En la presente tesis doctoral se han estudiado los factores implicados en el proceso de colonización fúngica (adhesión e invasión) de lentes de contacto nuevas y procedentes de usuarios.Para ello se ha diseñado una fase experimental en la que se han observado, mediante microscopía óptica, las lentes procedentes de usuarios a fin de determinar la presencia o ausencia de hongos. Cuando se detectaron, y para poder proceder a su identificación taxonómica, se procedió a la realización de cultivos.Las lentes nuevas fueron cultivadas con diferentes cepas de hongos y se valoró la frecuencia y densidad de colonización según las condiciones de cultivo establecidas.La caracterización morfológica de las diferentes especies fúngicas, experimentadas y encontradas, se realizó utilizando microscopía óptica, confocal y electrónica de barrido.Los resultados obtenidos indican que: el 11,82% de las lentes procedentes de usuarios mostraban contaminación. Entre los hongos contaminantes encontrados se describen tres nuevas citas, Gliomastix, Humicola y Phoma. Así mismo, se ha establecido que únicamente las cepas 93150 de Candida albicans y 2700 de Aspergillus niger fueron cepas de colonizar las lentes nuevas.Por otra parte, se ha comprobado que la composición de los medios de cultivo, su evaporación y las características quimicas de las lentes (ionicidad e hidrofilia), son factores que influyen en la frecuencia de colonización y en la densidad de las hifas invasoras.Finalmente, la microscopía confocal por reflexión de luz ha permitido visualizar las colonias internas y su grado de penetración en el material de la lente.Las conclusiones obtenidas representan una contribución al conocimiento de los fenómenos determinantes del desarrollo de hongos en los diferentes polímeros estudiados. Suponen, además, un avance en el campo de la contactología clínica y explican algunos de los problemas.

In this thesis, we address specific efforts towards developing the precise aspect of ultrashort laser pulse measurement in the context of biomedical research. The motivation for pursuing these new developments was triggered by the vision of developing fundamental tools that will enable to control matter by means of light with exquisite precision with the added difficulty of being next to biological samples which are extremely sensitive and fragile. For this, light matter interaction needs to be extremely well controlled to avoid undesired effects, like cell damage due to the high peak intensity values of ultrashort laser pulses, as well as promoting specific physical processes like two-photon fluorescence excitation of a desired fluorophore embedded in some biochemical environment. We focus in the two major bottlenecks regarding ultrashort laser pulse measurements for multiphoton microscopy, that aim for developing (1) new techniques for full characterization of ultrashort pulses under different experimental conditions and (2) new material with specific nonlinear properties that enable to obtain ultrashort pulse measurements that properly catch the temporal shape of light and at the same time can be readily found in biomedical lab, specially cost effective, non fragile and non-toxic. Combination of these two complementary strategies provides a new ground where it is possible to characterise an ultrashort pulse at the sample plane of a multiphoton microscope in a regular biomedical research facility. Importantly, we approach ultrashort pulse characterisation by developing a different theoretical framework to the state-of-the-art and we propose few initial experiments that preliminary support our theoretical statements in the form of new optical techniques. These findings are then experimentally tested under different conditions, such different optical setups and different pulsed regimes in order to evaluate the feasibility of the tools to measure ultrashort pulses in conditions that were prohibitive at the time this thesis was started. The scope of this thesis outlines the potential of such techniques, but further efforts shall be addressed to assess feasibility, robustness and further limitations.

Derivation of analytical refraction, propagation and reflection equations for Higher Order Aberrations of wavefronts From literature the analytical calculation of Lower Order Aberrations (LOA) of a wavefront after refraction, propagation and reflection is well-known, it is for local Power and Astigmatism performed by the Coddington equation for refraction and reflection and the classical vertex correction formula for propagation. However, equivalent analytical equations for Higher Order aberrations (HOA) do not exist. Since HOA play an increasingly important role in many fields of optics, e.g. ophthalmic optics, it is the purpose of this study to extend the analytical Generalized Coddington Equation and the analytical Transfer Equation, which deals with second order aberration, to the case of HOA (e.g. Coma and Spherical Aberration). This is achieved by local power series expansions. The purpose of this PhD was to extend the analytical Generalized Coddington Equation and the analytical Transfer Equation, which deals with Lower Order Aberrations (power and astigmatism), to the case of Higher Order Aberrations (e.g. Coma and Spherical Aberration). In summary, with the novel results presented here, it is now possible to calculate analytically the aberrations of an outgoing wavefront directly from the aberrations of the incoming wavefront and the refractive or reflective surface and the aberrations of a propagated wavefront directly from the aberrations of the original wavefront containing both low-order and high-order aberrations.

