circuito abierto y el factor de llenado del dispositivo fotovoltaico. En esta tesis, incorporamos una nueva cavidad óptica planar unidimensional que presenta un carácter resonante en dos frecuencias no armónicas diferentes, y que llamamos two-resonance tapping cavity (TRTC). Con la TRTC demostramos que puede alcanzarse una captura óptima de la luz en celdas de capa fina sobre un amplio rango de longitudes de onda, mejorando en gran medida la fotocorriente sin sacrificar las propiedades eléctricas del dispositivo solar. La falta de estabilidad es otro obstáculo que puede prevenir la aplicación industrial de la tecnología de PSCs. De esta manera, en paralelo a un aumento de la eficiencia del dispositivo, se debe abordar el problema de su corta vida operativa. En esta tesis, llevamos a cabo varios experimentos para entender la física que determina el rápido deterioro de la nano-morfología de la capa activa en condiciones de iluminación. Además, proponemos e implementamos un nuevo procedimiento basado en la formación de una fase de PCBM altamente ordenada para eludir la vía de degradación y conseguir PSCs de alto rendimiento con una vida útil extendida. La tesis se ha dividido en cinco capítulos. El Capítulo 1 revisa brevemente algunas de las estrategias para la manipulación de fotones y las bases de los mecanismos de degradación en PSCs. El Capítulo 2 describe el concepto de TRTC y la implementación experimental de celdas de PTB7-Th: PC71BM incluyendo la TRTC. En el Capítulo 3 describimos el uso del concepto TRTC presentado en el Capítulo 2 para conseguir un balance óptimo entre voltaje a circuito abierto y fotocorriente en celdas flexibles de PBDBT: ITIC. En el Capítulo 4 demostramos una estrategia para mejorar la vida operativa de los PSCs basado en un tratamiento con luz UV para eliminar activamente el oxígeno quimisorbido en la capa de ZnO. Finalmente, en el Capítulo 5, se presenta un estudio en profundidad de la fast burn-in loss para las celdas de PTB7-Th:PC71BM, y se propone una nueva explicación de la ruta de degradación. Además, proponemos e implementamos una alternativa para evitar la degradación y conseguir celdas solares de alta eficiencia con una larga vida útil.

Thin film photovoltaics encompass a group of technologies able to harvest light within a few microns thickness. The reduced thickness allows a low cost of manufacture while making the films flexible and adaptable to different surfaces. This, combined with their low weight, positioned thin film solar cells as ideal candidates for building integrated photovoltaics. For the latter, organic solar cells (OSC) can provide a high quality semi-transparency that closely mimics the aesthetics of standard windows. Indeed, some unique features of organic solar cells make them the optimal solution for applications where standard Si technology cannot be used. However, for large-scale electricity production where efficiency is, perhaps, the most determining factor, newer thin film technologies like perovskites solar cells may be a more adequate option. At the moment of writing this thesis, state of the art efficiencies of single junction perovskites nearly double that of the best single junction organic solar cell. A limitation found in both technologies, especially in organics and to a lesser degree in perovskites, is the low mobility of the carriers. This, together with other processing shortcomings in the organic absorbers and perovskites limit their thickness to 100-130 nm, and 500-600 nm, respectively. In summary, light management must be an essential ingredient when designing device architectures to achieve the optimal performance in the specific application being considered. In this thesis, in order to achieve an optimal light harvesting and therefore increase the performance of thin film solar cells, we take two approaches. On one hand, we increase the total thickness of the absorber material used in the device without increasing the thickness of the single active material layer and, on the other hand, we combine complementary absorbers to cover a wider portion of the solar spectra. These approaches pose the double challenge of finding the optimal electromagnetic field distribution within a complicated multilayer structure containing two or more active layers, while at the same time implementing an effective charge collection or recombination in the intermediate layers connecting two adjacent sub-cells. In the case of OSC, we consider multi-junction cells where the same active material is used in all the junctions. This can be implemented by fabricating structures where the active layer thickness in each sub-cell does not exceed the 100 nm. For other types of thin film solar cells, we consider configurations using complementary absorbers. In both cases, but particularly in the former one, a systematic approach to optimize light absorption is needed. In order to obtain such optimal configurations, we implement an inverse integration approach combined with a transfer matrix calculation of the electric field. Furthermore, we develop several new approaches to optimize charge collection in the sub-cell interconnection layers which we apply to tandem, triple, 4-terminal and series-parallel configurations. The thesis has been organized into five chapters. Chapter 1 introduces concepts required for the development of the thesis work including the optical model. Chapter 2 describes the optical optimization and experimental implementation of current-matched multi-junction devices using PTB7:PC71BM, including applications. In order to profit from the advantage of electrically separated devices, Chapter 3 evaluates different types of 4-terminal architectures using PTB7:PC71BM and PTB7-Th:PC71BM. In one of the architectures we establish a serial-connection between sub-cells while in other we leave the sub-cells completely independent. Chapter 4 theoretically proposes a novel monolithic architecture combining perovskites and CIGS which does not require current-matching. Finally, in Chapter 5, an in-depth study of the semi-transparent inner electrodes is given that include vacuum-based and solution-processed layers., La fotovoltaica de capa delgada engloba un grupo de tecnologías capaces de capturar la luz en tan sólo unos pocos nanómetros de espesor. Su bajo costo de manufactura, flexibilidad y bajo peso, hace a las capas delgadas candidatas ideales para la integración en edificios. En particular, las celdas orgánicas pueden proveer una transparencia de alta calidad similar a las ventanas convencionales irrealizable con tecnologías basadas en Silicio. Sin embargo, para la producción de electricidad a gran escala en donde la eficiencia es, tal vez, el factor determinante, existen nuevas tecnologías como las celdas solares de perovskita que pueden resultar más adecuadas. Al momento de escribir esta tesis, las eficiencias de celdas de perovskita de simple unión casi duplican la de las mejores celdas orgánicas de simple unión. Una limitante de ambas tecnologías, en especial de las celdas orgánicas y en menor medida de las perovskitas, es la baja movilidad de las cargas. Esta, junto a otras desventajas de los absorbentes orgánicos y perovskitas limita su espesor al rango de los 100 a los 130 nm, y entre los 500 a 600 nm, respectivamente. En resumen, el manejo de la luz debe constituir un ingrediente esencial para el diseño de los dispositivos, tal que se consiga un desempeño óptimo en la aplicación para la cual sean considerados. En esta tesis, con el fin de alcanzar un aprovechamiento óptimo de la luz y por ende aumentar el desempeño de las celdas solares de capa delgada, utilizamos dos enfoques. Por un lado, aumentamos el espesor total de material absorbente dentro del dispositivo sin incrementar el espesor de las capas actives individuales y por otro lado, combinamos absorbentes complementarios para cubrir una porción más amplia del espectro solar. Estos enfoques conllevan al doble reto de encontrar la distribución de campo electromagnético óptima dentro de una estructura compleja de multicapas con dos o más capas activas, junto a la implementación de una recolección o recombinación de cargas efectiva por parte de las capas intermedias encargadas de conectar dos subceldas adyacentes. En el caso de las celdas orgánicas, consideramos celdas de multiunión usando el mismo material activo para todas las subceldas. Para implementarlas, se realizan estructuras cuyas capas activas no excedan los 100 nm. También estudiamos configuraciones donde los materiales tienen absorciones complementarias usando perovskitas. En ambos casos, sobretodo en el primero, se requiere un método sistemático para optimizar el aprovechamiento de la luz. Para obtener las configuraciones óptimas empleamos una estrategia de integración inversa junto con un cálculo del campo eléctrico basado en el modelo de matriz de transferencia. Además, desarrollamos nuevas estrategias para optimizar la colección de cargas en las capas de interconexión de las subceldas aplicables a dispositivos tipo tandem, triple, 4-terminales y serie-paralelo.

