Neste trabalho estudamos uma nova classe de materiais ópticos não-lineares, os cristais orgânicos, que possuem potencial para muitas aplicação em dispositivos. Determinamos diferentes propriedades ópticas lineares e não-lineares de três cristais orgânicos: a l-alanina, a l-treonina e a l-lisina. Esses cristais de aminoácido foram escolhidos por apresentarem propriedades de geração de segundo harmônico (GSH) e por serem materiais nunca estudados sob o ponto de vista da óptica não-linear. Estudamos algumas das propriedades ópticas lineares fundamentais desses cristais biaxiais transparentes, pois deles dependem os fenômenos não-lineares sobre os quais são feitas intensas pesquisas. Determinamos as propriedades lineares como os espectros de absorção, os índices de refração, os eixo ópticos, velocidade de propagação da luz num meio anisotrópico e os limiares de dano por radiação. Posteriormente, determinamos as condições de casamento de fase, a eficiência de GSH, e a auto-modulação de fase. A GSH e a auto-modulação de fase pertencem, respectivamente, a processos não-lineares de segunda e de terceira-ordem. Para o estudo de muitas dessas propriedades foram desenvolvidas novas técnicas experimentais. Uma dessas novas técnicas, a varredura-Z oscilante, teve sensibilidade para determinar, pela primeira vez, o índice de refração não oscilante, vez, o índice de refração não linear desses cristais orgânicos. Além disso, como a todos os processos de caracterização envolveram a preparação de amostras, tivemos a oportunidade de criar procedimentos padrões para manipulação e utilização desses novos materiais. / In this work we studied a new class of nonlinear organic crystals that are potential candidates for devices application. We determined several linear and nonlinear optical properties of three organic crystals: l-alanine, l-threonine and l-lysine. These aminoacid. These aminoacid because their nonlinearities were never studied before. We characterized some of the fundamental linear optical properties of these biaxial transparent crystals, because they have influence on nonlinear phenomena that attracted a lot of research. We determined linear properties like absorption spectra, indices of refraction, optical axes, the light speed propagation in anisotropy media and the optical damage threshold. Subsequently, we determined the phase-matching condition for SHG, efficiency of the SHG and self-phase modulation. The SHG and self-phase modulation belongs, respectively, to the second-order and third-order nonlinear processes. To study several of these properties we had to develop some new experimental techniques. One of them, the oscillatory Z-scan, allows enough sensibility to determine, for the first time, the nonlinear refraction index of these organic crystals. Besides, as all these optical characterization involve the preparation of the samples, we had the opportunity to establishing standard procedures for manipulation of these new materials.

Orientador: Carlos Lenz Cesar / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-17T16:12:03Z (GMT). No. of bitstreams: 1 Pelegati_VitorBianchin_M.pdf: 3778666 bytes, checksum: d19d947cc4b4206345d5c2da244362d6 (MD5) Previous issue date: 2010 / Resumo: Biologia celular é um novo mundo promissor com enorme impacto social, econômico e na saúde. Organismos vivos são capazes de, produzir a própria energia a partir da luz do sol, se reproduzir, de se auto-reparar, sinalizar e navegar por sinais bioquímicos, biomecânicos, luminosos, térmicos, e outros, e produzir materiais à temperatura ambiente. As possibilidades abertas por essa área incluem, desde bactérias e protozoários usados para destruir células de câncer, regeneração de órgãos inteiros, produção de etanol a partir de algas, e outros. Entretanto, para o entendimento da biologia em seu nível mais profundo, ferramentas de observação não destrutivas fazem-se necessária para seguir os processos celulares durante seu tempo de vida. A óptica tem a única onda não destrutiva capaz de fornecer informações em tempo real com suficiente resolução espacial de eventos acontecendo internamente à célula. Ademais, porque feixes de luz não colidem, a óptica permite a integração de diferentes técnicas capazes de reunir informações simultâneas de processos celulares. Óptica não linear é especialmente adequada para tal, pois não requer marcação ou processamentos especiais de amostras que poderiam destruir, ou modificar, os processos celulares. Além disso, técnicas elásticas, como a geração de segundo e terceiro harmônicos não liberam energia no material que é, portanto, preservado após a observação. O objetivo dessa tese é desenvolver uma plataforma multimodal para observação de processos biológicos pelo uso de microscopias de fluorescência excitada por absorção de dois fótons, geração de segundo harmônico e geração de terceiro harmônico no mesmo instrumento. Nosso grupo foi pioneiro em demonstrar a aquisição de imagens de geração de segundo harmônico no Brasil e, essa