Neste trabalho estudamos o processo de absorção de dois fótons em compostos azoaromáticos, abordando tanto aspectos fundamentais quanto aplicados. Moléculas azo-aromáticos têm recebido especial atenção nos últimos anos devido as suas potencias aplicações em dispositivos para armazenamento óptico de informação, modulação da luz e geração de relevos em superfície, processes estes associados ao mecanismo de foto isomerização. Os trabalhos apresentados nessa tese tiveram como principal foco explorar o fenômeno de absorção de dois fótons, utilizando a propriedade de foto-isomerização das moléculas azo-aromáticas. Nosso objetivo foi estudar as vantagens que podem ser obtidas pela utilização de pulsos intensos de luz, que interagem de maneira não linear com a matéria, para desenvolver aplicações utilizando compostos azo-aromáticos. Inicialmente, estudamos o processo de absorção de dois fótons em uma nova família de compostos azo-aromáticos, denominados Salen Dyes. Investigamos a influencia da presença de íons metálicos no processo de absorção de dois fótons dessas moléculas. Nossos resultados mostraram que, devido a ausência de conjugação entre as estruturas azo-aromáticas conectadas pelo metal, este tem pouca influência no processo não linear estudado. Pelo uso de um modelo de soma de estado para a seção de choque de absorção de dois fótons, fomos capazes de determinar os momentos de dipolo de transição destas moléculas. Em seguida, implementamos uma técnica para estudar o processo de armazenamento óptico, induzido através da orientação molecular dos compostos azo-aromáticos, via absorção de dois fótons. Observamos que de fato é possível se observar birrefringência induzida por dois fótons nestas moléculas, a qual esta confinada ao volume focal, conforme demonstrado pela obtenção de armazenamento tri-dimensional. Finalmente, exploramos o processo de controle coerente via formatação de pulsos ultra-curtos para manipular o processo de birrefringência induzida por dois fótons em compostos azo-aromáticos. Foi demonstrado que apenas a através da manipulação da fase do pulso ultra-curto, é possível modular a birrefringência induzida em filmes de compostos azo-aromáticos. / Here we investigated fundamental and applied aspects of the two-photon absorption process in azoaromatic compounds. Azoaromatic molecules have been studied in the last few years due to their applications in optical storage devices, light modulators and surface relief gratings, processes related to the photo-isomerization mechanism. The main focus of the results presented in this thesis was to explore the two-photon absorption process, taking advantage of the intrinsic photo-isomerization of the azo-aromatic chromophores. Our main goal was to study the advantages of utilizing intense pulses, which promote nonlinear excitation, to develop application with azo-aromatic compounds. Initially, we studied the two-photon absorption process in a new class of azo-aromatic compounds, named Salen Dyes. We investigated the influence of metal ions, presented in the Salen Dyes structure, on the two-photon absorption spectrum. Our results revealed that the metals have practically no influence in the nonlinear optical properties due to the lack of conjugation between the azo-moieties in the Salen Dyes. Through a theoretical model based on a sum-over-states calculation, we were able to determine the transition dipole moment of these molecules. Subsquently, we implememnted a technique to study the two-photon induced birefringence, resulting from the molecular re-orientation, in azo-aromatic compounds. We observed that the two-photon induced birefringence is confined to the focal volume, s demonstrated in a three dimensional optical storage experiment. Finally, we used coherent control processes to manipulate the two-photon induced birefringence in azo-compounds. It was demonstrated that it is possible to modulate the optically induced birefringence by manipulating the phase of the ultra-short pulses, via pulse shaping techniques.

Neste trabalho estudamos a forma de linha na absorção de dois fotons nas transições 4f7 de defeitos cristalinos superpostas com bandas de absorção multifonons. Escrevemos o perfil de linha sob a forma de Fano. O íon em estudo é Eu++ como defeito em um cristal de CaF2. Apresentamos o desenvolvimento da instrumentação necessária para realização do experimento, e obtemos resultados de ótima qualidade. / In this work we study the absorption of two photons in the electronic transition within the 4f 7configuration wich is superposed to the vibronic structure of 4f 7 - 4f65d transition. We describe the line shape with the Fano\'s formalism. The study was carried out on Eu+2 doped CaF2. The experimental apparatus was build and the results obtained are in good agreement with the theoretical predictions.