For many years, the fields of quantum optics and biology have rarely shared a common path. In quantum optics, most of the concepts and techniques developed over the years stand for systems where only a few degrees of freedom are considered and, more importantly, where the systems under study are assumed to be completely isolated from their surrounding environment. This situation is far from what we can find in nature. Biological complexes are, by definition, warm, wet and noisy systems subjected to environmental fluctuations, where quantum phenomena are unlikely to be observed. Notwithstanding, in recent years, this paradigm has begun to be questioned by several works where quantum-mechanical concepts have been introduced in order to describe the dynamics of important biological processes, such as energy transport in photosynthetic light-harvesting complexes. The goal of this thesis is twofold. Firstly, we will investigate how ideas and techniques routinely used in quantum optics can be exploited in order to develop new quantum-based spectroscopy techniques and, secondly, we will examine to what extent microscopic quantum phenomena could impact on the efficient transport behavior of photosynthetic light-havesting complexes. This problem is particularly relevant, because the understanding of the fundamental mechanisms that enable the highly efficient transport of energy in photosynthetic systems could lead us to the design of future quantum-inspired light-harvesting technologies, such as high-efficiency organic solar cells. / Por muchos años, los campos de la óptica cuántica y la biología raramente han compartido un mismo camino. En la óptica cuántica, la mayoría de los conceptos y técnicas desarrolladas a lo largo de los años son válidas sólo en sistemas donde un número pequeño de grados de libertad es considerado y, más importante aún, donde se asume que los sistemas bajo estudio están completamente aislados del medio ambiente que los rodea. Esta situación está muy lejos de lo que podemos encontrar en la naturaleza. Los complejos biológicos son, por definición, sistemas a altas temperaturas, sujetos a fluctuaciones, en los cuales se cree que los fenómenos cuánticos son imposibles de observar. Sin embargo, en años recientes, esta creencia ha sido cuestionada por diferentes trabajos en los que conceptos de mecánica cuántica han sido usados con el objetivo de describir la dinámica de procesos biológicos de gran importancia como, por ejemplo, el transporte de energía en los complejos de captación de luz en sistemas fotosintéticos. El objetivo de esta tesis se divide en dos. Primeramente, investigaremos cómo las ideas y técnicas usadas comunmente en óptica cuántica pueden ser explotadas con el objetivo de desarrollar nuevas técnicas de espectroscopía y, segundo, estudiaremos hasta que punto los fenómenos cuánticos microscópicos pueden influir en el comportamiento del transporte eficiente de energía en sistemas fotosintéticos de captación de luz. Este problema es particularmente relevante, pues el entender los mecanismos fundamentales que permiten un eficiente transporte de energía en sistemas fotosintéticos nos podría conducir al diseño de nuevas tecnologías de captación y recolección de energía como, por ejemplo, celdas solares orgánicas de alta eficiencia.