tese é a primeira a realizar a aquisição de imagens por geração de terceiro harmônico. Estas três técnicas juntas fornecem informações complementares a respeito da organização de células e tecidos. Enquanto a fluorescência pode ser específica para algumas proteínas alvo, o segundo harmônico pode observar a rede de colágeno da matriz extra celular e, o terceiro harmônico pode observar os núcleos e gotículas de lipídios internas às células. Esta tese descreve o sistema experimental para realizar essas aquisições multimodais de imagens, a física por trás dos sinais não lineares, importantes para entender seu significado biológico, e mostra aplicações das técnicas para diferentes amostras biológicas e inorgânicas / Abstract: Cell biology is promising a brave new world with enormous social economic and health impacts. Living organisms are capable of producing their own energy from sun light, reproduce, self-repair, signalize and travel in response to biochemical, biomechanical, light and thermal signals among others, and to produce materials at room temperature. The possibilities opened by this area range from bacteria and protozoa used to destroy cancer cells, whole organs regeneration, ethanol produced from algae, and others. However, to actually understand biology at its deepest level no destructive observation tools are necessary to follow cell processes during their time course. Optics is about the only wave capable to provide non destructive real time information with enough spatial resolution of the events happening inside the cells. Moreover, because light beams do not collide, optics allows the integration of different techniques capable to gather simultaneous information during a cell process. Non linear optics is specially suited for that in the sense that it does not require staining or special sample processing that would destroy, or change, the process. Besides, elastic techniques such as second and third harmonic generation do not release energy at the material which is therefore preserved after the observation. The objective of this thesis is to develop a multimodality platform for biology process observation by using Two Photon Excited Fluorescence, Second Harmonic Generation and Third Harmonic Generation Microscopy with the same instrument. Our group was the first one to demonstrate the acquisition of Second Harmonic Generation images in Brazil and this thesis is the first one to perform the acquisition of third harmonic generation images. These three techniques together provide complementary information respect to cell and tissue organization. While fluorescence can be specific target to some proteins, second harmonic can observe the collagen network of extra cellular matrix and the third harmonic can observe the nucleus and lipid droplets inside the cells. This thesis describe the experimental setup to perform these multimodal image acquisition, the physics behind the non linear signals, important to understand their biological mean, and shows applications of these techniques for different biological and inorganic samples / Mestrado / Física / Mestre em Física

Orientador: Carlos Lenz Cesar / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-11T10:58:51Z (GMT). No. of bitstreams: 1 Thomaz_AndreAlexandrede_M.pdf: 10062018 bytes, checksum: 1e19c55cb5a4e709c2015e2d90f3ac13 (MD5) Previous issue date: 2007 / Resumo: A pesquisa em fotônica biomedica está claramente tomando a direção do entendimento de processos biológicos a nível celular. A resolução necessária para atingir esse objetivo requer praticamente ferramentas fotônicas. Contudo, uma integração de diferentes ferramentes fotônicas e uma aproximação funcional serão necessárias para acessar os processos biomecânicos e bioquímicos celulares. Deste modo nós podemos observar eventos bioquímicos disparados mecanicamente ou eventos mecânicos disparados bioquimicamente, ou até mesmo observar simultâneamente eventos biomecânicos e bioquímicos disparados por outros meios, entre outros, eletricamente. Uma das grandes vantagens das ferramentas fotônicas é a sua facilidade de integração. Nós desenvolvemos uma ferramenta integrada incorporando pinça óptica simples com Microscopia Confocal "Single-photon" e Multifóton. O sistema consegue realizar microscopias de fluorescência excitada pela absorção de dois fótons e geração de segundo harmônico em conjunto com manipulações ópticas. Medidas de força, elasticidade e viscosidade de membranes esticadas podem ser monitoradas em tempo real pelas microscopias confocais, bem como protozoários capturados opticamente, como, por exemplo, Trypanosoma cruzi. Nós mostraremos vários exemplos do uso de tal ferramenta integrada e seu potencial para observar processos mecânicos e bioquímicos a nível celular / Abstract: The research in biomedical photonics is clearly evolving in the direction of the understanding of biological processes at the cell level. The spatial resolution to accomplish this task practically requires photonics tools. However, an