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Organic π-conjugated molecules and materials with large real parts and small imaginary parts of the third-order polarizability are of great interest for all-optical switching applications. In this dissertation, we use quantum-chemical and molecular-dynamics approaches to investigate the structure-property relationships that influence the nonlinear optical properties of π-conjugated molecules and materials. We begin with an overview of nonlinear optics, focusing in particular on the electronic properties of linear π-conjugated systems and some of the important problems that have limited device applications of these molecules to date. This is followed by a brief review of the computational methods employed in these studies. We then turn to the main results of the dissertation. Chapter 3 describes the structural dependence of the transition dipole moment between the first two polymethine excited states. Chapter 4 discusses the relationship between BLA, which depends on the geometric structure, and BOA, which probes electronic structure. Chapter 5 describes the benchmarking of computational methods to describe the symmetry-breaking of long polymethines and preliminary evidence regarding the role of vibrational modes in symmetry-breaking. Chapter 6 explains the negative third-order polarizability of tetraphenylphosphate and analogous systems. Chapter 7 focuses on molecular-dynamics studies of polymethine aggregation, particularly the relationships between chemical structure and the geometric and electronic structures of aggregates. Finally, Chapter 8 provides a synopsis of the work and discussion of further directions.

The past decades have witnessed the development of new materials with large nonlinear optical properties, which have made them attractive candidats for a broad spectrum of breakthrough applications in the electro-optic and photonic fields (e.g., telecommunication and computing). A deeper understanding of the relationship between, on the one hand, the chemical structure and, on the other hand, the electronic and (linear and nonlinear) optical properties has proven useful for the rational design of new efficient materials. Reaching such an understanding has attracted major interest in the scientific community worldwide in both academia and industry. Therefore, the development of new efficient NLO chromophores and materials along with commercial devices of high quality is helped via the establishment of multidisciplinary research teams combining: (i) the theoretical modeling using quantum-chemical computational calculations; (ii) the organic synthesis; (iii) the optical characterization; and (iv) the device fabrication. In this dissertation, quantum-chemistry is used to evaluate the second- and third-order NLO properties of series of new chromophores and take advantage of a feedback loop with the experimental team to understand the structure-property relationships.

Le contrôle de l’aimantation sans application de champ magnétique externe est un domaine de recherche en plein essor, étant prometteur pour les applications technologiques d’enregistrement magnétique et de spintronique. En 2007, Stanciu et al. ont découvert la possibilité de retourner l’aimantation dans un film fait d’alliage ferrimagnétique de GdFeCo en utilisant des impulsions laser femtoseconde. Longtemps cantonné aux alliages de GdFeCo, ce retournement tout-optique s’avère un phénomène plus général, puisqu’il a été mesuré plus récemment dans une large variété de matériaux ferrimagnétiques et ferromagnétiques. Cette découverte a ainsi ouvert la voie à l’intégration de l’écriture tout-optique dans l’industrie des mémoires magnétiques. Néanmoins, l’ensemble des modèles théoriques expliquant le retournement tout-optique dans le GdFeCo ne semblent pas s’appliquer aux autres matériaux magnétiques, mettant ainsi en question l’unicité de l’origine microscopique de ce phénomène. Au cours de cette thèse, nous avons étudié la réponse aux impulsions laser femtoseconde des alliages ferrimagnétiques et des multicouches ferromagnétiques, dans l'objectif d'élucider divers aspects du mécanisme du retournement optique. Nous avons élucidé expérimentalement les paramètres magnétiques gouvernant le retournement tout-optique. Nous avons montré que l’observation du retournement tout-optique nécessite des domaines magnétiques plus grands que la taille du faisceau laser pendant le processus de refroidissement, un critère qui est commun à la fois aux matériaux ferrimagnétiques et ferromagnétiques. En outre, nous nous sommes intéressés à l’intégration du retournement tout-optique dans des dispositifs de spintronique. Grâce à une caractérisation temporelle de l’aimantation dans des croix de Hall via l’effet Hall extraordinaire, nous avons distingué entre deux types de mécanismes du retournement optique. Le premier type est un