In the last decades, optics has become central in many applications in modern society. Nano-optics, which studies the behavior of light at the nanoscale, holds promise to do the same. However, when using traditional optical elements such as mirrors and lenses to control light propagation, there is a fundamental limit on the localization of the field which could a priori impinge on the ability to use optics at the nanometer scale. One way to improve the confinement of electromagnetic waves is to couple light with materials that have high dielectric permittivity. In this context, a particular interest has been devoted to metallic nanostructures made of gold, silver or aluminum. In this case the interaction can lead to a coherent collective electron oscillation and is known as a Surface Plasmon (SP). Such enhanced interaction results in both strong absorption and scattering of light. An inherent effect in such systems is the unavoidable damping of the SP. This causes internal Joule dissipation, which results in heat generation. Although this phenomenon is considered to be a drawback in many plasmonic applications, recent studies have shown the promise of nanoscale heat generation for both physical and biological application. This strategy is at the basis of a rapidly growing field called Thermo-Plasmonics. Controlling, exploiting and monitoring plasmonic heating is the topic of my PhD. In the first chapter of the thesis, the Joule effect induced in plasmonic nanoparticles is described. Thereafter, this concept is used to develop both physical and biological applications which rely on the ability to control nanoscale temperature in order to perturb the surrounding environment. A physical application consisting of a fast tunable Photothermal Lens based on plasmonic heating is described in the third chapter. To this end, we develop a model to predict the lens behavior. Next, experimental characterization of a fabricated thermal lens is performed. Finally, we show that such a system could be used for fast and accurate focal plane tunability as well as for adaptive optics applications. In the fourth chapter I describe another thermo-plasmonic based application which relies on the use of nanoscale heat generation to modify and even control the fluid flow in a micro-fluidic system. To do so, we numerically calculate the fluid convection that is induced by plasmonic heating at the micro and nanoscale. Next, an experimental implementation of a microfluidic pump based on plasmonic heating is presented. The applications mentioned above rely on the efficient opto-thermal conversion in gold nano particles to generate local heat sources. One advantage of gold is that it is nontoxic, allowing the application of thermo-plasmonics to bio systems. This is a pertinent line of investigation, because temperature is a basic parameter which influences many cellular biological processes. Nanotechnology is not only providing new ways to generate point like heat sources but also to accurately monitor the resulting temperature maps. Within this context, we developed a tool that permits the measurement of temperature at the nanoscale in biological systems. This is done by monitoring the fluorescence polarization anisotropy (FPA) of a fluorescent molecule. Specifically, in the fifth chapter I present the use of Green Fluorescent Protein (GFP) as a thermal nanoprobe suited for in vitro cellular temperature mapping. This is performed by monitoring the FPA of the GFP. We apply this method to measure the temperature generated by photothermal heating of gold nanorods inside and outside cells. Consequently, we extend this technique and perform the first in vivo intracellular thermal imaging. In the sixth chapter we demonstrate this method with GFP expressing neurons of a worm. In both cases we show that the method enables diffraction limited spatial resolution, good temperature accuracy and fast readout together with high bio-compatibility. / En las últimas décadas, la óptica ha sido esencial en muchas aplicaciones en la sociedad moderna. La nano-óptica, que estudia el comportamiento de la luz a escala nanométrica, promete tener el mismo impacto. Sin embargo, cuando se utilizan elementos ópticos tradicionales como espejos y lentes para controlar la propagación de la luz, existe un límite fundamental en la localización del campo electromagnético que puede a priori impedir la posibilidad de utilizar la óptica en la escala nanométrica. Una manera de mejorar el confinamiento de las ondas electromagnéticas es acoplándolas con materiales que tienen una alta permitividad dieléctrica. En este contexto, se ha dedicado un interés particular en las