integration of different photonic tools and a multimodal and functional approach will be necessary to access the mechanical and biochemical cell processes. This way we can observe mechanicaly triggered biochemical events or biochemicaly triggered mechanical events, or even observe simultaneously mechanical and biochemical events triggered by other means, e.g. electricaly. One great advantage of the photonic tools is its easiness for integration. Therefore, we developed such integrated tool by incorporating single Optical Tweezers with Confocal Single and Multiphoton Microscopies. This system can perform 2-photon excited fluorescence and Second Harmonic Generation microscopies together with optical manipulations. Force, elasticity and viscosity measurements of stretched membranes can be followed by real time confocal microscopies. Also opticaly trapped living protozoas, such as Trypanosoma cruzi. Integration with CARS microscopy is under way. We will show several examples of the use of such integrated instrument and its potential to observe mechanical and biochemical processes at cell level / Mestrado / Física / Mestre em Física

La nano-optique est un vaste domaine permettant d'étudier et d'exalter l'interaction lumière-matière à l'échelle nanométrique. Ce domaine couvre notamment la plasmonique, mais depuis quelques années, un effort est porté sur les nanostructures diélectriques à fort indice de réfraction (typiquement des semiconducteurs comme le silicium). Des effets similaires aux nanoparticules plasmoniques peuvent être obtenus, tels un comportement d'antenne et l'exaltation de phénomènes non linéaires (génération d'harmoniques), avec l'avantage de faibles pertes. Dans cette thèse, une analyse des propriétés optiques linéaires et non linéaires de nanostructures individuelles. Une première partie est dédiée aux nanofils de silicium qui supportent de fortes résonances optiques dont le nombre et la gamme spectrale, du proche UV au proche IR, sont fonction de leur diamètre. Dans ces conditions, l'exaltation du champ proche optique et un rapport surface sur volume élevé favorisent l'apparition de processus non linéaires. Ainsi la génération de seconde harmonique (SHG) par rapport au silicium massif est augmentée de deux ordres de grandeur. En outre, différentes contributions à l'origine de la SHG peuvent être adressées individuellement en fonction de la polarisation du laser d'excitation et de la taille des nanofils. Les résultats expérimentaux sont confrontés à des simulations numériques (méthode dyadique de Green, GDM), qui permettent d'identifier les différentes contributions. Dans une seconde partie, la méthode dyadique de Green est couplée à des algorithme évolutionnistes (EO) pour la conception et l'optimisation de propriétés optiques choisies de nanostructures semiconductrices ou métalliques, par exemple diffusion résonnante de différentes longueurs d'ondes pour différentes polarisations.Des échantillons de nanostructures de silicium, réalisés à partir des résultats de l'EO, vérifient avec succès les prédictions de l'algorithme d'optimisation, démontrant l'énorme potentiel de l'EO pour de nombreuses applications en nanophotonique requérant une optimisation simultanée de différents paramètres. / Nano-photonic structures offer a highly interesting platform to enhance light-matter interaction on a nanometer scale. Recently, high-index dielectric structures have gained increasing attention as possible low-loss alternatives to plasmonic nano-antennas made from noble metals. Furthermore, since non-linear effects offer many unique functionalities like the coherent up-conversion of photons, including the generation of harmonics, many efforts are being made to exploit such phenomena in nano-photonics. In this thesis, an analysis is presented on nonlinear optical effects in individual dielectric structures, specifically in silicon nanowires (SiNWs). Nanowires develop strong optical resonances in the visible and infrared spectral range. In this context, strong enhancement of the optical near-field together with a large surface to volume ratio support the appearance of nonlinear effects. We show that, compared to bulk Si, a two orders of magnitude increase in second harmonic generation (SHG) is feasible and furthermore unravel different polarization and size-dependent contributions at the origin of the SHG. Numerical simulations are carried out to reaffirm these experimental findings for which a numerical technique is presented to describe nonlinear effects on the basis of the Green Dyadic Method (GDM). In the last part of the thesis, the GDM is used together with evolutionary optimization (EO) algorithms to tailor and optimize optical properties of photonic nano-structures. We eventually fabricate samples, based on EO design, and successfully verify the predictions of the optimization algorithm. It turns out that EO is an extremely versatile tool and has a tremendous potential for many kinds of further applications in nano-optics.