Organic polymeric solar cells require thin absorber layers due to intrinsic low exciton diffusion distances, thus showing low absorption of light. They are also characterized by having a low absorption peak, whose position lies in the near-infrared, slightly affecting the visible spectrum. By depositing thin silver layers of 10 nm for the back electrode, a semi-transparent organic solar cell can be created. Attractive outcomes could result from these devices, like solar cell windows that would remain visually transparent, while still collecting energy from the sun, or tandem polymeric solar cells. The formation of a non-homogeneous and discontinuous silver layer when depositing an ultra-thin film turns into a high resistivity electrode, therefore limiting the area that such devices can reach and the maximum power that the cell can offer. The goal of this bachelor thesis was to up-scale semi-transparent organic solar cells by characterizing the thin electrode conductivity and enhancing its charge collection using thick grid-like layers of silver. The cause for such high resistivity was confirmed by finding silver agglomerations of 30 nm in size, and a lower limit for its value was found by studying the silver bulk resistivity. The upscaling losses were determined in terms of the increase in area, and charge-collecting grids were consequently tested. The building of organic solar cells by spin-coating and vapour depositions was proved functional on top of non-flat substrates with roughness of the order of 100 nm. Fill Factor enhancements were found by designing optimal grid structures compatible with highly transparent cells.

Considering that the most abundant renewable energy source is the Sun, photovoltaic technology possesses one of the highest potentials to provide environmental benign and sustainable energy worldwide. Currently, most of commercial available modules are fabricated from crystalline silicon because of its high efficiency. To lower fabrication costs and increase the functionality of the solar modules, several thin film technologies are under development. Among them, organic photovoltaics has created large expectations provided it possesses some intrinsic advantages, such as light weight, flexibility or semi-transparency. However, the low charge mobility in the majority of the organic semiconductor materials prevents the use of active layers thicker than a few hundred nanometers. This leads to a limited light harvesting capacity and, consequently, a limited conversion efficiency. Different optical approaches have been considered to enhance the absorption of organic solar cells and increase their efficiency. In this thesis, we propose a novel configuration based on the use of fiber arrays to effectively trap light and efficiently couple it into the active layer to enhance absorption. The thesis work is presented in five chapters. After an introductory chapter, in chapter 2 light absorption of an organic solar cell deposited on the backside of a fiber array is studied theoretically. A strong enhancement in light harvesting is predicted using such configuration. For small diameter fibers the enhancements originated from light coupling to some low quality whispering gallery modes, while for large diameter fibers light seemed to be effectively trapped inside the fiber structure. In chapter 3 and 4, we consider the dip-coating procedure, a fabrication technique that can be applied to deposit from a precursor solution, layers on a substrate irrespective of its shape. Its viability is demonstrated by applying it to different device architectures. The deposition on such non-flat substrates of the rest of the layers forming an organic solar cell is also examined. For instance, several relevant changes that had to be introduced to the ITO sputtering to obtain transparent electrodes with an optimal quality, both, optically and electrically, are discussed. Once the layer deposition is optimized to fulfill the electrical and optical requirements of organic solar cells, in chapter 5, we experimentally demonstrate that an enhanced light absorption can be achieved from such organic solar cells when deposited on fiber arrays. Optical fibers of 80 µm in diameter were used to fabricate the arrays to be used as the cell substrate. Such substrates were coated with an organic solar cell of evaporated small molecules. The implemented fiber array configuration is seen to be an effective light trapping method. Indeed, the photogenerated current from such devices is shown to increase by a 26%, which is a considerable percentage when compared to the majority of the optical approaches that were considered in the past to enhance absorption in organic cells., Si considerem que la font d'energia renovable més a abundant és el Sol, la tecnologia fotovoltaica posseeix un dels potencials més alts per poder produir l'energia mundial de forma sostenible i benigne amb el medi ambient. Actualment la majoria dels mòduls comercials estan fabricats de silici cristal.lí ja que aquest material té una gran eficiència. Per tal de rebaixar els costos de producció i incrementar la funcionalitat d'aquest panells solars, diverses tecnologies de capa prima s'estan desenvolupant. Entre elles, la tecnologia fotovoltaica amb materials orgànics ha creat grans expectatives gràcies a les seves propietats intrínseques, com per exemple la seva lleugeresa, flexibilitat o bé semi transparència. Per altra banda, la baixa mobilitat de les càrregues en la majoria dels semiconductors orgànics impedeix l'ús de capes actives no molt més gruixudes que uns pocs nanòmetres. Això provoca que tinguin una capacitat de col.lecció lumínica limitada i com a conseqüència, la eficiència de conversió energètica també ho és. S'han considerat diferents estratègies òptiques per tal de millorar l'absorció en les cel.les solars orgàniques i incrementar la seva eficiència. En aquesta tesi proposem una configuració innovadora basada en l'ús d'una matriu de fibres, les quals atrapen i acoblen la llum en la capa activa per millorar l'absorció d'aquesta. La present tesi consta de cinc capítols. Després d'un capítol introductori, en el capítol 2 s'estudia teòricament l'absorció de llum d'una cel·la solar orgànica dipositada en la part posterior de la matriu de fibres. Per diàmetres de fibra petits, les millores són degudes a l'acoblament d'uns modes recirculants de llum de baixa qualitat. Mentre que per diàmetres grans, la llum sembla estar atrapada de forma efectiva dins de l'estructura formada per les fibres. En els capítols 3 i 4 hem considerat el recobriment per immersió, aquesta tècnica de fabricació pot ser aplicada per dipositar capes des d'una solució precursora a un substrat, independentment de la seva forma. La validesa de la tècnica es demostra quan s'aplica a diferents arquitectures de cel·la. El dipòsit en aquestes estructures no planes de la resta de les capes que formen la cel·la solar orgànica també s'ha investigat. Per exemple, es discuteixen varis canvis rellevants, els quals s'han tingut que introduir per la polvorització catòdica del ITO, per tal d'obtenir elèctrodes transparents amb unes qualitats òptiques i elèctriques òptimes.

