This thesis descibes a theoretical study of nonlinear self-focusing as applied to the metrology of the nonlinear optical parameters of a medium. It also studies the phe- nomenon of optical power limiting which utilizes self-focusing e ects. As an analytical tool, a mode decomposition method which uses an orthogonal and complete set of Gaussian-Laguerre modes as a basis set is used to treat these problems. Nonlinear media both in the thin and thick limits are investigated. For thin media, a closed form expression is derived which describes the optical eld of an initally Gaussian beam that is perturbed by a thin nonlinear material which exhibits nonlinear absorption as well as nonlinear refraction. This result is valid for any regime of nonlinearity in the thin medium approximation. Thick media are treated using a numerical extension of the Gaussian-Laguerre Mode Decomposition technique. Spatial scanning techniques such as the Z-scan that rely on self-focusing e ects and that are used to measure the nonlinear optical parameters of a material are studied in detail. Optical limiting in both thick and thin media is also investigated.

>Magister Scientiae - MSc / Recent research shows that the study of optical properties of organic material natural dyes has gained much consideration. The specific functional groups in several natural dyes remain essential for the large nonlinear absorption expressed in terms of nonlinear optical susceptibilities or other mechanism of absorption such as two photon absorption (TPA), reverse saturable absorption (RSA) or intensitydependent refractive index characteristic. In this thesis we highlight the optical limiting responses of selected natural dyes as nonlinear response in the femtosecond regime. This technique refers to the decrease of the transmittance of the material with the increased incident light intensity.Three dyes derived from beetroot, flame flower and mimosa flower dyes were investigated. The results showed a limiting behaviour around 795 mW for the beetroot and the flame dye while there is total transmission in the flame dye sample. The performance of the nonlinearity i.e. the optical limiting is related to the existence of alternating single and double bonds (i.e. C-C and C=C bonds) in the molecules that provides the material with the electron delocalization, but also it is related to the light intensity.Beside nonlinearity study, crystallographic investigation was carried out for more possible applicability of the selected dyes and this concerned only the mimosa and flame flower dye thin film samples since the beetroot thin film was very sensitive to strong irradiation (i.e. immediately destroyed when exposed to light with high intensity). For more stability,dye solutions were encapsulated in gels for further measurements.

Limitadores ópticos são dispositivos que apresentam alta transmitância para baixas intensidades de luz incidente e, inversamente, baixa transmitância para altas intensidades, sendo usados como protetores contra danos em sensores ópticos e olhos humanos. Com o objetivo de estudarmos novos materiais com mecanismos não lineares eficientes para o processo de limitação, bem como novas geometrias ópticas, apresentamos neste trabalho o desenvolvimento de um limitador óptico baseado na absorção saturada reversa de porfirinas e metaloporfirinas. Construímos também um dispositivo baseado na configuração óptica bifocal, usando porfirina como absorvedor saturável reverso. Além disso, obtivemos parâmetros espectroscópicos do estado excitado pelos ajustes teóricos das curvas de transmitância, através da resolução de equações de taxa. / Optical limiters are devices with a high transmittance in the presence of low intensity light, and a considerable lower transmittance otherwise, been used as protectors against damage in optical sensors and human eyes. Aiming at searching for new materials and optical geometries efficient for optical limiting. We report on the development of an optical limiter based on the reverse saturable absorption in porphyrin and metalioporphyrins. We also built a device-based .on a cascade focus configuration, using free base porphyrin as the absorber. Besides, we obtain the excited state spectroscopy parameters by the theoretical fitting of transmittance curves, by solving a set of rate equations.

No description available.

Neste trabalho, com objetivo de desenvolvermos dispositivos de limitação óptica, estudamos o processo de absorção de estado excitados em um grupo de tetrapiridil porfirinas com diferentes substituintes centrais (2H+, Zn2+, Ni2+, Cu2+). Para isso utilizamos a técnica de varredura – Z com pulsos de femto e picossegundos bem como com trem de pulsos. Desta forma, determinamos a dinâmica de absorção de estados excitados, para estas moléculas, obtendo parâmetros espectroscópicos tais como tempo de cruzamento intersistema, e as seções de choque de absorção dos primeiros estados excitados singleto e tripleto. Além disso, visando aproveitar os processos ópticos acumulativos de alguns materiais não lineares, em aplicação de limitação óptica, desenvolvemos uma nova geometria óptica denominada de limitador óptico de múltiplas passagens, a qual utiliza dois espelhos esféricos confocais e um espelho plano posicionado paralelamente a linha que une os espelhos esféricos. / In this work, aiming to develop optical limiting devices, we have studied excited state absorption process in a group of tetrapyridyl porphyrins with different central substitutes (2H+, Zn2+, Ni2+, Cu2+). We have employed Z-scan technique with femto and picosecond pulses as well as with pulse trains to determine the excited state absorption dynamics and obtain some spectroscopic parameters like intersystem crossing time and triplet and singlet excited state absorption cross-sections. Moreover, we have developed a new geometry for an optical limiter, named multipass optical limiter. It is formed by two confocal spherical mirrors together with a plane mirror placed in parallel with the line joining them.