nanoestructuras metálicas compuestas de oro. En este caso, la interacción puede llevar a una oscilación coherente y colectiva de los electrones del metal, la cual es conocida como el Plasmón de Superficie (PS). Esta interacción conlleva a un aumento tanto de la absorción como la dispersión de la luz. Un efecto inherente en dichos sistemas es la inevitable amortiguación del PS. Esto causa la disipación interna por efecto Joule, la cual resulta en la generación de calor. A pesar que este fenómeno es considerado como una desventaja en muchas aplicaciones en plasmónica, estudios recientes han demostrado la promesa de la generación de calor en la nanoescala. Esta estrategia está en la base de un campo llamado Termo-Plasmónica. Controlar, explotar y monitorizar el calentamiento plasmónico es el tema de mi tesis doctoral. En el primer capítulo de la tesis se describe el efecto Joule inducido en nanopartículas plasmónicas. Después, este concepto es utilizado para desarrollar aplicaciones físicas y biológicas que dependen de la habilidad de controlar la temperatura en la nanoescala para perturbar el ambiente local. Una aplicación física consistiendo de un Lente Fototérmico basado en el calentamiento plasmónico y con afinación rápida está descrito en el tercer capítulo. Para este fin, desarrollamos un modelo para predecir el comportamiento del lente. A continuación, se realizó la caracterización experimental de un lente termal. Finalmente, mostramos que dicho sistema puede ser utilizado para la variación local rápida y precisa del plano focal. En el cuarto capítulo describo otra aplicación que utilizala generación de calor a la nanoescala para modificar e incluso controlar el flujo de un fluido en un sistema microfluidico. Para ello, hemos calculado numéricamente la convección de un fluido que es inducida por el calentamiento plasmónico en la micro y nanoescala. Luego, se presenta una implementación experimental de una bomba microfluidica basada en el calentamiento plasmónico. Las aplicaciones mencionadas anteriormente dependen de la conversión opto-térmica eficiente en nanopartículas de oro para generar fuentes de calor locales. Una ventaja del oro es que no es tóxico, permitiendo así aplicaciones de la termo-plasmónica en sistemas biológicos. Con esto en mente, hemos desarrollado una herramienta que permite medir la temperatura en la nanoescala en sistemas biológicos. Esto se lleva a cabo al monitorizar la anisotropía de la polarización de la fluorescencia (APF) de una molécula fluorescente. Específicamente, en el quinto capítulo mapeamos la temperatura celular in-vitro monitorizando el APF de la Proteína Fluorescente Verde (GFP). Aplicamos este método para medir la temperatura generada por el calentamiento fototérmico de nanobarras de oro dentro y fuera de células. Consecuentemente, extendemos esta técnica y realizamos los primeros mapas de temperatura intracelular in vivo. En el sexto capítulo demostramos este método en gusanos que contienen neuronas que expresan la GFP. En ambos casos demostramos que el método permite resolución espacial en el límite de difracción, buena precisión de temperatura y una lectura rápida, todo esto junto con una alta biocompatibilidad.

The efficient interaction of photons with single quantum emitters like nitrogen vacancy (NV) centres is essential for the elaboration of future integrated quantum optical devices. A promising strategy towards this goal capitalizes on the latest advances of nano-optics to boost the interaction with single emitters as well as strengthen coupling between several of them. However, fully exploiting the capabilities of this marriage between NV centres and optical nanostructures requires suitable tools to accurately control their interaction. In this thesis, we use optical manipulation to trap and manipulate in 3D individual nanodiamonds containing a single NV. We first demonstrate the use of optical tweezers as a tool to achieve deterministic trapping and three-dimensional spatial manipulation of individual nanodiamonds hosting a single NV spin. Remarkably, we find that the NV axis is nearly fixed inside the trap and can be controlled in situ by adjusting the polarization of the trapping light. By combining this unique spatial and angular control with coherent manipulation of the NV spin and fluorescence lifetime measurements near an integrated photonic system, we demonstrate individual optically trapped NV centers as a novel route for both three-dimensional vectorial magnetometry and sensing of the electromagnetic local density of states. In a second step, our manipulation technique is further developed to deterministically position a single nanodiamond into