Disordered media with refractive index variations can be found in the atmosphere, the ocean, and in many materials or biological tissues. Several technologies that make use of such random media, as image formation, satellite communication, astronomy or microscopy, must deal with an unavoidable light scattering or diffusion. This is why for many years light propagation through random media has been a subject of intensive study. Interesting phenomena such as speckle, coherent backscattering or random lasing have been discovered and studied. More recently, researchers are beginning to investigate mechanisms to control light propagation through such media to enhance light transmission and sharpen the focus. On the other hand, it has been known for several years that nonlinear random structures are able to generate light in an ultra-broad frequency range, without the need of angle or temperature tuning. Particularly interesting is the nonlinear light diffusion observed from materials with no change in the refractive index and which appear to be fully diffusion less to linear light propagation. However, a comprehensive understanding of the scattering when a nonlinear interaction takes place has not yet been given. The core of the thesis focuses on the study of the nonlinear light generation and propagation from crystalline structures with disordered nonlinear domains but with a homogenous refractive index. A random distribution of non-linear domains is found naturally in the Strontium Barium Niobate (SBN) ferroelectric crystal. As opposed to other mono-domain nonlinear optical crystals commonly used for frequency up-conversion, such as Potassium Titanyl Phosphate (KTP) or Lithium Niobate (LiNbO3), in SBN the nonlinear domain size is, typically, on the order of the coherence length or many times smaller than the size of the whole crystal. Such domains are usually several times longer in the c-axis direction relative to the plane perpendicular to that axis. Adjacent domains exhibit antiparallel polarization along such crystalline axis, with no change in refractive index. In Chapter 1 we give a brief introduction to light generation and propagation in random media, describing the speckle, light manipulation and second harmonic generation (SHG). In chapter 2, we study the nonlinear light generation and manipulation from a transparent SBN crystal. In its theoretical description we use a two-dimensional random structure consisting of a homogeneous background polarized in one direction with uniform rectangular boundaries, and a group of square reverse polarization domains with random sizes and located in random positions. The SHG from each domain is obtained using the Green's function formalism. In the experiments, we alter the ferroelectric domain structure of the SBN crystal by electric field poling or thermal treatments at different temperatures. The SBN crystal structures after such different treatments are shown to be characterized by their SHG patterns. In chapter 3, by measuring the spatial distribution of the second harmonic light in the c-plane, we demonstrate that the randomness in the nonlinear susceptibility results in a speckle pattern. We explain the observations as a result of the linear interference among the second harmonic waves generated in all directions by each of the nonlinear domains. In chapter 4, we report on our experimental implementation of the wave-front phase modulation method to control and focus the SHG speckle from the random SBN crystal. This research creates a bridge between light phase modulation and nonlinear optics. Finally we perform a theoretical analysis to demonstrate enhanced efficiencies for nonlinear light focusing by the wave-front phase modulation method in different directions. Various types of nonlinear structures are considered, including the homogeneous rectangular crystal, the group of random domains, and the combination of both.

In recent years, it has been proposed that circular microresonators may become an important element in the core of many photonic devices. The high Q-factors seen in fused silica micro-spheres and micro-toroids for light coupled in the whispering gallery modes (WGMs) inside the micro-resonator led to many new developments in a diversity of fields. Indeed, WGM micro-resonators have found applications in laser oscillation, optical filtering, bio and chemical sensing, frequency stabilization, quantum electrodynamics experiments, nonlinear parametric conversion and in many other light-matter interaction processes where light recirculation is an essential ingredient. For second and third order nonlinear optical phenomena a high-Q micro circular cavity is an ideal framework to lower the light intensity or material density and still obtain a measurable interaction. This may become particularly useful when the nonlinear interaction is considered on the sphere surface because at an interface centro-symmetry is always broken. In this thesis, we approach the problem of obtaining SHG with the smallest amount of material possible. Our goal is to demonstrate that WGMs in micro-sphere resonators are an optimal option to consider such type of non-linear interaction. SHG from a small amount of material may found interesting applications in high sensitivity unmarked detection of low numbers of very small objects such as molecules, viruses or other types of nano-particles. The different experimental and theoretical developments we implemented to achieve such goal are reported in the four chapters of the current thesis. In chapter 1 we introduce basic concepts of spherical micro-resonators an their interest. Theoretical aspects of light propagation and nonlinear light generation in the whispering gallery modes in such micro-resonators are discussed in Chapter 2. A new method to obtain patterns of non-linear material is presented in Chapter 3. In Chapter 4, the developments presented in the previous chapters are combined to obtain second harmonic generation in the whispering gallery modes of microspheres. In this chapter we report the design and fabrication of a nonlinear spherical resonator to experimentally measure SHG from molecules deposited on its surface. Such nonlinear interaction is quasi-phase matched by implementing the periodical patterning reported in Chapter 3 on a molecular layer deposited on the surface of a micro-sphere. By coupling laser light pulses at the fundamental frequency into the whispering gallery modes of the high-Q spherical micro-resonators we demonstrate that a signal at the second harmonic (SH) frequency can be measured when less than 100 molecules contribute in the nonlinear interaction. Finally, applications of such type of generation in highly sensitive sensing are discussed., En años recientes los microresonadores circulares han sido propuestos como un elemento central para formar parte de muchos dispositivos fotónicos. El alto factor de calidad observado en microesferas o microtoroides de sílice cuando la luz se propaga en modos "whispering gallery" (WG) ha dado lugar a un gran número de nuevos desarrollos en campos muy diversos. En efecto, los micro resonadores con modos WG han encontrado aplicación en la oscilación laser, en el filtrado óptico, en sensores bioquímicos, como estabilizadores de frecuencia, en experimentos de electrodinámica cuántica, en la conversión paramétrica no lineal y en muchas otros procesos donde la recirculación de luz es un ingrediente esencial para su interacción con la materia. En fenómenos ópticos no lineales de segundo y tercer orden, la micro cavidad circular con un alto factor de calidad constituye una estructura