Neste trabalho, com objetivo de desenvolvermos dispositivos de limitação óptica, estudamos o processo de absorção de estado excitados em um grupo de tetrapiridil porfirinas com diferentes substituintes centrais (2H+, Zn2+, Ni2+, Cu2+). Para isso utilizamos a técnica de varredura – Z com pulsos de femto e picossegundos bem como com trem de pulsos. Desta forma, determinamos a dinâmica de absorção de estados excitados, para estas moléculas, obtendo parâmetros espectroscópicos tais como tempo de cruzamento intersistema, e as seções de choque de absorção dos primeiros estados excitados singleto e tripleto. Além disso, visando aproveitar os processos ópticos acumulativos de alguns materiais não lineares, em aplicação de limitação óptica, desenvolvemos uma nova geometria óptica denominada de limitador óptico de múltiplas passagens, a qual utiliza dois espelhos esféricos confocais e um espelho plano posicionado paralelamente a linha que une os espelhos esféricos. / In this work, aiming to develop optical limiting devices, we have studied excited state absorption process in a group of tetrapyridyl porphyrins with different central substitutes (2H+, Zn2+, Ni2+, Cu2+). We have employed Z-scan technique with femto and picosecond pulses as well as with pulse trains to determine the excited state absorption dynamics and obtain some spectroscopic parameters like intersystem crossing time and triplet and singlet excited state absorption cross-sections. Moreover, we have developed a new geometry for an optical limiter, named multipass optical limiter. It is formed by two confocal spherical mirrors together with a plane mirror placed in parallel with the line joining them.

The demand for protection of eyes and various types of optical sensors from laser-beam pulses has resulted in the search for optical limiting devices that have the property of being transparent at low intensity of light (normal light), but non-transparent towards high intensity (laser) light. This type of protection may be obtained by using an organic material that displays nonlinear optical (NLO) properties. Examples of NLO effects that can be used for optical limiting are reverse saturable absorption (RSA), two-photon absorption (TPA) and nonlinear refraction. The advantage of using compounds that show such NLO effects is that they can have very fast response and are self-activating, that is, there is no need for externally controlled switching to obtain optical limiting. In this work, several dialkynyl substituted thiophenes and some thiophenyl-alkynyl-platinum(II)-complexes were synthesized and tested for nonlinear absorption of light. A palladium-copper mediated coupling (Sonogashira coupling) was utilized for all reactions between terminal alkynes and aryl halides. Molecular orbital calculations were used in order to screen for suitable properties, such as the second hyperpolarizability, in compounds of interest. A quantitative structure-activity relationship (QSPR) study using a PLS approach were performed in order to identify important molecular electronic variables for optical limiting of organic compounds.

This thesis described the synthesis and characterization of new perylene diimide (PDI)-based photonic and electronic materials. In the first part of this thesis, PDI-based polynorbornenes, including PDI-grafted homopolymers and block-copolymers (BCPs) were synthesized and characterized as alternative acceptors for fullerenes for organic electronics. It was found that the PDIs on the polymer side-chains affect π-π stacking with the neighboring PDIs, which has implications for the use of these materials for organic field-effect transistors (OFETs) and organic photovoltaic devices (OPVs). It should be noted that the performance of solar cell based on these materials was poor, like other similar materials. The major reasons could be the challenge in controlling the molecular alignment of the PDI-based materials, which leads to lower electron mobilities in films compared to devices with fullerene-based acceptors. One PDI-grafted BCP showed better OPV performance compared to the other BCPs and respective homepolymer blends, presumably due to favorable morphology. In the second part of this thesis, photo-induced charge-separation in blends of poly-3-hexyl-thiophene (P3HT) and various PDI derivatives have been studied. Probing of long-lived photo-generated PDI radical anions provided insight on these photo-induced processes and their use for OPVs. In the third part of this thesis, the use of photo-generated PDI radical-anion absorption was shown to be effective for optical limiting of nanosecond laser pulses between 650 - 800 nm. In Chapter 5, an effective approach for two-photon absorption (2PA)-induced optical limiting using donor-PDI dyads through which donors and acceptors can be independently chosen to maximize optical suppression at particular wavelengths has been demonstrated. In Chapter 6, conjugated polymers with PDI pendants and poly(carbazole-alt-2,7-fluorene) main-chains were synthesized for optical limiting using the photo-generated PDI radical anion via PDI aggregate excitation and/or 2PA from the polymer backbones. It was also found that nitro-phenyl group or similar derivatives could be good candidates to incorporate into those donor-conjugated polymers, which have significant overlap between their 2PA band and respective polaron absorptions for 2PA-indced optical limiting. / Thesis advisor has approved the addition of errata to this item. Corrections were made to pages 95, 98 and 101.