Le poling thermique est une technique consistant à appliquer un fort champ électrique (DC) à un substrat de verre chauffé. Après traitement, un champ électrique est figé au sein de la matrice vitreuse, brisant sa centrosymmétrie. La présence de ce champ permet d’accéder à des propriétés d’optique nonlinéaire du second ordre, habituellement interdite dans un milieu centrosymmétrique tel que le verre. En plus des propriétés d’optique nonlinéaire, la présence du champ électrique a été associée à des modifications structurelles et compositionnelles mais également à des changements de propriétés de surface. Notre objectif a été d’utiliser cette technique pour modifier les propriétés de réactivité de surface et optique de verres d’oxyde (borosilicate et borophosphate de niobium (BPN)) et de verres de chalcogénures. Après poling, les modifications structurelles ont été caractérisée par spectroscopie vibrationnelle Raman et infrarouge. L’intensité et la localisation du champ électrique ont été caractérisées par des techniques de génération de seconde harmonique (SHG) : une analyse quantitative avec les franges de Maker et une d’imagerie μSHG. Le traitement a permis d’augmenter localement la réactivité de surface du verre borosilicate. Dans les verres BPN et chalcogénures, le traitement a permis de contrôler les propriétés optiques à la fois linéaire et nonlinéaire à l’échelle micrométrique. Ces résultats permettent d’envisager l’utilisation du poling thermique pour des applications en photonique intégrée. / Thermal poling is a technique which consists in the application of a strong DC electric field to a heated glass substrate. Following the treatment, a static electric field is frozen inside the glass matrix, effectively breaking its centrosymmetry. Presence of the electric field allows for second order non-linear optical properties usually forbidden in centrosymmetric medium such as glasses. In addition to nonlinear optical properties, the presence of the electric field has been associated with structural/compositional modifications as well as surface property changes. Our objective was to use this technique to tailor surface reactivity and optical properties in oxide (borosilicate and niobium borophosphate) and chalcogenide glasses. After poling, structural modifications were investigated using Raman and infrared spectroscopy. Strength and localization of the electric field were characterized by Second Harmonic Generation (SHG) techniques: quantitative Maker fringes analysis and μSHG imaging. The treatment has successfully allowed to locally enhanced the surface reactivity of a borosilicate glass. In niobium borophosphate and chalcogenide glasses, the treatment has permitted to control optical properties both linearly and non-linearly at the micrometric scale. These results show that thermal poling could be used to create functional devices for applications in integrated photonics.

Le manuscrit présente une étude de la réponse non linéaire de films moléculaires chiraux. Ces recherches s'inscrivent dans le contexte général d'une meilleure compréhension de la brisure de symétrie dans les films moléculaires et l'apparition de la chiralité. Ces systèmes moléculaires laissent envisager des perspectives intéressantes notamment le développement d'une nouvelle technologie de guides optiques chiraux.Dans un premier temps, la description des systèmes moléculaires, la fabrication des films ainsi que la modélisation de la réponse non linéaire sont présentées. Dans une seconde partie, les études menées sur des monocouches moléculaires formées sur une interface liquide ont démontré la possibilité de suivre la transition entre une chiralité intrinsèque à la molécule vers une chiralité supramoléculaire induite par une compression mécanique de la monocouche. Il s'avère que ce processus est réversible si la compression du film n'est pas maintenue. Dans une troisième partie, l'analyse du signal optique non linéaire provenant de films solides minces a mis en avant la présence d'une structuration et d'une orientation particulière des molécules dans ce film / The manuscript presents a study of the nonlinear response of chiral molecular films. This research is within the general context of a better understanding of the symmetry breaking in molecular films and the emergence of chirality. These molecular systems presage interesting perspectives including the development of a new chiral optical guides technology.Initially, we describe molecular systems, films formation as well as the modeling of the nonlinear response. In the second part, studies of molecular monolayers formed on a liquid interface demonstrated the ability to monitor the transition between intrinsic chirality of molecule to a supramolecular chirality induced by mechanical compression of the monolayer. It turns out that this process is reversible if the films compression is not maintained. In the third part, the analysis of the nonlinear optical signal from thin solid films bring out the presence of a particular structure and orientation of molecules in the film

Ce manuscrit décrit le processus optique non linéaire de Génération de Second Harmonique (acronyme anglais SHG pour Second Harmonic Generation) réalisé en phase liquide. En particulier, la propriété de cohérence de ce processus est étudiée en détail. En effet, en raison de la parité du processus SHG, cette cohérence est perdue dans les liquides. Ces études portent ainsi sur plusieurs géométries afin d’accéder aux réponses de volume et de surface. Les avantages d’une méthode combinée sont aussi discutés. Dans une première partie, la configuration classique de la Diffusion Hyper Rayleigh (acronyme anglais HRS pour Hyper Rayleigh Scattering ou SHS Second harmonic scattering) est utilisée pour l’étude de systèmes simples comme les solvants purs. Cette étude porte en particulier sur les méthodes de normalisation des hyperpolarisabilités moléculaires. Une voie alternative est proposée basée sur l’introduction d’une section efficace HRS ou SHS. Par la suite, les effets liés aux ordres non linéaires supérieurs sont démontrés et interprétés, ceux-ci pouvant altérer les valeurs absolues qui peuvent être mesurées.La seconde partie