ideal para poder obtener una interacción medible incluso cuando se consideran pequeñas intensidades de luz o bajas densidades de materia. Esto puede resultar particularmente útil en la superficie de la microesfera ya que en la interface entre dos materiales se rompe la simetría de inversión incluso cuando los materiales son centro simétricos. En esta tesis abordamos la generación de segundo armónico con una cantidad mínima de material. Nuestra meta es demostrar que los modos WG en resonadores de microesfera son una opción óptima para poder considerar este tipo de interacción no lineal. La generación de segundo armónico con una cantidad muy pequeña de material puede encontrar aplicaciones interesantes en la detección de muy pocos objetos pequeños tales como moléculas, viruses o cualquier otro tipo de nanopartículas. Los diferentes desarrollos experimentales y teóricos que implementamos para alcanzar nuestro objetivo están explicados en los cuatro capítulos de esta tesis. En el Capítulo 1 introducimos conceptos básicos de microresonadores esféricos y su interés. Aspectos teóricos de la propagación y generación no lineal de luz de los modos WG en dichos resonadores se discuten en el Capítulo 2. Un método nuevo para generar patrones de material no lineal se presenta en el Capítulo 3. En el Capítulo 4, los desarrollos de los capítulos previos presentados se combinan para implementar la generación de segundo armónico en los modos WG de las microesferas. En este capítulo reportamos el diseño y la fabricación del resonador esférico no lineal para llegar a medir experimentalmente la generación de segundo armónico de las moléculas depositadas en su superficie. Dicha interacción no lineal se obtiene en la configuración "quasi-phase matched" implementando el mecanismo de escritura de patrones reportado en el Capitulo 3, sobre una capa molecular depositada en la superficie de la microesfera. Mediante el acoplamiento de pulsos de luz láser a la frecuencia fundamental en los modos WG de un microresonador esférico con un alto factor de calidad Q, demostramos que la señal a la frecuencia de segundo armónico puede ser medida, menos de 100 moléculas contribuyen a esta interacción no lineal. Finalmente, se discuten aplicaciones de ese tipo de generación para la detección altamente sensible.

Organic photovoltaic (OPV) technology has emerged as a potential cost-effective solution to produce electrical energy. The foreseen low manufacturing costs combined with features as semi-transparency or mechanical flexibility give to OPV devices a strong potential for industrial applicability. However, the commercial implementation of this technology faces the challenge of increasing the relatively low power conversion efficiency of the current state-of-the-art OPV devices. This thesis presents an optical based approach to enhance the performance of OPV devices by effectively controlling sunlight photons. Such control is possible because of the coherent interaction between light and the multilayered structure constituting the OPV device. Accordingly, we studied the dependence of the optical field distribution inside the solar cell relative to the optical properties of the different layers including their refractive index , extinction coefficient , and thickness. This optical study led to the prediction of optimal OPV device structures. The first implementation of a photon control was done by changing the relative thicknesses of the different layers in the device. An optimal combination of thicknesses was found and confirmed experimentally. A significant reduction of the energy lost in the device was demonstrated. As a consequence, the photon harvesting improved, which led to a close matching between the external and internal quantum efficiencies in a broad wavelength range. A second photon control strategy to enhance the performance of OPV cells was implemented by modifying the complex refractive index of the nonactive device layers. Both and were changed in specific layers by considering new materials. Three different cases were considered: in the first example a BCP layer was used to replace calcium as electron transporting layer. The parasitic absorption induced by the highly absorptive calcium layer was diminished almost to zero after replacing this layer with BCP, a material whose extinction coefficient is null for a broad wavelength range. A 19% performance enhancement was demonstrated. In the second example, an ultrathin nickel oxide layer was used to replace the commonly used PEDOT layer as hole transporting layer. Very thin layers of nickel oxide could be used for a better photon distribution and harvesting in the photoactive layer. In the last case, a metallic cupper/nickel semi-transparent electrode was used to replace an ITO electrode. This new metallic electrode in combination with the back aluminum electrode enabled the formation of an optical cavity which resulted in a stronger localization of the field in the active layer. Finally, several of the concepts considered above to effectively localize the field in the active layer were used in conjunction with a photonic structure integrated in the OPV architecture to achieve an optically optimized semi-transparent OPV device. In particular, a one-dimensional non-periodic photonic crystal was designed and added to a semi-transparent OPV device in order to re-harvest UV and IR photons while keeping a high transmission for the visible photons. A power conversion efficiency enhancement larger than 56% was achieved while maintaining the device luminosity around 30%. An additional feature of the integration of such photonic crystal was the possibility of tuning the color transmitted by the device which was also demonstrated. In summary, in this thesis we demonstrate experimentally and theoretically that optics plays a very relevant role for enhancing the power conversion efficiency of OPV devices. The methods presented are perfectly compatible with a more oriented material science approach to achieve the final objective of obtaining a performance-competitive OPV technology., La tecnología fotovoltaica orgánica (OPV) ha surgido como una solución potencial rentable para producir energía eléctrica. Los bajos costos de manufactura previstos combinados con propiedades como semi-transparencia o flexibilidad mecánica le dan a los dispositivos OPV un gran potencial de ser aplicados industrialmente. Sin embargo, la implementación comercial de esta tecnología se enfrenta al reto de incrementar la relativamente baja eficiencia de los dispositivos OPV del estado del arte. Esta tesis presenta una aproximación óptica para aumentar la eficiencia de los dispositivos OPV mediante un control efectivo de los fotones de la radiación solar. Tal control es posible debido a la interacción coherente entre la luz y la estructura de multi-capas que constituye el dispositivo OPV. Consecuentemente, en esta tesis se estudia la dependencia de la distribución del campo óptico dentro de la celda solar con las propiedades ópticas de las diferentes capas. Entre esas propiedades se incluyen el índice de refracción , el coeficiente de extinción y espesor de cada una de las capas. Este estudio óptico ha permitido predecir estructuras óptimas para los dispositivos OPV. La primera implementación del control de fotones fue hecha al cambiar los espesores relativos de las diferentes capas en el dispositivo. Una combinación óptima fue encontrada y confirmada experimentalmente. Una reducción significativa de la energía perdida por reflexión especular fue demostrada y como consecuencia, la recolección de fotones fue mejorada lo cual condujo a la