retournement purement thermique obtenu avec une impulsion unique dans les alliages ferrimagnétiques de GdFeCo, tandis que le deuxième type est un retournement cumulative et à deux régimes dans les alliages ferrimagnétiques de TbCo et les multicouches ferromagnétiques de Co/Pt. Ce dernier consiste en une formation indépendante de l’hélicité de multidomaines magnétiques suivie d'une ré-aimantation dépendante de l'hélicité sur plusieurs dizaines de millisecondes. / The control of magnetization without external magnetic fields is an emergent field of research due to the prospect of impacting many technological applications such as magnetic recording and spintronics. In 2007, Stanciu et al. discovered an intriguing new possibility to switch magnetization in a ferrimagnetic GdFeCo alloy film using femtosecond laser pulses. This all-optical switching of magnetization had long been restricted to GdFeCo alloys, though it turned out to be a more general phenomenon for a variety of ferromagnetic and ferromagnetic materials. This discovery paved the way for an integration of the all-optical writing in storage industries. Nevertheless, the theoretical models explaining the switching in GdFeCo alloys films do not appear to apply in the other materials, thus questioning the uniqueness of the microscopic origin of all-optical switching. In this thesis, we have investigated the response of femtosecond laser pulses in ferrimagnetic alloys and ferromagnetic multilayers to the action of femtosecond laser pulses, in order to elucidate several aspects of the all-optical switching mechanism. We have experimentally studied the magnetic parameters governing the all-optical switching. We showed that the observation of all-optical switching requires magnetic domains larger than the laser spot size during the cooling process; such a criterion is common for both ferrimagnets and ferromagnets. Furthermore, we have investigated the integration of all-optical switching in spintronic devices via the anomalous Hall effect. Through a time-dependent electrical investigation of the magnetization in Hall crosses, we distinguished between two types of all-optical switching mechanisms. The first type is the single-pulse helicity-independent switching in ferrimagnetic GdFeCo alloy films as shown in previous studies, whereas the second is a two regimes helicity-dependent switching in both ferrimagnetic TbCo alloys and ferromagnetic Co/Pt multilayers. The latter consists in a step-like helicity-independent multiple-domain formation followed by a helicity-dependent remagnetization on several tens of milliseconds.

Depuis une quinzaine d’années, de nombreuses solutions différentes ont été proposés afin de modifier l’aimantations de matériaux sans aucun champ magnétique extérieur appliqué. La manipulation d’aimantation à moindre coût énergétique, de préférence à des échelles de temps ultracourtes, est devenu un enjeu fondamental avec des implications pour les technologies d’enregistrement magnétique et de nouvelles sortes de stockage. Sur ce chemin, le type d’interaction découverte par Stanciu et al. ouvre la voie à l’utilisation de la lumière comme moyen d’exciter et de sonder directement les matériaux magnétiques. La description des théories et modèles existants dans ce domaine permet de nous rendre attentif sur les différents paramètres impliqués par l’interaction des lasers ultrarapides et matériaux magnétiques. L’entrelacement spécifique des impulsions de chaleur et de moment angulaire propre aux lasers ultrarapides est mise en avant afin de discuter de leur rôle dans les phénomènes observés. Le délai des interactions responsables de l’état final de l’aimantation est abordé et notamment la manière dont celle-ci ont un impact sur la façon dont le système se stabilise après une excitation laser. En outre, nous nous sommes intéressés à la relation entre les paramètres matériels et l’état final de l’aimantation obtenue avec un laser ultrarapide. Grâce aux nombreuses classes de matériaux magnétiques existantes les paramètres magnétiques peuvent être ajustés dans une grande gamme de valeurs et de manière entièrement contrôlés. Notre installation d’imagerie magnétique est alors capable de sonder les caractéristiques optiques et la stabilité des domaines après l’excitation. Nous avons finalement démontré que le retournement optique dépendent de l’hélicité peut être observée non seulement dans un grand nombre de couches minces d’alliages de terre rare-métaux de transition (RE-TM) mais aussi dans une variété beaucoup plus large de matériaux, y compris les multicouches et hétérostructures de RE-TM. Nous montrons en outre que les hétérostructures ferrimagnétiques dépourvues de terres rares présentent également un retournement optique. Nous avons en plus développé le contrôle optique de multicouches ferromagnétiques dont des films granulaires actuellement explorés pour l’enregistrement