Neste trabalho apresentamos resultados de birrefringência opticamente induzida via absorção de um e dois fótons em filmes casting de poli (metil metacrilato) (PMMA), contendo uma nova série de compostos azoaromáticos chamados de corantes Salen, os quais podem ser usados em aplicações de memória óptica. Para a birrefringência opticamente induzida por um fóton, foi necessário um tempo de escrita maior para os compostos com metal na estrutura em comparação com o composto Salen que não contém metal, provavelmente devido a agregações do grupo azoaromático na estrutura do polímero diminuindo a mobilidade molecular. Além disso, como uma novidade em memórias ópticas deste tipo, birrefringência opticamente induzida por dois fótons pôde ser realizada nos filmes de Salen, o que pode permitir sua aplicação em memória óptica tridimensional. / In this work we present the one- and two-photon optically induced birefringence in cast films of amorphous poly(methyl methacrylate) (PMMA), containing a new series of organometallic azoaromatic compounds, referred to as Salen dyes, which may be used in optical storage applications. For the one-photon optically induced birefringence, longer writing times were required for the organometallic compounds in comparison with the nonmetal one, probably due to the higher degree of aggregation in the azoaromatic moieties that precludes molecular orientation. Furthermore, as a novelty concerning optical storage, two-photon optically induced birefringence could be achieved in the Salen dyes guest-host films, which allows their application in three-dimensional optical memories.

Neste trabalho, demonstramos a fabricação de microambientes tridimensionais para investigar o crescimento celular. Inicialmente, desenvolvemos um sistema de microfabricação que utiliza fotopolimerização via absorção de dois fótons, com o qual se pode fabricar um conjunto de microestruturas com formas e espaçamentos pré-determinados. Este sistema de fabricação utiliza pulsos de femtossegundos, provenientes de um laser de Ti:safira operando em 790 nm. A intensidade destes pulsos é alta o bastante para induzir a absorção de dois fótons no fotoiniciador, o qual é responsável por promover a polimerização em uma resina acrílica. A natureza não linear da absorção de dois fótons confina a excitação ao volume focal, permitindo a fabricação de estruturas tridimensionais com alta resolução espacial. Para a obtenção dos microambientes, foi necessário o desenvolvimento de um sistema opto-mecânico de movimentação, tanto do feixe quando do substrato da amostra. Com esta técnica, fabricamos microambientes compostos de estruturas com diferentes formas (paralelepípedos, cilindros e cones) e espaçamentos, os quais foram caracterizados através de microscopia óptica e eletrônica. Para demonstrar a viabilidade destes microambientes para a investigação do crescimento celular, estes foram utilizados para monitorar o desenvolvimento da célula Michigan Cancer Foundation-7 (MCF-7), uma linhagem celular de adenocarcinoma de mama que apresenta fenótipo tumoral amplamente utilizada como modelo de estudo para câncer de mama. Observamos, via microscopia óptica de transmissão e fluorescência, o desenvolvimento das células MCF-7 nos distintos microambientes. Nossos resultados indicam uma melhor aderência e, portanto, desenvolvimento celular nas microestruturas cilíndricas. Observamos ainda uma maior densidade de células nos microambientes com estruturas separadas de 12 µm, a qual diminui com o aumento do espaçamento, de tal forma que para o microambiente com 30 µm, por exemplo, poucas células são observadas. Portanto, nossos resultados demonstram que os microambientes desenvolvidos são viáveis para estudos mais aprofundados em biologia celular, com potenciais aplicações em engenharia de tecido. / In this work we have demonstrated the fabrication of tridimensional microenvironments for the investigation of cell growth. Initially we have developed a two-photon absorption photopolymerization microfabrication system, which allows producing a set of microstructures with predetermined forms and spacing. This fabrication system uses femtosecond pulses from a Ti: sapphire laser operating at 790 nm. These pulses are intense enough to induce two-photon absorption by the photoinitiator, that is responsible for promoting the polymerization in an acrylic resin. The nonlinear nature of the two-photon absorption confines the excitation to the focal volume, allowing the fabrication of tridimensional structures with high spatial resolution. In order to obtain the microenvironments, it was necessary to develop a movement system for both the laser beam and the sample substrate. With this technique we have fabricated microenvironments composed by structures with different geometries (parallelepipeds, cylinders and cones) and spacing, which were characterized by optical and scanning electron microscopes. To demonstrate the feasibility of the microenvironments for the investigation of cell growth, the samples were used to monitor de development of the Michigan Cancer Foundation-7 cell (MCF-7), a lineage of breast adenocarcinoma that has a tumoral phenotype and is highly used as a model in breast cancer studies. We have observed, through conventional optical and fluorescence microscopy, the growth of the MCF-7 cells in the various microenvironments. Our results indicate a better adhesion and, therefore, better development of cells on the cylindrical microstructures. We also observe a higher cell density in the microenvironments with microstructures having a spacing of 12 µm, which reduces as the distance between microstructures increases, in such a way that for the microenvironment with 30 µm spacing, for example, just a few cells are observed. Thus, our results demonstrate that the produced microenvironments are applicable in deeper studies in microbiology, with potential application in tissue engineering.