the hotspot of a plasmonic antenna. The gradient force of electromagnetic field of the excited plasmon acts as localized optical tweezers to drive the functionalized nanodiamonds to the regions of largest field enhancement of the antenna, where they are adsorbed. The proximity of the immobilized NV to the nano-antenna is corroborated by the observed decrease in its fluorescence lifetime. Last but not least, we observe a NV fluorescence decrease upon near-infrared (NIR) illumination. We identify the promotion of the excited electron to a so far unknown dark band with a fast decay channel as the origin of the fluorescence decrease. This assumption is verified by the excellent agreement between our simple rate equation model and the experiment. With this mechanism we demonstrate that a single NV can operate as an efficient and fast optical switch controlled through an independent NIR gating laser. Furthermore the hybrid system formed by a single NV coupled to a gold gap antenna enhances the modulation depth. The results presented in this thesis show the ability to manipulate and position NV centres in nanodiamond with optical tweezers. This paves the way towards spin based magnetic field and temperature sensing in liquid environment. Furthermore, the control of positioning and coupling to photonic and plasmonic nanostructures may play a role for potential applications in all-optical circuits or quantum optical devices. / La interacción de fotones con emisores cuánticos individuales como los centros de nitrógeno-vacante (NV) es esencial para la elaboración de futuros dispositivos integrados de óptica cuántica. Una estrategia prometedora para alcanzar este objetivo es de aprovechar de los últimos avances de la nano-óptica para aumentar la interacción con emisores individuales, así como fortalecer el acoplamiento entre varios de ellos. Sin embargo, para aprovechar al máximo las capacidades de este matrimonio entre centros de NV y nano-estructuras ópticas se requiere de herramientas adecuadas para controlar con precisión su interacción. En esta tesis, se utiliza la manipulación óptica para atrapar y manipular en 3D nano-diamantes individuales que contienen un solo centro de NV. En primer lugar, demostramos el uso de pinzas ópticas como una herramienta para lograr la captura precisa y manipulación espacial tridimensional de nano-diamantes individuales conteniendo un solo centro de NV. Sorprendentemente, encontramos que el eje del centro de NV está casi fija dentro de la trampa y puede controlarse in situ mediante el ajuste de la polarización de la luz del láser de captura. Combinamos este control espacial y angular con la manipulación coherente del espín del centro NV y con medidas de tiempo de vida de fluorescencia de un sistema fotónico integrado. Demostramos que los centros NV atrapados ópticamente pueden servir como una nueva ruta para ambos magnetometría vectorial tridimensional y de detección de la densidad local de estados electromagnéticos. En un segundo paso, nuestra técnica de manipulación se desarrolló aún más hacia el posicionamiento de un nano-diamante individual en una antena plasmónica. La fuerza de gradiente del campo electromagnético del plasmon excitado actúa como una pinza óptica local para atraer los nano-diamantes funcionalizados a las regiones de mayor aumento del campo de la antena, donde quedan adsorbidos. La proximidad del centro NV inmovilizado en la nanoantena es corroborado por la disminución observada del tiempo de vida de la fluorescencia. Por otra parte, se observa una disminución de la fluorescencia NV tras la iluminación infrarroja. Identificamos como origen de la disminución de la fluorescencia la promoción del electrón excitado a una banda, que tiene un canal de decaimiento rápido. Esta hipótesis es comprobada por el excelente acuerdo entre nuestro modelo simple de ecuación cinética y el experimento. Por último, demostramos que un centro NV puede funcionar como un interruptor óptico eficiente y rápido controlado a través de un láser de control infrarrojo independiente. Además, el sistema híbrido formado por un solo NV acoplado a una nano-antena de oro aumenta la profundidad de modulación. Los resultados presentados en esta tesis demuestran la capacidad de manipular y posicionar nano-diamantes conteniendo un centro NV con pinzas ópticas. Esto allana el camino hacia un sensor de campo magnético y de temperatura en ambiente liquido usando el espín del centro NV. Además, el control de posicionamiento y acoplamiento a nano-estructuras fotónicas y plasmónicas podría tener un impacto para aplicaciones potenciales en circuitos completamente ópticos o dispositivos de óptica cuántica .