magnétique ultra-haute densité de demain. Notre découverte montre que la manipulation de l’aimantation dans des matériaux magnétiques est un phénomène beaucoup plus général que précédemment suspecté et peut avoir un impact majeur sur l’enregistrement magnétique et le stockage de l’information grâce à l’intégration nouvelle de ce type de contrôle optique dans des bits ferromagnétiques / The possibilities of modifying magnetization without applied magnetic fields have attracted growing attention over the past fifteen years. The low-power manipulation of magnetization, preferably at ultrashort timescales, has become a fundamental challenge with implications for future magnetic information memory and storage technologies. In particular the interplay of laser and magnetism recently discovered by Stanciu et al. opens up new way for light to be used as an excitation and a probe of magnetic materials. A description of the current models and frameworks developed in the field requires a careful look at the different parameters involved through the interaction of ultrafast lasers and magnetic materials. The specific and complex interplay between heat and angular momentum transfer is highlighted in order to discuss the role of each of them in the phenomena observed. The timescales of the different interactions responsible for the final state of magnetization are presented and will impact the way the system recovery after a laser excitation. Besides we were interested in exploring the relation between the material parameters such as anisotropy, ordering temperature and exchange coupling on the final state of magnetization obtained with a laser. Indeed thanks to the many different magnetism classes existing the magnetic parameters can be tuned widely and in a controlled manner. Our imaging setup then is able to probe the optical characteristics and domain stability after the laser excitation. We finally demonstrated that all-optical helicity-dependent switching (AO-HDS) can be observed not only in selected rare earth-transition metal (RE-TM) alloy films but also in a much broader variety of materials, including RE-TM alloys, multilayers and heterostructures. We further show that RE-free Co-Ir-based synthetic ferrimagnetic heterostructures designed to mimic the magnetic properties of RE-TM alloys also exhibit AO-HDS. We further developed the optical control of ferromagnetic materials ranging from magnetic thin films to multilayers and even granular films being explored for ultra-high-density magnetic recording. Our finding shows that optical control of magnetic materials is a much more general phenomenon than previously assumed and may have a major impact on data memory and storage industries through the integration of optical control of ferromagnetic bits

Pendant les 20 dernières années, le nanomagnétisme a suscité un intérêt grandissant au sein de la communauté scientifique du fait de ses nombreuses applications pour les mémoires magnétiques. A l’échelle nanométrique beaucoup de propriétés des matériaux magnétiques découlent de leurs interfaces avec d’autres matériaux (magnétiques ou non). Cela explique l’omniprésence des hétérostructures composées de plusieurs couches d’épaisseur nanométrique dans le domaine du nanomagnétisme. Dans les hétérostructures que nous étudions, ces propriétés interfaciales sont le décalage d’échange, l’interaction Dzyaloshinskii-Moriya, l’anisotropie magnétique perpendiculaire et l’échange entre deux couches ferromagnétiques. D’abord nous étudions la modification du champ de décalage d’échange dans une bicouche [Pt/Co]xN/IrMn lorsque l’on l’expose à des impulsions laser de lumière polarisée circulairement. Nous montrons que le champ de décalage d’échange après exposition au laser résulte de la configuration du ferromagnétique [Pt/Co]xN. Nous étudions ensuite les conditions nécessaires à un retournement tout optique dépendant de l’hélicité d’un matériau ferrimagnétique de synthèse composé de deux couches de CoFeB /Pt /CoFeB et Co couplées antiferromagnétiquement et concluons que les facteurs clés qui gouvernent le renversement de l’aimantation totale sont les températures respectives des deux couches. Nous nous sommes aussi concentrés sur la propagation de parois de domaine de Néel de même chiralité stabilisées par interaction Dzyaloshinskii-Moriya dans des multicouches de [Pt/Co/Ni]N. Nous avons finalement démontré la possibilité de générer des bulles skyrmioniques par le laser femtoseconde / During the last 20 years, nanomagnetism has attracted a growing interest in the scientific community due to its multiple applications for magnetic memories. At the nanometer scale, many of the properties of the magnetic materials arise from their interfaces with other materials (magnetic or non-magnetic). This explains the omnipresence of heterostructures composed of several layers of thicknesses in the range of