Highly correlated quantum chemical methods have been appliedto study the photophysical properties of substituted benzenes.With the inclusion of spin-orbit coupling, the phosphorescencesof these molecules have been calculated usingMulti-CongurationalSelf- Consistent Field (MCSCF) quadraticresponse theory. The Herzberg-Teller approximation has beenadopted to evaluate the vibronic contributions tophosphorescence. The performance of hybrid density functional theory (DFT) atthe B3LYP level is examined in comparison to the MP2, CCSD andCCSD(T) methods for the geometry and permanent dipole moment ofp-aminobenzoic acid. The time-dependent DFT/B3LYP method isapplied to calculate the two-photon absorption of a series ofZinc-porphyrin derivatives in combination with a two-statemodel. The transitions between excited singlet and tripletstates of Zinc and Platinum based organometallic compounds havebeen computed using DFT quadratic response theory. The resultsare used to simulate the non-linear propagation of laser pulsesthrough these materials utilizing a dynamical wave propagationmethod.

This thesis presents a novel method of modulating silicon photonic circuits using 3D printed microfluidic devices. The fluids that pass through the microfluidic device interact directly with the silicon waveguides. This method changes the refractive index of the waveguide cladding, thus changing the effective index of the system. Through using this technique we demonstrate the shift in resonant wavelength by a full free spectral range (FSR) by increasing the concentration of the salt water in the microfluidic device from 0% to 10%. On a 60 μm microring resonator, this equals a resonant wavelength shift of 1.514 nm when the index of the cladding changes by 0.017 refractive index units (RIU), or at a rate of 89.05 nm/RIU. These results are confirmed by simulations that use both analytical and numerical methods. This thesis also outlines the development of a process that uses two-photon absorption(TPA) in silicon to produce a photoelectrochemical (PEC) etching effect. TPA induces free carriers in silicon that then interact with the Hydroflouric Acid (HF) solution that the wafer is submerged in. This interaction removes silicon away from the wafer, which is the etching observed in our experiments. Non-line-of-sight PEC etching is demonstrated. The optical assemblies used in these experiments are presented, as are several of the results of the etching experiments.

This dissertation reports on the investigation of two-photon absorption (2PA) in a series of fluorenyl molecules. Several current and emerging technologies exploit this optical nonlinearity including two-photon fluorescence imaging, three-dimensional microfabrication, site-specific photodynamic cancer therapy and biological caging studies. The two key features of this nonlinearity which make it an ideal candidate for the above applications are its quadratic dependence on the incident irradiance and the improved penetration into absorbing media that it affords. As a consequence of the burgeoning field which exploits 2PA, it is a goal to find materials that exhibit strong two-photon absorbing capabilities. Organic materials are promising candidates for 2PA applications because their material properties can be tailored through molecular engineering thereby facilitating optimization of their nonlinear optical properties. Fluorene derivatives are particularly interesting since they possess high photochemical stability for organic molecules and are generally strongly fluorescent. By systematically altering the structural properties in a series of fluorenyl molecules, we have determined how these changes affect their two-photon absorbing capabilities. This was accomplished through characterization of both the strength and location of their 2PA spectra. In order to ensure the validity of these results, three separate nonlinear characterization techniques were employed: two-photon fluorescence spectroscopy, white-light continuum pump-probe spectroscopy, and the Z-scan technique. In addition, full linear spectroscopic characterization was performed on these molecules along with supplementary quantum chemical calculations to obtain certain molecular properties that might impact the nonlinearity. Different designs in chemical architecture allowed investigation of the effects of symmetry, solvism, donor-acceptor strengths, conjugation length, and multi-branched geometries on the two-photon absorbing properties of these molecules. In addition, the means to enhance 2PA via intermediate state resonances was investigated. To provide plausible explanations for the experimentally observed trends, a conceptually simple three level model was employed. The subsequent correlations found between chemical structure and the linear and nonlinear optical properties of these molecules provided definitive conclusions on how to properly optimize their two-photon absorbing capabilities. The resulting large nonlinearities found in these molecules have already shown promise in a variety of the aforementioned applications.