In this thesis we have investigated and proposed acoustic superLattices (ASLs) made of periodically poled ZX-cut lithium niobate (PPLN) associated ith coplanar electrodes as an effective alternative for surface acoustic wave (SAW) generation. In order to examine the acoustic transduction in the SL transducer we have developed and employed two modeling techniques. i.e . scalar approximation and finite element method (FEM) analysis (2-D nd 3-D modeling) implemented in COMSOL MULTIPHYSICS. Both techniques give similar results with respect to the characteristics of generated SAW modes. The calculated results obtained with the 3-D FEM simulation confirmed the Rayleigh nature of the generated SAW, showing that the excited SAW in the ASL is similar to that of the standard interdigitated transducer (IDT). From the propagation analysis in 2-D FEM simulation. we obtained that in addition to the SAW, the ASL transducer excites longitudinal bulk acoustic wave (L-BAW). Moreover. the 2-D model showed that the SAW excited by the ASL transducer does not propagate but it is rather confined within the transducer. Several ASL transducers, with different lattice periods, have been realized and characterized, also to validate the modeling and design tools . One and two-portelectrical measurements have been performed to evaluate the electro-acoustic response. In addition laser interferome try has been carried out o determine the out-of-plane component of the SAW displacement. With respect to the standard IDT configuration using the same crystal orientation, he efficiency of the SAW generation in the proposed designs is similar, while, for the same grating period, the resonance frequency that can be achieved is two times larger. In addition to the SAW, unlike its lOT counterparts, the ASL transducer can excite the L-BAW, as it had been predicted by the modeling. The two-port measurements have shown that the SAW signal collected at the receiver is small, indicating that the SAW energy remains essentially confined within the transmitter. This experimental result too is in agreement with the aforementioned theoretical predictions. After the experimental validation of the modeling, we designed and fabricated acousto-optic (AO) filters incorporating the ASL transducer. ASL struc tures with 20 tm period and coplanar electrodes have been realized along with 6 m wide Ti-LiNb03 optical channel waveguide. In that way monolithic and integrated (waveguide) SAW based AO filters and L-BAW based bulk AO filters have been demonstrated. As for integrated SAW based 0 filter, a 3-dB optical bandwidth of 2.5 nm, center wavelength of 1456 nm, and -20 dB distance of 14.49 nm are obtained at the SAW resonance frequency of 189.94 MHz. Such filter requires 1 W RF power to achieve nearly complete polarization conversion. As for the bulk AO filter. a center Wavelength of 1472.5 nm is measured at the L-BAW resonance frequency of 328 MHz. In the bulk AO filter. full conversion cannot be reached even at RF power up to 2 W. Comparing to the integrated (waveguide) filter. the efficiency of the bulk filter is significantly lower. The AO filter's central wavelength can be tuned by changing the RF power. We investigated several designs to improve the AO filter efficiency. By optimizing the ASL transducer (coplanar electrodes with electrode width of 70 urn and gap of 20 t m), nearly complete optical switching at very low electrical powers (50mW) has been obtained, this corresponding to an improvement factor of 20 compared to previous results. An appropriate mass-loading, placed on the surface and in between the electrodes of a coplanar LiNb03 ASL, has allowed achieving strong lateral confinement of the acoustic field, thus leading to a significant improvement of the AO filter performance.

Optical solitons provide unique opportunities for the control of light‐bylight. Today, the field of soliton formation in natural materials is mature, as the main properties of the possible soliton states are well understood. In particular, optical solitons have been observed experimentally in a variety of materials and physical settings, including media with cubic, quadratic, photorefractive, saturable, nonlocal and thermal nonlinearities. New opportunities for soliton generation, stability and control may become accessible in complex engineered, artificial materials, whose properties can be modified at will by, e.g., modulations of the material parameters or the application gain and absorption landscapes. In this way one may construct different types of linear and nonlinear optical lattices by transverse shallow modulations of the linear refractive index and the nonlinearity coefficient or complex amplifying structures in dissipative nonlinear media. The exploration of the existence, stability and dynamical properties of conservative and dissipative solitons in settings with spatially inhomogeneous linear refractive index, nonlinearity, gain or absorption, is the subject of this PhD Thesis. We address stable conservative fundamental and multipole solitons in complex engineered materials with an inhomogeneous linear refractive index and nonlinearity. We show that stable two‐dimensional solitons may exist in nonlinear lattices with transversally alternating domains with cubic and saturable nonlinearities. We consider multicomponent solitons in engineered materials, where one field component feels the modulation of the refractive index or nonlinearity while the other component propagates as in a uniform nonlinear medium. We study whether the cross‐phase‐modulation between two components allows the stabilization of the whole soliton state. Media with defocusing nonlinearity growing rapidly from the center to the periphery is another example of a complex engineered material. We study such systems and, in contrast to the common belief, we have found that stable bright solitons do exist when defocusing nonlinearity grows towards the periphery rapidly enough. We consider different nonlinearity landscapes and analyze the types of soliton solution available in each case. Nonlinear materials with complex spatial distributions of gain and losses also provide important opportunities for the generation of stable one‐ and multidimensional fundamental, multipole, and vortex solitons. We study onedimensional solitons in focusing and defocusing nonlinear dissipative materials with single‐ and double‐well absorption landscapes. In two‐dimensional geometries, stable vortex solitons and complexes of vortices could be observed. We not only address stationary vortex structures, but also steadily rotating vortex solitons with azimuthally modulated intensity distributions in radially symmetric gain landscapes. Finally, we study the possibility of forming stable topological light bullets in focusing nonlinear media with inhomogeneous gain landscapes and uniform twophoton absorption.