the nanometer in the field of nanomagnetism. In the heterostructures we study, those interfacial properties are the exchange bias, the Dzyaloshinskii-Moriya interaction, the perpendicular magnetic anisotropy and the interlayer exchange between two ferromagnetic layers. First we study the modification of the exchange bias field in a [Pt/Co]xN/IrMn bilayer when we expose it to laser pulses of a femtosecond circularly polarized light. We demonstrate that the final exchange bias field after laser pulses results from the magnetic configuration of the [Pt/Co]xN multilayer. We then study the conditions required for a helicity-dependent all optical switching of a synthetic ferromagnetic material composed of a CoFeB /Pt /CoFeB and a Co ferromagnetic layers coupled antiferromagnetically and conclude that the key factors that drive the switching of the total magnetization are the Curie temperatures of both layers. We focused also on the field-driven propagation of Néel domain walls of the same chirality stabilized by the Dzyaloshinskii-Moriya interaction in [Pt/Co/Ni]xN multilayers. We finally demonstrated the possibility to generate skyrmionic bubbles with the femtosecond laser

Development of the faster and denser magnetic memory storage elements has been an active area of research since early 20th century. The path of research on magnetization manipulation began with firstly changing the magnetization state of a medium in an external magnetic field, then heating of a medium and magnetizing with a permanent magnet was explored, while the latest efforts have been focused on switching the magnetization only by a polarized laser light. Nowadays due to the technological advancement of lasers and material fabrication methods, the search and development process of magnetic memory elements is much faster. The implementation of such technologies, however, relies on finding suitable magnetic materials which would allow for a fast magnetization writing and read-out processes and would remain magnetized, even with the reduced dimensions. Ferrimagnetic rare Earth - transition metal (RE-TM) alloys have been used for fabricating magneto-optical recording media already since the 1990’s. Relatively recently, in 2007, it was demonstrated that the ferrimagnetic GdFeCo alloy magnetization state can be switched using only circularly polarized laser light. Hence, ferrimagnetic RE-TMalloys could be suitable candidates for all-optical light-induced magnetization switching (AOS), without any external magnetic field. Another combination of RE-TM alloys that was shown to exhibit AOS is ferrimagnetic amorphous alloys containing terbium and cobalt (Tb:Co). They have attracted attention due to their strong out-of-plane magnetic anisotropy, high magneto-optical activity and amorphicity, which makes them attractive from a fabrication point of view since a variety of substrates and buffer layers could be used for growing such layers. In this Thesis, TbCo alloys are investigated in order to examine how the magnetic, optical and magneto-optical properties could be tuned by varying the elemental ratio and film thickness. The main question that was addressed here was whether such a system is suitable for fabrication of nanosized magnetic elements as the building blocks for the magnetic memory applications. TbCo alloys were prepared as thin films by magnetron co-sputtering method onto different substrates and buffer layers. Films were characterized using a variety of techniques such as an ion beam analysis, an x-ray reflectivity and diffraction, and magneto-optical characterization techniques. It was observed that the properties of such alloys depend not only on the Tb:Co ratio but also on the film thickness and an underlying buffer layer. Magnetization compensation point, at which the magnetization of a film is zero, as in an antiferromagnet, can be modified depending on the buffer layer. All-optical switching (AOS) of magnetization experiments were performed on the fabricated samples. It was determined that AOS with at least 50-100 laserpulses can be achieved for the films grown directly onto fused silica substrates and with the compositions above the magnetization compensation point at room temperature, in the range of 24 - 30 at.% Tb. In the Outlook, the initial efforts of patterning the films into the arrays of nanosized elements are presented. It is demonstrated that after the lithographic patterning of the films, the resulting nanosized elements remained out-of-plane magnetized. In this work it is shown that the ferrimagnetic TbCo alloy system is a potential candidate material for bothfacilitating AOS and the fabrication of arrays of nanomagnets. Combining the TbCo alloys,which show AOS, together with a suitable buffer layer and patterning the hybrid structure,could enable selective element-by-element magnetization switching for the magnetic memorystorage devices.