porte sur l’introduction de plusieurs améliorations dans les mesures HRS ou SHS. En particulier, l’évolution vers des géométries non conventionnelles s’éloignant de la géométrie standard ainsi que sur des évolutions permettant d’accéder aux aspects dynamiques sont proposées. Des expériences sont réalisées sur des systèmes simples comme les solvants purs, des composés moléculaires et des nanoparticules, tous présentant des spécificités propres. Cette partie finit sur l’exploration des systèmes aléatoires diffusants et les problèmes associés. Enfin, dans une dernière partie, la réponse HRS ou SHS de solutions aqueuses de sels est discutée. Les mesures portent plus particulièrement sur une étude comparée des aspects cohérents et incohérents et les propriétés qu’il est possible de mesurer dans ces systèmes / This manuscript describes the nonlinear optical process of Second Harmonic Generation (SHG acronym for Second Harmonic Generation) carried out in liquid phase. In particular, this process coherence property is studied in detail. Indeed, due to the parity of the SHG process, this coherence is lost in liquids. These studies covers several geometries to access the volume and surface responses. The benefits of a combined method are also discussed. In the first part, the typical configuration of the Hyper Rayleigh Scattering(HRS or SHS for Second harmonic scattering) is used for the study of simple systems like pure solvents. This particular study focuses on methods of standardization of molecular hyperpolarizabilities. An alternative route is proposed based on the introduction of a HRS or SHS cross. Subsequently, the effects of the higher nonlinear orders are demonstrated and interpreted, they can alter the absolute values that can be measured.The second part deals with the introduction of several improvements in the HRS or SHS measurement. In particular, the trend towards unconventional geometries away from the standard geometry as well as developments for accessing dynamic aspects are proposed. Experiments are performed on simple systems such as pure solvents, molecular compounds and the nanoparticles, all presenting specificities. This part ends on exploring the random scattering systems and associated problems. Finally, in the last part, the HRS or SHS response of aqueous salt solutions is discussed. The measures focus specifically on a comparative study of coherent and incoherent aspects and properties that can be measured in these systems

Pas de résumé en français / Being compared with oxide glasses, chalcogenide glasses have fine infrared transmissivity and higher optical nonlinearity, and also could be drawn into optical fibers. So chalcogenide glasses and fibers have potential wide applications in the fields of all-optical information processing, infrared lasers, nonlinear optical devices, and so on, the studies of their optical nonlinearity are one of the attractive subjects in the area of optoelectronics at present. The main purpose of this paper is to improve the stability and enhance the intensity of nonlinearity in chalcogenide glasses and fibers by means of exploring new glass compositions, optimizing the external field poling method, designing and fabricating fibers with special structures, all of these will promote their real applications. The main results are concluded as follows . The glass-forming region of GeS2-GA2S3-AgX (X=Cl, Br, I) and GeS2-Ga (In)2S3-CuI systems were determined , the maximal content of the additive halides are 70% and 12% respectively. In both two systems glasses, with the increasing addition of halides, the thermal stability reduce, density and linear refractive index increase, the ultraviolet cut-off edges shift to longer wavelength, while the infrared cut-off edges keep almost the same. 30GeS2 35Ga2S3 35AgCl and 47.5GeS2 17.5Ga2S3 35AgCl surface- and bulk-crystallized glasses that contain AgGaGeS4 nonlinear optical crystallites were prepared. Obvious second harmonic generation (SHG) could be observed in these crystallized glasses, and their intensity relate to the distribution and size of the precipitated AgGaGeS4 crystals, the maximal second-order nonlinearity coefficients is as high as 12.4pm/V. These crystallized glasses have good chemical and SHG stability. For GeS2-Ga (In)2S3-CuI systems glasses, due to their small glass-forming region, they are not suit for the preparation of crystallized glasses that contain CuGaS2 or CuInS2 nonlinear optical crystals. According to the structural studies of two system glasses, the main structural units of theses glasses are [YS4-xXx] (Y=Ge, Ga, In. X=Cl, Br, I) mixed anion tetrahedrons, they form a three-dimensional glassy network through bridging sulphur bonds. When the contents of halides MX(M=Ag, Cu. X=Cl, Br, I) are low, some [XxS3-xGe(Ga)S3-xXx] (X=Cl, Br, I) mixed ethane-like structural units exist in the glass network, and they will gradually transform to [YS4-xXx] (Y=Ge, Ga, In. X=Cl, Br, I) mixed anion tetrahedrons with the increasing content of halides, till totally disappear. Both two system glasses have ultrafast (~150fs) third-order optical nonlinearity and reverse saturation absorption, they belong to self-focusing medium. The third-order optical nonlinearity mainly originate from the distortion of electron cloud of Y-X (Y=Ge, Ga, In, X=Cl, Br, I, S) bonds in the structural units. For GeS2-GA2S3-AgX (X=Cl, Br, I) system glasses, the largest nonlinear susceptibility n2 is 10.50x10-18 m/W, the smallest figure of merit (FOM) is 0.606. In addition, the relation of n2 with n0 do not obey Miller’s rule, but in accordance with the structural variation. Among the glass compositions with different additive halogens, Br-containing glasses have relatively best third-order nonlinearities. For GeS2-Ga (In)2S3-CuI system glasses, the largest nonlinear susceptibility n2 is 9.37x10-18 m/W, the smallest figure of merit (FOM) is 2.237. High purity AS2S3 glass performs and low loss single index fibers with diameter of 100~400µm that drawn form these performs were prepared, the transmission losses between 2~6 µm is only 0.5dB/m. AS2S3 tapered fibers have a uniform diameter of taper wasit, fine surface smoothness, and sharp taper transition part.