High-power, continuous-wave (cw) optical parametric oscillators (OPOs), from the ultraviolet (UV) and visible to the near- and mid-infrared (IR) wavelength range, are of interest for a variety of applications such as spectroscopy, trace-gas detection and remote sensing. As such, it is desired to investigate OPOs and different frequency conversion techniques, to cover the spectral regions that are inaccessible to lasers, and that too in a compact and low-cost design. This thesis presents the development of high-power cw OPOs, and frequency conversion sources, spanning the UV to mid-IR spectral range, employing different designs, experimental configurations and nonlinear crystals, making them compact and cost-effective. Commercial high-power cw lasers at 1940 nm, 1064 nm and 532 nm have been exploited as the pump sources, in the work presented in this thesis. We have demonstrated a fiber-based cw source at 970 nm, in a simple and practical design. Using direct single-pass second-harmonic-generation (SHG), 13.1 W of output power at 970 nm has been generated in a high-beam-quality, narrow-linewidth, linearly-polarized beam. Further, a technique based on the use of an antiresonant ring (ARR) interferometer for the attainment of optimum output coupling in a cw singly-resonant OPO has been investigated. The technique was deployed in a Yb-fiber-laser-pumped cw OPO based on MgO:PPLN. To extend the tunability of the 1-µm-pumped OPO from the mid-IR to near-IR, SHG of the intracavity signal has been performed in fanout-grating MgO:sPPLT. This compact cw source, tunable across 775¿807 nm, provides >3 W of near-infrared power across 56% of SHG tuning range, in high spatial beam quality. We have also generated output in the UV, down to 355 nm, using single-pass configuration based on sum-frequency-generation of fundamental at 1064 nm and the generated SHG at 532 nm, in BiB3O6. Further, we demonstrated an architecture comprising two cw OPOs coupled together with an ARR interferometer, generating two pairs of signal and idler wavelengths, that can be independently and arbitrarily tuned to indefinitely close spacing, through degeneracy, and beyond, across the wavelength range of 870-1370 nm. The OPOs, based on identical MgO:sPPLT crystals, were pumped by a single cw laser at 532 nm. On the other hand, we also demonstrated active mode-locking of cw OPOs using direct low-frequency electrooptic phase-modulation (EOM), opening up the possibility of avoiding the need for ultrafast laser sources and synchronous pumping. We have generated picosecond pulses in doubly- and singly-resonant configuration. Also, a technique based on the deployment of the EOM in combination with an ARR interferometer internal to the cw OPO has been investigated for active modelocking.