Back in 1894 Lord Kelvin coined the term "chiral" in order to refer to molecules whose mirror images were not superimposable with themselves. Over the years, research has demonstrated the important role that chiral molecules play in life, chemistry, and biology as well as their importance in the development of new drugs and technologies. The efforts to understand chiral systems have been mainly driven by spectroscopic methods that leverage on the opposite responses that enantiomers have to linear or circularly polarized light of both handedness. More specifically, Electronic Circular Dichroism (ECD) which measures the differences in linear absorption of left and right circularly polarized light has been the method par excellence for the spectroscopic characterization of chiral compounds. Unfortunately, the fact that ECD is based on linear absorption severely limits the use of this method in the near to far UV region. This is mainly due to the interferences generated by the strong linear absorption of common organic solvents and buffers in this portion of the light spectrum. Nevertheless, the fact remains that many chiral biomolecules of interest related to deceases like Alzheimer and Parkinson, exhibit most of their linear absorption in the near to far UV region where ECD cannot be employed for their study. Therefore, it has become an urgent necessity to develop spectroscopic methods to study chiral molecules that can circumvent the limitations of ECD at shorter wavelengths. In order to overcome the existent limitations in linear chiral spectroscopy, the nonlinear equivalent of ECD arises as a promising alternative, i.e. Two-Photon Circular Dichroism (TPCD). Although, this phenomenon was theoretically predicted in 1975, it was not until 2008, with the introduction of the double-L scan, that a reliable and versatile method for the measurement of TPCD was introduced. The high sensitivity of this method is based on the use of "twin" pulses that allow accounting for fluctuations in the excitation source that prevented the experimental realization of the measurement. The first measurement of a full TPCD spectrum was performed on BINOL enantiomers and the results were supported and discussed with the help of theoretical calculations. After that seminal work, we embarked in expanding the understanding of the structure-property relationship of TPCD by performing, systematically, a series of theoretical-experimental studies in chiral biaryl derivatives and compounds with helical chirality. In Chapter 2 we present the theoretical-experimental study of the effect of the π-electron delocalization curvature on the TPCD of molecules with axial chirality. The targeted molecules for this part of our investigation were S-BINOL, S-VANOL, and S-VAPOL. Our findings revealed that an increase in the TPCD signal, within this series of compounds, was related to the curvature of the π–electron delocalization. The contributions of the different transition moments to the two-photon rotatory strength support our outcomes. Then, in Chapter 3 we introduce the development of the Fragment-Recombination Approach (FRA) for the calculation of the TPCD spectra of large molecules. This simple but powerful method is based on the additivity of the TPCD signal, and is subject to a strict conditional fragmentation approach. FRA-TPCD is demonstrated, theoretically, in two hypothetical molecular systems from the biaryl derivatives family. Afterward, in Chapter 4 we show the first experimental demonstration of FRA-TPCD through the conformational analysis of an axially-chiral Salen ligand in solution (AXF-155). The FRA-TPCD spectra calculated for the different isomers of AXF-155 allowed narrowing the number of possible isomers of this complex molecule in THF solution to only two. This represents a significant improvement from previously reported results using ECD. Subsequently, in Chapter 5 we present the study of the effect of intramolecular charge transfer (ICT) in S-BINAP, an axially dissymmetric diphosphine ligand with strong ICT. The evaluation of the performance of two different exchange-correlation functional (XCF) confirmed that in order to properly predict the theoretical TPCD spectrum of a molecule exhibiting strong ICT, it is required to use an XCF such as CAM-B3LYP. In addition, our findings revealed the importance of considering an adequate number of excited states in order to be able to fully reproduce the experimental TPCD spectrum, thus avoiding wrong assignments of theoretical transitions to experimental spectral features. Finally, and expanding on our previous study, in Chapter 6 we investigated the effect of the nature of ICT on two hexahelicene derivatives. Our investigation demonstrated that the TPCD signal of chiral molecules with strong ICT does not only depend on the strength of this effect but on its nature, i.e. extension of the π–electronic delocalization increasing beyond (EXO-ICT) or within (ENDO-ICT) the helicene core. In summary, with the results presented in this thesis we closed a first loop in the understanding of the structure-property relationship of TPCD. In the future, we expect to deepen in our knowledge of the structure-property relationship of this phenomenon by studying further helicene derivatives with donor-acceptor motif, and through the application of FRA-TPCD to the conformational analysis of amino acids in peptides. We foresee numerous applications of TPCD for the study of optically active molecules with implications in biology, medicine, and the drug and food industry, and applications in nanotechnology, asymmetric catalysis and photonics.