Nonlinear optical microscopy is a collection of very powerful imaging techniques. Linear optical microscopes probe the refractive index and absorption, which both stem from the first-order linear electric susceptibility. Especially in biological tissue, the variation in the refractive index is often small and the tissue is, in many cases, transparant. Nonlinear optical microscopes on the other hand probe the nonlinear higher-order susceptibilities, which can be chemically sensitive, leading to the capability to achieve label-free imaging. Nonlinear optical microscopes have been in development for more than thirty years and they are based on numerous nonlinear optical processes. The ones I will concentrate on in this thesis are second harmonic generation (SHG), coherent anti-Stokes Raman scattering (CARS), and stimulated Raman Scattering (SRS). The first technique is commonly used to image collagen as those molecules have a particularly large second-order nonlinear susceptibility due to their chiral structure. CARS and SRS on the other hand are often used because they resonantly target vibrational resonances in molecules, giving rise to the aforementioned label-free imaging. Deep understanding of the nonlinear imaging process is crucial to the interpretation of the images these techniques produce. Computational tools are exceptionally suited for this task as they allow studying the electromagnetic field anywhere in the sample as well as the far-field, and one can change any of the material properties to study their effect. One such tool is finite-difference time-domain (FDTD) that our group developed for nonlinear optical microscopy simulations. It is a direct discretization of Maxwell's equation. While computationally costly, it does allow any arbitrary shaped sample to be simulated. The sample can have frequency dependent refractive indexes, and also nonlinear media with third-order nonlinearities such as Kerr media and Raman-active media, but also second-order nonlinearities for SHG. The code is designed in such a way that it can run on thousands of CPUs on a wide variety of compute cluster which allows our group to obtain nanoscale resolution. Another computational tool I use is the free-space Green's function solution to the Helmholtz equation, which can be used to calculate the Hertz vector in the frequency domain, both in the near- and far-field, based on the induced nonlinear polarization. The electric field is then calculated from this Hertz vector. This technique is much faster then FDTD and also allows for arbitrary shapes of the nonlinear electric susceptibility in the sample. However, it assumes a homogeneous refractive index throughout the entire spatial domain and requires complete knowledge of the input beam or beams that induce the nonlinear polarization. In this thesis, I use these tools to study the image formation process of various nonlinear optical processes mentioned earlier. For example, I study the effect of an inhomogeneous refractive index on the images produced by these microscopes. In literature the index of refraction is almost always assumed to be homogeneous, because, as mentioned before, the inhomogeneity of the refractive index is often small. However, I show that these small differences in the index of refraction can have a significant effect on the measured far-field intensity signal. For example, in SRS and CARS images, the measured signal can increase by an order of magnitude depending on the index mismatch and structure of the sample. Additionally, significant shifts in perceived position occur. Even nonresonant nonlinear signals can be evoked purely through a mismatch in linear refractive index. Computational modelling can also help reveal additional detail. As SHG is a coherent process, subwavelength information can be inferred through the phase information. Our experimental collaborators built an interferometric SHG (I-SHG) microscope for exactly that purpose. We used this to image collagen fibrils, which are all aligned in a parallel fashion. However, because collagen fibrils have a chiral molecular structure, they can point either ``up'' or ``down''. Using my Green's function simulations of the SHG imaging process of collagen fibrils, I was able to predict the standard deviation in the measured phase and link it to the orientation of collagen fibrils in the focal spot of the probing laser beam, even though the diameters are far below the minimum resolvable capabilities of the microscope. We found that the ``upwards'' fibrils make up 46--53% of the sample. Even with a normal SHG microscope that does not measures phase, additional subresolution information is obtainable. With our collaborators we measured the ratio of the forward SHG intensity signal