Los solitones ópticos son paquetes de luz (haces y/o pulsos) que no se dispersan gracias al balance entre difracción/dispersión y no linealidad. Al propagarse e interactuar los unos con los otros muestran propiedades que normalmente se asocian a partículas. Las propiedades de los solitones ópticos en fibras ópticas y cristales han sido investigadas en profundidad durante las últimas dos décadas. Sin embargo, los solitones en mallas, o redes, ópticas, que podrían ser usados para procesado y direccionamiento totalmente óptico de señales, se han convertido en una nueva área de investigación. El principal objetivo de esta tesis es el estudio de nuevas técnicas para controlar solitotes en medios no lineales en mallas ópticas.El capítulo 2 se centra en ciertas propiedades de los solitones ópticos en medios no lineales cuadráticos. La primera sección presenta en detalle la existencia y estabilidad de tres familias representativas de solitones espacio temporales en dos dimensiones en series de frentes de onda cuadráticos no lineales. Se asume, además de la dispersión temporal del pulso, la combinación de difracción discreta que surge debido al acoplamiento débil entre frentes de onda vecinos. La otra sección da cuenta de la existencia y estabilidad de vórtices de solitones multicolores en retículo, consistentes en cuatro jorobas principales dispuestas en una configuración cuadrada. También se investiga la posibilidad de generarlos dinámicamente a partir de haces de entrada Gaussianos con vórtices anidados. La técnica de inducción de mallas ópticas ofrece un sinfín de posibilidades para la creación de configuraciones de guía de ondas con varios haces de luz no difractantes. El capítulo 3 presenta el concepto de estructuras reconfigurables ópticamente inducidas por haces no difractantes de Bessel mutuamente incoherentes en medios no lineales de tipo Kerr. Los acopladores de dos nucleos son introducidos y se muestra cómo calibrar las propiedades de conmutación de estas estructuras variando la intensidad de los haces de Bessel. El capítulo también discute varios escenarios de conmutación para solitones lanzados al interior de acopladores direccionales multinucleares ópticamente inducidos por apropiadas series de haces de Bessel. Es más, la propagación de solitones es investigada en redes reconfigurables bidimensionales inducidas ópticamente por series de haces de Bessel no difractantes. Se muestra que los haces anchos de solitones pueden moverse a través de redes con diferentes topologías casi sin pérdidas por radiación. Finalmente, se estudian las propiedades de las uniones X, que se crean a partir de dos haces de Bessel intersectantes. La respuesta no local de los medios no lineales puede jugar un papel importante en las propiedades de los solitones. El capítulo 4 trata el impacto de la no localidad en las características físicas exhibidas por los solitones que permiten los medios no lineales de tipo Kerr con una retícula óptica integrada. El capítulo investiga propiedades de diferentes familias de solitones en mallas en medios no lineales no locales. Se muestra que la no localidad de la respuesta no lineal puede afectar profundamente la movilidad de los solitones. Las propiedades de los solitones de gap también se discuten en el caso de cristales fotorefractivos con una respuesta de difusión no local asimétrica y en presencia de una malla inducida.El capítulo 5 trata del impacto de la no localidad en la estabilidad de complejos de solitones en medios no lineales de tipo Kerr uniformes. En primer lugar, se muestra que la diferente respuesta no local de los materiales tiene distinta influencia en la estabilidad de los complejos de solitones en el caso escalar. En segundo lugar, se da cuenta de una serie de resultados experimentales sobre solitones multipolares escalares en medios no lineales fuertemente no locales en 2D, incluyendo solitones dipolares, tripolares y de tipo pajarita, organizados en series de puntos brillantes fuera de fase. Finalmente, el capítulo estudia la interacción entre la no linealidad no local y el acoplamiento vectorial, enfatizando especialmente la