This thesis analyses the effects of two-photon interference in a polarisation squeezed state under two different points of view: on one hand, it presents a new method to obtain the temporal wavefunction of a state of two photons; on the other hand, it studies the microscopic entanglement properties of a collective nonclassical polarisation state, such as the polarisation squeezed state. The complete characterisation of an unknown quantum state often requires complicated reconstruction methods due to its complex nature: in the first part of this thesis, we describe a new technique to recover completely the wavefunction of a state with two photons (a ¿biphoton¿) with just few simple measurements, thanks to the interference with a coherent reference. With this technique, we reconstruct successfully the wavefunction of single-mode biphotons from a low-intensity narrowband squeezed vacuum state. Many large collective systems that feature nonclassical properties, e.g. superconductivity and squeezing, show entanglement among their components at their microscopic level. Here we report the first direct study of this kind of entanglement for light polarisation. In analogy with the spin-squeezing inequalities that connect squeezing to entanglement for atomic ensembles, we derive an inequality valid for states with classical polarisation correlations, whose violation implies pairwise entanglement among the photons in the state. We consider a polarisation squeezed state that results from the combination in the same spatial mode of a squeezed vacuum state, generated by an optical parametric oscillator (OPO), and a coherent state with orthogonal polarisations: we find that this kind of state always violates our inequality within the coherence time of the squeezed vacuum state. We also quantify the entanglement between the photon pairs by computing the concurrence of the two-photon reduced density matrix: we find that the states that exhibit higher entanglement satisfy the condition for higher visibility of the two-photon interference. We also find that the concurrence is larger for lower squeezing levels, in agreement with the monogamy of entanglement and in analogy to the atomic case. This translation of spin-squeezing inequalities to the optical domain enables us to test directly the squeezing-entanglement relationship. We generate a squeezed vacuum state with an OPO and we combine it with a coherent state to generate a polarisation squeezed state and we measure the photon pair counts for different polarisation bases. We recover the density matrices corresponding to different realisations of the polarisation squeezed state via quantum tomography: all the density matrices that we reconstruct with this method are entangled, with concurrence up to 0.7. Our measurements confirm several theoretical predictions, including entanglement of all photon pairs within the squeezing coherence time. / En esta tesis se analizan los efectos de la interferencia de dos fotones en un estado comprimido en polarización desde dos puntos de vista: por un lado, se presenta un nuevo método para obtener la función de onda temporal de un estado de dos fotones; por el otro, se estudian las propiedades de entrelazamiento microscópico de un estado colectivo de polarización no clásico, como el estado comprimido en polarización. La completa caracterización de un estado cuántico desconocido requiere frecuentemente métodos de reconstrucción complicados debido a su compleja naturaleza: en la primera parte de esta tesis describimos una nueva técnica para recuperar completamente la función de onda de un estado con dos fotones (un bifotón) usando pocas medidas sencillas, gracias a la interferencia con un estado coherente de referencia. Con esta técnica, reconstruimos con éxito la función de onda de los bifotones que pertenecen a un estado de vacío comprimido de banda estrecha y de baja intensidad. Muchos sistemas colectivos con un gran número de partículas que presentan propiedades no clásicas, como por ejemplo superconductividad y estados comprimidos, muestran entrelazamiento entre sus componentes a nivel microscópico. Aquí describimos el primer estudio directo de este tipo de entrelazamiento para los estados de polarización de la luz. En analogía con las desigualdades para estados comprimidos en espín, derivamos una desigualdad válida para estados con correlaciones clásicas en polarización, cuya violación implica entrelazamiento en parejas entre los fotones del estado. Consideramos un estado comprimido en polarización, que es el resultado de la combinación en el mismo modo espacial de un estado de vacío comprimido generado por un oscilador óptico paramétrico (OPO) y de un estado coherente con polarización ortogonal al primero: hallamos que estos estados violan nuestra desigualdad siempre que nos encontremos dentro del tiempo de coherencia del estado de vacío comprimido. Cuantificamos también el entrelazamiento entre las parejas de fotones calculando la concurrencia de la matriz de densidad reducida de dos fotones: observamos que los estados que tienen mayor entrelazamiento satisfacen la condición para la visibilidad máxima de la interferencia entre bifotones. Hallamos también que la concurrencia es mayor para niveles de compresión menores, en acuerdo con la monogamia del entrelazamiento, siendo este resultado análogo al caso atómico. El trasladar estas desigualdades para los estados comprimidos en espín al dominio óptico nos permite observar directamente la relación entre estados comprimidos y entrelazamiento de manera experimental. Con este fin generamos un estado de vacío comprimido con un OPO y lo combinamos con un estado coherente para obtener un estado comprimido en polarización y contamos las parejas de fotones en diferentes bases de polarización. Con estas medidas reconstruimos las matrices de densidad que corresponden a diferentes versiones del estado comprimido en polarización usando tomografía cuántica: todas las matrices de densidad que hemos obtenido con este método están entrelazadas, mostrando valores de concurrencia de hasta 0.7. Nuestras medidas confirman las predicciones teóricas, entre las que se encuentra el entrelazamiento de todas las parejas de fotones dentro del tiempo de coherencia del estado entrelazado.