to that in the backward direction and with my simulations, we are able to link this to the fibril diameters in collagen tissue. Thus we inferred that the fibril diameter increases as a function of tissue depth. Furthermore, a computational technique called ptychography is able to retrieve phase information without an interferometric reference beam. Additionally, it increases resolution to the theoretical limit, independent of the laser focal spot size, and corrects for distortions in the input beam as well. I have developed this technique for use with nonlinear optical microscopy and was able to show it is a viable alternative to I-SHG by imaging simulated rat tail tendon at the diffraction limit while retrieving the orientation of the fibrils through the phase of the SHG signal. I also implemented the algorithm for CARS, where the phase information can be used to greatly increase the signal-to-noise ratio by reducing the nonresonant background radiation that results from competing nonlinear optical processes. I showed an example of this by imaging a simulated fibroblast cell where the CARS process was tuned to the lipid droplets inside of the cell. I am currently in talk with experimentalists to apply this theoretical technique to experiments as that would further demonstrate the impact of my work. Finally, keeping in theme with the collagen fibrils, I show that the ratio of the forward SHG signal to the backward signal, the F/B ratio, is affected by a mismatch in the refractive index for fibrils larger than 100nm. This measure is an indicator of fibril diameter and thus important for making qualitative predictions. Single fibrils are generally too small to be significantly affected by near-field effects, but the bigger fibrils can be. Fibrils in rat tail tendon have a distribution of fibrils diameters and the large fibrils occur infrequent. However, I found that the large fibrils are largely responsible for the forward as well as backward signal, thus refractive index mismatches still affect the F/B ratio significantly despite their infrequency. The F/B ratio for a collection of fibrils placed in a n=1.47 medium was found to be 31.8±0.7% higher than for those in a n=1.33 medium. Our experimental colleagues have done preliminary measurements on mouse tail tendon where they found an increase of 40±20%, in line with the value of 28.1±0.6% that I found for simulations with mouse tail tendon. In conclusion, the theoretical tools I have used in my thesis have provided me with the ability to study nonlinear optical image formation processes with a level of detail that would be near-impossible to do experimentally. I have used this ability to show how refractive index mismatches, such as those found in biological tissue, can significantly distort the far-field intensity signals. I have shown this for SRS and CARS where the far-field intensity signal appeared an order-of-magnitude larger compared to the same sample without a refractive index mismatch with the background medium. Additionally, shifts in the perceived position of the object under investigation were observed and I showed the presence of a nonresonant background signal in AM-SRS. Likewise I showed that in the SHG imaging of collagen fibrils significant changes in the F/B ratio can occur. All of these effects have important implications as these types of images as biomedical researches rely on the correct interpretation of nonlinear optical microscopy images for both research and diagnostics. Apart from showing the effect of a refractive index mismatch, I have also shown that computation modelling can be used to infer subwavelength features in SHG imaging experiments of collagen fibril such as fibril orientation and fibril diameter. These methods have the potential to aid medical researchers as changes in the structure of collagen are often an early indicator of diseases such as osteoarthritis. Finally, I showed that the ptychography algorithm I developed for nonlinear optical microscopy is able to retrieve phase information of the nonlinear electric susceptibility in SHG and CARS imaging while also enhancing the resolution and correcting for distortions in the input beams. I can also use much larger laser spot sizes than in conventional experiments without compromising the obtained resolution, thus fewer measurements are required. The technique is not limited to SHG and CARS either; it will work for other nonlinear optical processes as well. Experimental verification of nonlinear ptychography will be done soon. This technique has to potential to significantly improve current imaging techniques since access to the phase information allows one to observe additional information about the sample as we showed with the I-SHG microscope.