estabilización de efectos vectoriales en complejos de solitones en medios no lineales no locales.Por último, el capítulo 6 resume los principales resultados obtenidos en la tesis y discute algunas cuestiones abiertas. / Optical solitons are light packets (beams and/or pulses) that do not broaden because of the proper balance between diffraction/dispersion and nonlinearity. They propagate and interact with one another while displaying properties that are normally associated with real particles. The properties of optical solitons in optical fibers and crystals have been investigated comprehensively during the last two decades. However, solitons in optical lattices, which might be used for all-optical signal processing and routing have recently emerged a new area of research. The main objective of this thesis is the investigation of new techniques for soliton control in nonlinear media with/without an imprinted optical lattice. Chapter 2 focuses on properties of optical solitons in quadratic nonlinear media. The first section presents in detail the existence and stability of three representative families of two-dimensional spatiotemporal solitons in quadratic nonlinear waveguide arrays. It is assumed in addition to the temporal dispersion of the pulse, the combination of discrete diffraction that arises because of the weak coupling between neighboring waveguides. The other section reports on the existence and stability of multicolor lattice vortex solitons, which comprise four main humps arranged in a square configuration. It is also investigated the possibility of their dynamical generation from Gaussian-type input beams with nested vortices. The technique of optical lattice induction opens a wealth of opportunities for creation of waveguiding configurations with various nondiffracting light beams. Chapter 3 puts forward the concept of reconfigurable structures optically induced by mutually incoherent nondiffracting Bessel beams in Kerr-type nonlinear media. Two-core couplers are introduced and it is shown how to tune the switching properties of such structures by varying the intensity of the Bessel beams. The chapter also discusses various switching scenarios for solitons launched into the multi core directional couplers optically-induced by suitable arrays of Bessel beams. Furthermore, propagation of solitons is investigated in reconfigurable two-dimensional networks induced optically by arrays of nondiffracting Bessel beams. It is shown that broad soliton beams can move across networks with different topologies almost without radiation losses. Finally, properties of X-junctions are studied, which are created with two intersecting Bessel beams.Nonlocal response of nonlinear media can play an important role in properties of solitons. Chapter 4 treats the impact of nonlocality in the physical features exhibited by solitons supported by Kerr-type nonlinear media with an imprinted optical lattice. The chapter investigates properties of different families of lattice solitons in nonlocal nonlinear media. It is shown that the nonlocality of the nonlinear response can profoundly affect the soliton mobility. The properties of gap solitons are also discussed for photorefractive crystals with an asymmetric nonlocal diffusion response and in the presence of an imprinted optical lattice.Chapter 5 is devoted to the impact of nonlocality on the stability of soliton complexes in uniform nonlocal Kerr-type nonlinear media. First, it is shown that the different nonlocal response of materials has different influence on the stability of soliton complexes in scalar case. Second, experimental work is reported on scalar multi-pole solitons in 2D highly nonlocal nonlinear media, including dipole, tripole, and necklace-type solitons, organized as arrays of out-of-phase bright spots. Finally, the chapter addresses the interplay between nonlocal nonlinearity and vectoral coupling, specially emphasizing the stabilization of vector effects on soliton complexes in nonlocal nonlinear media.Finally, Chapter 6 summarizes the main results obtained in the thesis and discusses some open prospects.