concordancia entre las eficiencias cuánticas externa e internas en un amplio rango de longitudes de onda. Una segunda estrategia de control de fotones para mejorar el desempeño de los dispositivos OPV fue implementada tras modificar las propiedades ópticas de las capas en el dispositivo distintas a la capa activa. Tanto como fueron cambiados en capas específicas tras considerar nuevos materiales. Tres casos diferentes fueron considerados: en el primer caso, una capa de BCP fue usada para reemplazar el calcio como capa transportadora de electrones. La absorción parásita inducida por el elvevado coeficiente de extinción de la capa de calcio fue reducida casi hasta cero tras reemplazar esta capa con una de BCP, un material cuyo coeficiente de absorción es prácticamente cero para un amplio rango de longitudes de onda. Se demostró un aumento en el desempeño de los dispositivos de hasta el 19%. En el segundo ejemplo, una capa ultra-delgada de óxido de níquel fue usada para reemplazar la comúnmente empelada capa de PEDOT como capa transportadora de huecos. Estas capas de óxido de níquel permitieron una mejor distribución y recolección de fotones en la capa foto-activa. En el último caso, un electrodo semi-transparente hecho de cobre/níquel fue usado para reemplazar un electrodo de ITO. Este nuevo electrodo metálico en combinación con el electrodo de aluminio posterior del dispositivo permitió la formación de una cavidad óptica la cual resultó en una mayor localización del campo en la capa activa. Finalmente, varios de los conceptos considerados anteriormente para localizar efectivamente el campo en la capa activa fueron usados en combinación con una estructura fotónica integrada en la estructura para obtener un dispositivo OPV semitransparente ópticamente optimizado. Concretamente, un cristal fotónico unodimensional no-periódico fue diseñado y añadido al dispositivo OPV semi-trasparente con la intención de recolectar fotones UV e IR y al tiempo manteniendo una alta transmisión de los fotones visibles. Una mejora en el desempeño de los dispositivos superior al 56% fue obtenida preservando la luminosidad del dispositivo alrededor del 30%. Una propiedad adicional aportada por la integración de tales cristales fotónicos fue la posibilidad de modular el color transmitido por el dispositivo lo cual fue también demostrado. En síntesis, en esta tesis se demostró experimental y teóricamente que la óptica juega un papel relevante para aumentar la eficiencia de los dispositivos OPV. Los métodos presentados son perfectamente compatibles con la aproximación que se realiza desde la perspectiva de la ciencia de los materiales al objetivo final de obtener una tecnología OPV competitiva.

In this century some of our main issues are energy shortage and pollution. This work will briefly describe these problems, proposing a plan of action combining energy saving and different sustainable energy sources. Within different types of renewable energy sources, solar energy is the most abundant one. To make solar energy a more sustainable and cost effective technology we focus on enhancing the optical characteristics of thin film solar cells. In this category, organic solar cells are good options for their exiguous amount of material and the low energy needed for the fabrication process. This technology can be lightweight, transparent, flexible and conformal in order to be applied to and integrated in various architectural solutions and consumer electronics. After a study of the physics of such devices and on how to optically enhance their performances, we will show some examples where we theoretically and experimentally collect the solar radiation with optical antennas. We report, for the first time in literature, a nanogap antenna that efficiently couples the light in our active material thin film. Finally, we elaborate on the concept of building integrated photovoltaics introducing some examples of solar façades. Based on our research, we are able to design and fabricate an organic transparent solar cell with a visible transparency above 20% and an optically enhanced photon-electron conversion efficiency remarkably similar to its opaque equivalent., En el presente siglo, algunas de las prioridades son la escasez de la energía y la contaminación. Este trabajo describirá brevemente estos problemas y propondrá un plan de acción que combina el ahorro energético con diferentes fuentes sostenibles de energía. Dentro de estas fuentes de energía renovables, la energía solar es la más abundante. Con el objetivo de hacer la tecnología solar más sostenible y eficiente económicamente nos concentramos en aumentar las características ópticas en celdas solares de película delgada. Dentro de esta categoría, las celdas solares orgánicas son una buena opción porque su desarrollo requiere bajas cantidades de materiales y su fabricación es de baja energía embebida. Adicionalmente, esta tecnología puede ser liviana, transparente, flexible mecánicamente y modular para ser aplicada e integrada en varias soluciones arquitectónicas y de electrónica de consumo. Luego de estudiar los procesos físicos en tales dispositivos y de determinar las metodologías para aumentar ópticamente sus desempeños, mostraremos algunos ejemplos donde teórica y experimentalmente se colecta la radiación solar mediante antenas ópticas. Se reporta por primera vez, una antena de nanogap que acopla eficientemente la luz en la capa activa de la celda solar. Finalmente, se desarrolla el concepto de tecnología fotovoltaica integrada en edificaciones tras introducir algunos ejemplos de fachadas solares. Basados en nuestra investigación, fue posible diseñar y fabricar una celda solar orgánica transparente cuya transparencia en el rango visible estuvo por encima del 20% y una eficiencia de conversión foton-electron aumentada ópticamente que resulto notoriamente similar a la celda solar orgánica opaca equivalente., La rareté grandissante des ressources en énergie associée à une augmentation de la pollution font partie des enjeux plus importants de ce siècle. Cette thèse décrira brièvement ces deux problématiques et proposera un plan d’action combinant économie d’énergie et diversité des sources d’énergies renouvelables. Parmi les formes d’énergies renouvelables disponibles, l’énergie solaire est la plus abondante. Pour faire de l’énergie solaire une ressource plus durable et plus rentable économiquement, nous proposons d’amplifier les propriétés optiques de cellules solaires en couches minces. Dans cette catégorie, les cellules solaires organiques représentent un choix pertinent de part la faible quantité de matériau nécessaire ainsi que la faible énergie nécessaire au procédé de fabrication. Cette technologie peut être légère, transparente et flexible de sorte qu’elle peut être utilisée dans différentes solutions architecturales s’adaptant à des produits électroniques pour le grand publique. Suivra la théorie sous jacente à ces dispositifs et l’explication de la manière dont leurs performances sont améliorées. Nous présenterons quelques exemples où l’on collecte la radiation solaire avec une antenne optique. Ainsi, nous faisons la toute première démonstration d’une antenne auto-assemblée qui couple efficacement la lumière dans le matériau constituant la couche mince que nous utilisons. Finalement, nous développons le concept de cellules photovoltaïques intégrées en présentant différents cas de façades solaires. Ces travaux nous ont permis de concevoir et de fabriquer une cellule solaire organique transparente avec une transparence dans le visible de 20% et une efficacité de conversion photon-électron améliorée, similaire