Ce travail concerne le développement d’un nouveau type de microscopie optique en champ proche (SNOM) basé sur la mise en œuvre de sondes dite actives qui utilisent le signal de génération de seconde harmonique (SHG) d’un petit nombre de molécules orientées. L’orientation de ces molécules est obtenue par l’application d’un champ électrique statique dans une jonction constituée d’une pointe métallique effilée placée à proximité d’un substrat conducteur et immergée dans une solution de molécules dipolaires non-linéaires. L’excitation laser de ces molécules localement orientées permet d’obtenir une polarisation non-lineaire à fréquence double qui constitue une nanosource de lumière intrinsèquement localisée et pouvant interagir avec le champ proche du substrat. Nous nous sommes intéressés à l’imagerie de nano-objets lithographiés par cette technique de SNOM-SHG. Nous avons pu démontrer la possibilité d’obtenir une résolution de l’ordre de 200 nm, soit une résolution meilleure d’un facteur 2 par rapport à la limite de diffraction.Nous avons ensuite étudié les moyens d’optimiser les performances de ce nouveau type de sondes SNOM-SHG. Une voie consiste à exploiter les propriétés d’antenne optique de pointes métalliques effilées, qui peuvent être le siège d’effets d’exaltation du champ électromagnétique résultant de la singularité géométrique de ces objets (extrémité effilée) ou de l’excitation de résonances plasmons. Afin de pouvoir quantifier ces effets, nous avons entrepris la caractérisation, par luminescence à 2 photons (TPL), de nanofils d’or considérés comme objets de référence pour mimer une pointe. Des fils lithographiés ainsi que des fils issus de chimie colloïdale ont été étudiés de façon à mieux comprendre à la fois l’influence de la forme et de la cristallinité des objets sur les exaltations de champ. Des études simultanées de la géométrie et des propriétés optiques d'un nanofil unique ont été menées au moyen d'un microscope optique inversé associé à une excitation laser et couplé à un microscope à force atomique (AFM) dont la pointe est préalablement réglée pour coïncider avec le spot laser. En balayant l’échantillon, nous pouvons directement confronter l’image topographique de l’objet à la cartographie de points chauds enregistrés à sa surface, le signal de TPL étant directement corrélé à la densité locale d’états électromagnétiques. Nous avons pu montrer que les fils lithographiés et les fils colloïdaux présentaient des facteurs d’exaltation locale de champ différents, la cristallinité des objets pouvant aussi être révélée que via l’analyse spectrale du signal de TPL émis. Enfin, un dernier volet important de mon travail a consisté à faire évoluer le banc expérimental précédemment développé au laboratoire de façon à pouvoir réaliser simultanément des caractérisations de type SNOM-SHG et des caractérisations topographiques. Dans ce but, nous avons travaillé à l’intégration d’une tête AFM diapason sur notre banc de microscopie non-linéaire. Au-delà des aspects électroniques liés à l’optimisation du fonctionnement de ce diapason, le couplage du faisceau laser dans le microscope a également été entièrement reconfiguré. / This work deals with the development of a new kind of scanning near-field optical microscopy (SNOM) based on the realization of so-called active probes taking advantage of the second harmonic generation (SHG) signal coming from a few oriented molecules. The orientation of these molecules is obtained by applying a static electric field in a junction made of a sharp metallic tip placed close to a conductive substrate and immersed in a solution containing dipolar non-linear molecules. A second order nonlinear polarization is obtained from these locally oriented molecules following their excitation with a laser beam finally leading to a nanosource of light intrinsically localized and able to interact with the near-field of the substrate.We have investigated this SNOM-SHG technique to image nano-objects made by e-beam lithography. We were able to demonstrate that a resolution of about 100 nm could be reached, which appears better (of a factor2) than the diffraction limit.We have then been focusing on the way to improve the capabilities of this new type of SNOM-SHG probes. One approach consists in taking advantage of the optical antenna effects that can occur at the end of sharp tips, where the electromagnetic field can be enhanced due to geometrical effects (sharp extremities) or due to the excitation of plasmon resonances. In order to quantify these field enhancements, we have carried out the characterization of gold nanowires using two-photon luminescence (TPL) ; considering these wires as reference objects that can mimic tips. Nanowires made by e-beam lithography and nanowires synthesized by colloidal chemistry have both been studied in order to have a better understanding of the influence of the shape and the crystallinity on the field enhancements. Simultaneous analysis of the geometry and the optical properties of a single nanowire has been carried out using an inverted microscope associated to a laser excitation and coupled to an atomic force microscopy (AFM) which tip is previously aligned with the laser spot. When scanning the sample, we can directly correlate the topographic image of the object to the mapping of the hotspots recorded on its surface, the TPL signal being directly linked to the electromagnetic local density of states. We were able to evidence that both nanowires made by e-beam lithography or synthesized by colloidal chemistry exhibit different field enhancement factors, the crystallinity of the objects being also revealed following the spectral analysis of the emitted TPL signal.Finally, a last important part of my work has dealt with the evolution of the experimental setup previously developed in the laboratory in order to be able to achieve simultaneously SNOM-SHG type and topographic characterizations. We have therefore been working on the integration of an AFM tuning fork head to our nonlinear optical bench. Above the electronic aspects related on the optimization of the tuning fork implementation, the coupling of the laser beam in the microscope has also been reconfigured.