Esta tesis constituye un estudio detallado y exhaustivo de las propiedades de una variedad específica de ondas ópticas solitarias. Observadas experimentalmente por primera vez en 1995, estas ondas estan formadas por un haz óptico a frecuencia fundamental y su segundo armónico que están ligados entre sí y viajan juntos en el material cuadrático; y son debidas al equilibrio entre la difracción lineal que sufre el haz al propagarse y un término no lineal de segundo orden en la susceptibilidad del medio. Las llamamos por eso solitones ópticos en medios cuadráticos o simplemente 'solitones cuadráticos'. También se les conoce como 'Solitones Multicolor' aludiendo al hecho de que requieren de haces a diferentes frecuencias para formarse.Un medio no-lineal cuadrático tiene por fuerza que ser no-centrosimétrico, lo cual es una variedad de anisotropía. Una gran parte de los materiales no-lineales cuadráticos (los que tienen mayor interés para la industria) son uniaxiales lo que significa que presentan un eje de simetría que suele llamarse eje óptico. De la dirección de un haz relativa a ese eje óptico dependen las características de la propagación del haz en el medio cuadrático no-lineal. Una consecuencia de eso en configuraciones de interés es un desvío ('walk-off') sufrido por el haz respecto a su dirección de propagación inicial al entrar en el material no-lineal.Las propiedades de los solitones cuadráticos 'caminantes' son también estudiadas en la tesis, estableciendo que existe una relación entre la potencia inyectada en el medio y el ángulo de desvío (walking angle).Una parte importante de la tesis está dedicada al estudio a través de exhaustivos experimentos numéricos del potencial de estas ondas solitarias para constituir la base de dispositivos de conmutación y encaminamiento totalmente ópticos que puedan hacer realidad la promesa de la red transparente totalmente óptica. Los experimentos han permitido identificar varias configuraciones de interés con niveles de potencia y dimensiones que permiten plantearse el diseño y construcción de dispositivos comerciales de conmutación y encaminamiento totalmente ópticos basados en solitones ópticos cuadráticos. / This thesis is a comprehensive study of the fundamental properties of a specific kind of optical spatial solitary waves. First observed experimentally in 1995, these solitary waves are formed by an optical beam at a fundamental frequency and its second harmonic which propagate together and are mutually entangled; and are due to a balanced interplay between the beams' linear diffraction and a second-order nonlinear susceptibility of the medium. They are thereby referred as 'Optical Solitons in Quadratic Nonlinear Media' or simply 'Quadratic Solitons', They are also known as 'Multicolor Solitons' recalling that they are formed by beams at different frequencies.A quadratic nonlinear media needs to be non centrosymmetric which is a special kind of anisotropy. A great deal of quadratic nonlinear materials (the most used by industry such as lithim niobate, KTP, etc.) are uniaxial meaning that they feature a symmetry axis known as 'optical axis'. The direction of propagation of an optical beam relative to that axis determines the characteristics of the beam's propagation through the quadratic nonlinear material. A main result of that in some configurations of interest is a walk-off suffered by the beam as it enters the quadratic material.The properties of the families of quadratic solitons in the presence of a linear walk-off (quadratic walking solitons) are studied as well in the thesis stating that there is a relationship between the power injected into the medium and the walking angle, suitable to applications of all-optical switching and routing.An important last part of the thesis is devoted to the study from a practical viewpoint and through extensive numerical experiments of the potential of these solitary waves as the basis of practical all-optical switches and routers which could take the all-optical transparent network to a reality. The experiments have allowed to identify several configurations of interest with power level and dimensions suited to practical applications which could allow the production of commercial all-optical switching and routing devices based on quadratic solitons.