à une cellule équivalente opaque., La difficile reperibilità di risorse energetiche e l’inquinamento sono alcuni dei problemi più importanti di questo secolo. In questo lavoro saranno presentati brevemente questi temi proponendo un piano d’azione che abbini il risparmio energetico alle differenti fonti di energia rinnovabili. Nell’insieme delle fonti energetiche rinnovabili l’energia solare è senz’altro la più abbondante. Con l’obbiettivo di rendere lo sfruttamento di tale energia più sostenibile ed economicamente vantaggioso, ci premuriamo di migliorare le caratteristiche ottiche di celle fotovoltaiche a film sottile. In questa categoria utilizziamo, tra le diverse opzioni, le celle solari organiche in quanto la loro fabbricazione richiede una quantità di materiale minimo e un basso consumo energetico. Inoltre questi tipi di dispositivi possono essere leggeri, trasparenti, flessibili e conformabili alle superfici su cui sono applicati. Questa è una tecnologia che potrebbe essere implementata e integrata in varie soluzioni architettoniche o nell’ elettronica di consumo. Dopo aver presentato i principi fisici di tali dispositivi e determinato le metodologie ottiche per aumentarne le prestazioni, vengono illustrati alcuni esempi dove, teoricamente e sperimentalmente, riusciamo a intercettare la radiazione solare con antenne ottiche. Riportiamo, per la prima volta in letteratura, un’antenna ottica con nano-gap che accoppia efficacemente la luce solare nel nostro materiale attivo a film sottile. Nell’ultima parte sviluppiamo il concetto di tecnologia solare integrata negli edifici, introducendo alcuni esempi di facciate solari. Basando il design sulla nostra ricerca, è possibile realizzare una cella solare fotovoltaica organica trasparente, con una trasparenza superiore del 20% e un’ efficienza di conversione fotone-elettrone migliorata grazie all’ottica, che risulta molto vicina all’ equivalente cella fotovoltaica organica non trasparente.

Projecte fet en col.laboració amb l'Institut de Ciències Fotòniques Joining organic molecules and metallic nanoparticles into coupled heterostruc- tures allows tuning their optical properties and o®ers novel possibilities for manipulating their emission patterns. This Master Thesis is aimed at probing and understanding the nature of the optical coupling between plasmonic nanoparticles and organic molecules. We carry out second harmonic spectroscopy and interferometry experiments on thin flms of the nonlinear dye crystal violet deposited on glass and on top of silver nanoparticles to study the interplay between the resonance of the dye and the plasmon modes of the nanoparticles. We find a small redshift of second harmonic emission from the nanoparticles, while the molecular second harmonic resonance displays a large blueshift of tens of nanometers in the vicinity of the nanoparticles. Across both resonances a characteristic shift of ¼ in the phase of the generated second harmonic light is observed by second harmonic interferometry. To explain our ¯nd- ings, a phenomenological model with two coupled damped anharmonic oscillators is proposed. With the model, we are able to reproduce the main features of the experimental data.

Els materials nanoestructats periòdics han ofert, en les dues darreres dècades, un nou marc per a l'estudi de la interacció entre la radiació electromagnètica i la matèria. Aquestes estructures permeten modelar les propietats electromagnètiques dels materials i han esdevingut una eina idònia per confinar, guiar, suprimir, localitzar, dividir, dispersar, i filtrar la llum. L'abast del control de radiació electromagnètica va des de la propagació fins a la generació de la llum. Els cristalls fotònics han demostrat ser eficients per suprimir o afavorir mecanismes de generació de llum com l'emissió espontània o els processos no lineals.L'eix central d'aquesta tesi se centra en investigar els efectes fintis i fins a quin punt les propietats d'estructures ideals infinites o infinitament periòdiques es mantenen per a estructures que tenen un caràcter finit. Fins fa poc, els desenvolupaments tant experimentals com teòrics en el camp de cristalls fotònics es basaven, principalment, en càlculs que consideraven estructures ideals amb condicions de contorn perfectament periòdiques. Des dels inicis del camp, però, es van observar desajustos a les prediccions fetes amb aquestes condicions. Tanmateix, alguns d'ells resten, en gran part, inexplicats. En aquest el treball, tractem alguns d'aquests aspectes relacionats amb la propagació i generació del llum en cristalls fotònics finits reals, és a dir, com els que es fabriquen. Amb aquest propòsit, en realitzem un estudi tant teòric com experimental. Estudiem els efectes fintis tant en la regió de la primera banda de reflexió de Bragg com en el rang d'energies altes, on la longitud d'ona de la llum és de l'ordre o més petita que el paràmetre de xarxa.En concret, part del treball es dedica a l'estudi dels cristalls col·loïdals en el rang d'energies baixes. Desenvolupem un model vectorial 3D en l'aproximació de Rayleigh-Gans per simular estructures amb contrasts d'índexs baixos. Aquest model contempla aspectes rellevants dels cristalls reals com són ara les condicions de contorn, inclou una lleugera dispersió en el diàmetre de les esferes com també una absorció eficaç que descriu la difusió de Rayleigh i la dispersió inelàstica causada per la presència d'imperfeccions. Aquest model s'utilitza per estudiar la propagació dins de nanoestructures fotòniques reals, i per determinar-ne les propietats dispersives. Les prediccions del model es contrasten amb mesures experimentals dependents de la polarització de l'estructura de bandes parcial d'un cristall col·loïdal real. També determinem el temps de confinament del fotons, a l'extrem de la primera banda fotònica prohibida, per mitjà de la deformació d'un pols provocada per canvis en la velocitat de grup que acompanya l'atrapament dels fotons.Per explicar la propagació de llum en el rang d'energies altes, utilitzem el model vectorial KKR que ens permet determinar la velocitat de grup d'òpals artificials prims. Trobem que, per determinades freqüències, la velocitat de grup pot ser superlumínica, positiva, negativa o tendir a zero depenent del guix del cristall i la seva absorció. Aquest comportament es pot atribuir al carácter finit de l'estructura i explica observacions experimentals presents a la literatura. La mateixa propagació amb velocitats de grup anòmales pot explicar l'observació experimental de l'augment de la generació de segon harmònic en un òpal prim no lineal. Confirmant així que la disminució de velocitat de grup proporciona un mecanisme que afavoreix els processos no lineals.En darrer lloc, considerem una altra configuració en què la interacció no lineal quadràtica té lloc en una capa de menys d'una longitud d'ona de gruix. Demostrem que, en presència d'una superfície reflectora, la contribució de termes que no conserven el moment lineal de la llum, i que no tindrien cap contribució en un medi infinitament llarg, són els més determinants., Photonic periodic nanostructures have offered, in the last two decades, a new framework for the study of the interaction between electromagnetic radiation and matter. Such structures can engineer the electromagnetic properties of materials and have become a powerful tool used to confine, route, suppress, localize, split, disperse, and filter light. The scope of the electromagnetic radiation control can be extended to light propagation and generation. Photonic crystals have successfully been used as host materials to suppress or enhance light generation