This dissertation reports on the investigation of the relationships between molecular structure and two-photon absorption (2PA) properties for a series of polymethine and squaraine molecules. Current and emerging applications exploiting the quadratic dependence upon laser intensity, such as two-photon fluorescence imaging, three-dimensional microfabrication, optical data storage and optical limiting, have motivated researchers to find novel materials exhibiting strong 2PA. Organic materials are promising candidates because their linear and nonlinear optical properties can be optimized for applications by changing their structures through molecular engineering. Polymethine and squaraine dyes are particularly interesting because they are fluorescent and showing large 2PA. We used three independent nonlinear spectroscopic techniques (Z-scan, two-photon fluorescence and white-light continuum pump-probe spectroscopy) to obtain the 2PA spectra revealing 2PA bands, and we confirm the experimental data by comparing the results from the different methods mentioned. By systematically altering the structure of polyemthines and squaraines, we studied the effects of molecular symmetry, strength of donor terminal groups, conjugation length of the chromophore chain, polarity of solvents, and the effects of placing bridge molecules inside the chromophore chain on the 2PA properties. We also compared polymethine, squaraine, croconium and tetraon dyes with the same terminal groups to study the effects of the different additions inserted within the chromophore chain on their optical properties. Near IR absorbing squaraine dyes were experimentally observed to show extremely large 2PA cross sections ([approximately equal to] 30000GM). A simplified three-level model was used to fit the measured 2PA spectra and detailed quantum chemical calculations revealed the reasons for the squaraine to exhibit strong 2PA. In addition, two-photon excitation fluorescence anisotropy spectra were measured through multiple 2PA transitions. A theoretical model based on four-levels with two intermediate states was derived and used for analysis of the experimental data.

The purpose of this dissertation is to provide a study and possible applications of two-photon absorption (2PA), in direct-gap semiconductors and quantum-well (QW) semiconductor structures. One application uses extremely nondegenerate (END) 2PA, for mid-infrared (mid-IR) detection in uncooled semiconductors. The use of END, where the two photons have very different energies gives strong enhancement comapared to degenerate 2PA. This END-2PA enhanced detection is also applied to mid-IR imaging and light detection and ranging (LIDAR) in uncooled direct-gap photodiodes. A theoretical study of degenerate 2PA (D-2PA) in quantum wells, QWs, is presented, along with a new theory of ND 2PA in QWs is developed. Pulsed mid-IR detection of femtosecond pulses is investigated in two different semiconductor p-i-n photodiodes (GaAs and GaN). With the smaller gap materials having larger ND-2PA, it is observed that they have better sensitivity to mid-IR detection, but unwanted background from D-2PA outweighs this advantage. A comparison of responsivity and signal-to-background ratio for GaAs and GaN in END-2PA based detection is presented. END-2PA enhancement is utilized for CW IR detection in uncooled GaAs and GaN p-i-n photodiodes. The pulsed mid-IR detection experiments are further extended to perform mid-IR imaging in uncooled GaN p-i-n photodetectors. A 3-D automated scanning gated imaging system is developed to obtain 3-D mid-IR images of various objects. The gated imaging system allows simultaneous 3-D and 2-D imaging of objects. The 3-D gated imaging system described in the dissertation could be used for examination of buried structures (microchannels, defects etc.) or laser written volumetric structures and could also be suitable for in-vivo imaging applications in biology in the mid-IR spectral region. As an example, 3-D imaging of buried semiconductor structures is presented. A theoretical study of D-2PA of QWs for transverse electric (TE) and transverse magnetic (TM) fields is carried out and an analytical expression for the D-2PA coefficient in QWs using second-order perturbation theory is derived. A theory for ND-2PA in QW semiconductor using second-order perturbation theory is developed for the first time and an analytical expression for the ND-2PA coefficient for TE, TM, and the mixed case of TE and TM is derived. The shape of the 2PA curve for the D-2PA and ND-2PA for QWs in the TE case is similar to that of bulk semiconductors. As governed by the selection rules both the D-2PA and ND-2PA curves for the TE case does not show a step-like signature for the density of states of the QWs whereas 2PA curve for the TM case shows such step like sharp features. The ND-2PA coefficient for TE, TM, and the mixed case is compared with that obtained for bulk semiconductors. Large enhancement in ND-2PA of QW semiconductors for the TM case over bulk semiconductors is predicted.