This thesis is concerned with the material processing, photophysical and third-order nonlinear optical responses, and applications of a set of conjugated polymers in the telecommunication regions. Polyacetylene-based third-order nonlinear optical materials were chosen as candidates for all-optical signal and image processing. Substituted polyacetylenes were obtained using ring-opening metathesis polymerization of mono-substituted cyclooctatetraenes. Polymerization and processing conditions have been developed to generate thick, large-area films possessing large third-order nonlinearities in the telecommunication bands. The good optical quality of a 200 μm thick substituted polyacetylene film allowed for image correlation via off-resonant degenerated four-wave mixing with improved diffraction efficiency. Poly(2-methoxy-5-(2-ethyl-hexyloxy)-(phenylene vinylene)) (MEH-PPV) and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) composites showed strong nonlinear absorption and potential as optical limiters in the region of 700-900 nm. High optical quality, thick film of MEH-PPV:PCBM with the plasticizer dioctylphthalate (DOP) were made. Optical limiting of femtosecond and nanosecond pulses in the near infrared on these composites showed strong power suppression over a broad temporal regime. Femtosecond and nanosecond transient studies on the same thick MEH-PPV:PCBM:DOP composite films and the experimental results showed evidence for the photogeneration of radical ions as being responsible for the enhanced nonlinear absorption and strong optical suppression in the near infrared. Dithienopyrrole-based donor-acceptor copolymers with narrow bandgap showed strong nonlinear absorption and potential as optical limiters in the telecommunication wavelengths. Molecular engineering was applied to manipulate the spectral overlap of two-photon absorption and subsequent nonlinear absorptions. Femtosecond transient spectroscopy showed near infrared transient absorption and 22 - 61% yields of photogenerated charge-transfer species depending on donor-acceptor coupling strength. Torsional fluctuations of the backbone structure potentially affected the excited state behavior. Evidence suggests that ultrafast relaxation occurs to ground state and to long-lived charge-transfer state from the initially excited state. The dispersion of nonlinear absorption measured using the Z-scan method revealed large two-photon absorption cross sections of these polymers in the telecommunication region. Large suppression of nanosecond pulses at 1064 nm was achieved.

Le procédé sol-gel permet la réalisation de matériaux optiquement performants et la possibilité d’intégrer diverses molécules dans ces systèmes ouvre les portes à des applications dans de nombreux domaines. Nous nous sommes intéressés en particulier à la réalisation de matériaux sol-gel monolithiques, fortement dopés avec des molécules actives, dans le cadre de la réalisation de limiteurs optiques performants dans le visible et dans l’infrarouge.La mise au point de procédés et de matrices sol-gel compatibles avec divers types de chromophores a tout d’abord été effectuée. Une investigation des paramètres expérimentaux et de la nature des précurseurs influençant la microstructure des matériaux a été réalisée, ainsi que la mise au point de méthodes de gélification accélérée capables de piéger efficacement les chromophores dans les matrices sol-gel même à de très hautes concentrations.Le dopage des matrices obtenues avec différents chromophores pour la limitation optique dans le visible et dans l’infrarouge s’en est suivi, avec une étude de l’impact des matrices sur les propriétés optiques des dopants. L’évaluation des performances en limitation a révélé les performances exceptionnelles des systèmes étudiés dans le visible, et des résultats prometteurs pour l’infrarouge.Enfin, la synthèse de nanoparticules d’or isotropes et anisotropes et leur incorporation dans les matériaux préparés a permis de mettre en évidence des effets d’exaltation importants sur les propriétés non-linéaires des chromophores au sein des matrices sol-gel et permettent d’améliorer les performances en limitation de manière considérable. / Sol-gel chemistry is a potent approach for the realization of optical materials, and enables chromophores incorporation in materials making the soft process a gateway for many applications. In this work, we focused on monolithic sol-gel materials, heavily doped with optically active chromophores for the realization of solid-sate optical power limiting devices in the visible and near-infrared range.The elaboration and optimization of compatibles sol-gel matrices with those chromophores was the first step. An investigation of the different synthesis parameters and the nature of the precursors on the materials nanostructure were done, together with the creation of high speed gelation processes to enable the incorporation of very high chromophores contents.Using these approaches, the preparation of highly doped materials for optical power limiting in the visible and the near infrared range was successful, and the impact of the matrices on chromophores properties was studied. Evaluation of optical limiting properties of these materials revealed exceptional performance in the visible range and promising results for the NIR range.Finally, isotropic and anisotropic gold spheres were prepared and incorporated inside the materials. The resulting composites showed enhancements of the non-linear properties of the chromophores inside the matrix, and dramatic improvement of the optical power limiting efficiency was achieved.