mechanisms such as spontaneous emission or nonlinear processes.The aim of this thesis is to investigate finite-size effects and to what extent the properties of ideal infinite or infinitely periodic structures hold for structures that are finite in size. Until recently, experimental as well as theoretical developments in the field of photonic crystals have been based, mostly, on calculations that consider ideal structures with perfectly periodic boundary conditions.Deviations from the behavior predicted form such assumptions were already observed when the field was born. However, some of them remained, for the most part, unexplained. In the present work, we tackle some of these aspects related to light propagation and generation in the real finite photonic crystals that can be fabricated. With this purpose, we perform such study from, both, an experimental as well as a theoretical perspective. We study finite-size effects in the region of the first order Bragg reflection band as well as in the high energy range where the wavelength of light is on the order or smaller than the lattice parameter.To be more specific, part of the work is devoted to the study of colloidal crystals at the range of low energy. We develop 3D full wave vector calculation in the Rayleigh-Gans approximation to simulate low index contrast structures. This model accounts for relevant real crystal's aspects such as boundary conditions, a slight dispersion in the spheres diameter and includes an effective absorption accounting for Rayleigh scattering and inelastic diffusion due to imperfections. This model is used to study light propagation within real photonic nanostructures, and to determine their dispersive properties. The predictions of the model are contrasted with experimental polarization dependent measurements of the partial band structure of an actual colloidal crystal. We also determine the experimental photon's lifetime, at the edge of the first order pseudogap, by means of the pulse reshaping induced by changes in the group velocity accompanied by the photon trapping.To explain light propagation in the high energy range, we use a vector KKR calculation that we apply to understand the group velocity of light propagating in artificial opals slabs. We show that for certain frequencies, the group velocity can either be superluminal, positive or negative or approach zero depending on the crystal size and absorption. Such behavior can be attributed to the finite character of the structure and accounts for previously reported experimental observations. The same propagation at anomalous group velocity may explain the experimental observation of second harmonic generation enhancement of light from a nonlinear opal film. Indeed, the group velocity slowing-down provides an enhancement mechanism for nonlinear processes.We finally consider another configuration such that the quadratic nonlinear interaction occurs within a sub-wavelength layer. In the presence of a nearby reflecting surface we demonstrate that the contribution of terms that do not conserve light momentum, and that would vanish in an infinitely long medium, is the most relevant one.

Les interaccions no lineals de segon ordre són, probablement, uns dels processos de l'òptica no lineal més utilitzats i rellevants en quant a les seves aplicacions. Aquestes interaccions només són eficients en materials que presenten una simetria d'inversió i que permeten aconseguir-hi un mecanisme de phase matching. Això succeeix, en general, quan s'utilitzen materials amb un alt grau d'anisotropia, fet que imposa limitacions importants en les seves aplicacions. Des de fa temps s'utilitzen una gran varietat de cristalls inorgànics, com el LN o el KTP, en dispositius òptics comercials. Tanmateix aquests materials inorgànics tenen alguns inconvenients o limitacions com ara el cost, dificultats en el processat i poca flexibilitat per modificar-los i incorpora'ls-hi d'altres propietats. En aquest sentit les molècules orgàniques poden aportar solucions, però la dificultat d'assolir cristalls orgànics no centrosimètrics prou grans com per assolir eficiències semblants pel que fa als processos no lineals, n'ha limitat la seva aplicabilitat. Donada l'elevada no linealitat que s'obté amb algunes molècules orgàniques, les interaccions no lineals de superfície, són una de les possibilitats per aquest tipus de materials. Si bé l'eficiència d'un procés de superfície no és elevada, quan un és capaç de sumar coherentment els efectes d'un gran nombre de processos superficials, la interacció resultant pot ser eficient.Els cristalls fotònics són materials nanoestructurats amb la capacitat d'exercir un control ampli sobre la generació i propagació de la llum. Aprofitant els efectes en la propagació de la llum que es donen al llindar de les bandes prohibides, és possible exercir aquest control sobre les interaccions no lineals.Això, juntament amb el fet de que en un cristall fotònic hi ha un gran nombre de interfícies on dur a terme les interaccions no lineals, fa que sigui de gran interès realitzar un estudi exhaustiu de diferents interaccions paramètriques no lineals que s'hi poden considerar.En aquesta tesi es presenten estudis experimentals i teòrics sobre diferents interaccions no lineals considerades en el si de cristalls fotònics col·loïdals i d'òpals. En el cas dels cristalls col·loïdals, el treball es centra, majoritàriament, en l'estudi de les interaccions no lineals de segon ordre. Es demostra, que aquests processos que es poden aconseguir en el si d'una estructura centrosimètrica, són de superfície. Pel que fa als òpals, l'interès està centrat en conèixer com els efectes que aquests materials tenen sobre la velocitat de grup, poden aprofitar-se per incrementar l'eficiència de les interaccions no lineals.Fent ús de tècniques de síntesi en fase sòlida, s'ha pogut enllaçar un gran nombre de molècules no lineals a la superfície de nanoesferes de poliestirè. Aquestes esferes de làtex tenen la capacitat d'autoordenar-se en una xarxa cristal·lina centrosimètrica. Es demostra experimentalment que, gràcies a poder dur a terme una interacció no lineal de superfície en un material amb propietats de cristall fotònic, es poden assolir unes eficiències, 6 ordres de magnitud superiors a les assolides fins ara.Aquest treball comença amb una introducció, dels aspectes més rellevants dels cristalls fotònics i de l'òptica no lineal de segon ordre. Al capítol II es presenten les interaccions no lineals de segon ordre en cristalls col·loïdals. S'explica com es fabriquen aquestes estructures no lineals i es demostra experimentalment que la generació de segon harmònic en un cristall col·loïdal és un procés de superfície.Al capítol III de la tesi s'estudien la suma de freqüències contrapropagants i la generació de tercer harmònic en cristalls col·loïdals. Al capítol IV s'estudia, experimentalment, com es poden aprofitar les anomalies que apareixen en la velocitat de grup, quan la llum s'acobla a les bandes altes d'un òpal, per tal d'incrementar la generació de segon harmònic en aquestes estructures. Finalment, es presenten les conclusions del treball., Second order nonlinear interactions are, among, the most relevant nonlinear interactions between light and matter when one considers their applications. Such interactions are only efficient in noncentrosymmetric materials and materials or material structures that provide a phase matching mechanism. This is the case, for instance, in highly anisotropic crystals. However such anisotropy sets important limitations to the application scope of these materials.In the last decades, a large variety of inorganic crystals, such as, for instance, LN or KTP, have been used in optics devices. However, these inorganic materials have several drawbacks like their cost, processing difficulties and limitations to their flexibility and capability to hold new properties. Organic molecules may provide some alternatives, but the difficulties in getting a noncentrosymmetric organic crystal, large enough to hold an efficient nonlinear interaction, has restricted their applicability. Because the high nonlinearity of some organic molecules, one may consider surface nonlinear interaction as a good nonlinear mechanism for these molecules.Although the efficiency of surface interaction is low, when many of this surfaces interactions are coherently added, the whole process can be efficient. Photonic crystals have the capability of controlling the propagation and generation of light. Such control is larger in the neighbourhood of a forbidden band. In fact, at the edge of the band it is possible to control the nonlinear interactions. The high number of interfaces present in the photonic crystal structure, where a quadratic nonlinear interaction may occur, and the band edge effects, make it interesting to focus our study into some of such second order nonlinear interaction.In this thesis, we present experimental and theoretical results related to different second order nonlinear interactions in the framework of nonlinear colloidal photonic crystals, and nonlinear opals. For the colloidal crystals we mostly consider second order nonlinear processes, and the surface origin of these interactions is demonstrated. In the case of opals we focus our work on the effects that the group velocity anomalies present in the high bands of the photonic crystals, and show how we can take advantage of them for a nonlinear interaction enhancement.Using solid face methods, we have been able to covalently link a large amount of nonlinear organic molecules to the surfaces of polystyrene nanospheres. These latex spheres have the capability to self organise in a centrosymmetric lattice. We experimentally demonstrate that, given the photonic crystal properties of this material and the possibility of holding surface nonlinear interactions in the interfaces of the nanospheres, efficiencies up to 6 orders of magnitude larger than the ones obtained in the past, can be achieved.An introduction to relevant aspects of photonic crystals and nonlinear optics can be found in chapter I. In chapter II second order nonlinear interactions in photonic crystals are described.We explain how to fabricate these colloidal nonlinear crystals, and then experimentally demonstrate that second harmonic generation in the framework of colloidal photonic crystals is a surface phenomenon. In chapter III, counter-propagating sum frequency generation and third harmonic generation are discussed. In chapter IV, we experimentally demonstrate that, using an opal made of nonlinear polystyrene spheres, the enhancement of second harmonic generation is possible if one takes advantage of the group velocity anomalies presents on the edges of flat bands that are opened at higher frequencies. The main conclusions of the work are summarized in the last chapter.

Photonic crystals emerged at the end of the last decade as a new frame to control the interaction between radiation and matter. The potential advances that such structures could report in photonics technology has lead to an increasing research focused on the implementation of photonic crystals possessing full photonic band gaps, hindering the fact that more simple structures, possessing band gaps in selected directions of space, may also provide strong control of the electromagnetic radiation leading to the observation of many new interesting phenomena. In fact, the scope of this control is not limited to a linear interaction and can be extended to nonlinear interactions of any order. In this work we present a study of the second order nonlinear interaction from nonlinear organic molecules placed within two different types of photonic crystals. First, we will discuss the enhancement and inhibition of the radiation at the second-harmonic frequency of a sheet of dipoles embedded in a 1D photonic crystal. The experimentally observed reflected second-harmonic intensity as a function of the angle of incidence shows sharp resonances corresponding to the excitation of the SH field in a local mode within the forbidden band in the structure, which position depends on the size of the defect, and additional resonance at the high angular band edge, which position is independent of the size of the defect. Comparison among these results and the SH intensity reflected by the same monolayer in free space (which presents a bell shaped radiation pattern as a function of the angle of incidence), shows an enhancement of the radiation at the resonances, and strong inhibition of the radiation at other angles within the gap. Theoretical simulation of the experiment shows a good agreement with the experimental results.A detailed analysis of the enhancement and inhibition phenomena occurring in these structures shows a clear dependence of the resulting intensity with the position of the monolayer within the defect and with the dipole orientation. The change in phase difference between the oscillating dipoles and the field at the SH frequency at the monolayer as it is moved within the defect is found to play a determining role in the final energy transfer to the second-harmonic field. The resulting enhancement and inhibition of the radiation may be studied in terms of a nonsymmetric contribution of the different components of the field to the energy transfer process.The second configuration studied in the present work consider the experimental demonstration of second-harmonic generation in a 3-dimensional macroscopically centrosymmetric lattice formed by spherical particles of optical dimensions. In such photonic crystals, the local breaking of the inversion symmetry at the surface of each sphere, allows for the existence of a nonvanishing second order interaction. The growth of the SH radiation is provided by the phase-matching mechanism caused by the bending of the photon dispersion curve near the Bragg reflection bands of this photonic crystal. Experimental evidence of this phase-matching mechanism, inherent of such crystals, is reported in this work. By measuring the SH intensity radiated from several crystals with different concentrations, we obtained the angular dependence of this type of emission and confirmed the surface character of the nonlinear interaction. A simplified theoretical model shows very good agreement with the experimental results. It is important to notice that in this mechanism of SHG, the nonlinearity of the molecule is independent of the phase-matching mechanism, that is inherent to the periodicity of the crystal. In conclusion, the results obtained show a clear influence of the photonic crystals in the radiated SH intensity, resulting in enhancement and inhibition of the dipoles radiation.

At the moment the use our society makes of the available energy sources is far from optimal. Only a small 5% fraction of the energy used for the electricity production comes from new renewable energy sources. For over several decades, in an attempt to maximize sunlight energy harvesting, researchers in thin film devices have been searching for the optimal materials. Out of the many thin film cell options available one of the most promising is the one based on perovskites. According to the NREL chart the efficiency of such cells almost doubled in just two years. These attrac