Στην παρούσα εργασία, γίνεται αρχικά η μελέτη τριών χρωστικών ως προς την απόδοσή τους στη διφωτονική απορρόφηση. Με βάση τα αποτελέσματα της μέτρησης αυτής, επιλέχθηκαν οι δύο χρωστικές που παρουσίασαν ισχυρότερα το φαινόμενο, ώστε να μελετηθούν σχετικά με τον αποχρωματισμό τους. Για το λόγο αυτόν, τα δείγματα που παρασκευάστηκαν από τις ουσίες, δέχτηκαν ακτινοβολία μεγάλης έντασης, μέσω πολύ στενών παλμών laser. / Photobleaching of organic dyes after two photon excitation

Nonlinear optics has had extensive application into a vast array of scientific fields. One such nonlinear process, two-photon absorption (TPA), has had a wildly successful adoption into the field of biological imaging and microscopy. As far and as fast as this field is progressing, limitations stemming from the use of ultrafast lasers are starting to appear. In this work, an alternative nonclassical light source will be motivated for the application of low photon-flux two-photon microscopy. The origin and properties of the chosen nonclassical source, spontaneous parametric downconversion (SPDC), will be discussed along with the spatial and spectral properties modelled. Nonlinear processes such as TPA and sum frequency generation (SFG) will be viewed as "ultrafast coincidence measurements" of two photons arriving at a molecule within the time window of excitation. These ultrafast coincidence measurements will be viewed in an alternative manner: in terms of the second-order coherence from a light source. This degree of second-order coherence can be subdivided into two categories arising from different combinations of correlations within and between entangled photon pairs. Of interest, the energy-time correlations within the photon pair allow for enhancements in ultrafast coincidence rates over coherent light sources. The makings of an experimental setup to demonstrate enhanced rates from ultrafast two-photon coincidences taking place in SFG in a nonlinear crystal will be discussed.

This thesis presents time-dependent density functional theory studies on electronic and vibronically resolved linear and nonlinear optical absorption and dichroism spectra of organic molecules. Special attention has been paid to the influence of solvent environment and molecular vibrations on one-, two- and three-photon absorption and one- and two-photon circular dichroism. It is found that dielectric medium as described by polarizable continuum model can enhance remarkably three-photon absorption cross section of a highly conjugated fluorene derivative, for which the simplified two-state model is shown to be largely inadequate. Origin-invariant density functional calculations on one- and two-photon circular dichroisms of a chiral molecule confirm that the recently developed CAMB3LYP functional performs better than the popular B3LYP functional for Rydberg-states. The first experimental measurement of TPCD spectra is performed on an axial chiral system in tetrahydrofunan, where the double L-scan technique is applied. Theoretical calculations well reproduce the experimental profiles when both the electron correlation and the solvent effect are taken into account. Vibronically resolved one- and two-photon absorption spectra of charge-transfer molecules have been obtained using a Linear Coupling model, where the 'borrowing mechanism' for the so-called Herzberg-Teller contribution is analyzed in detail. It is shown that Herzberg-Teller contribution can introduce a change of sign to the chiral responses of a molecule without the involvement of different electronic states, which has important consequences for the assignment of absolute configurations of chiral molecules. Adiabatic harmonic Franck-Condon model is also applied to simulate vibronically resolved one- and two-photon circular dichroism spectra of the same chiral system, where the sign-inversion and the interference between Franck-Condon and Herzberg-Teller contributions are also observed. / QC 20100727

Quantum information science has shown that computers which exploit the quantum nature of particles, namely quantum computers, can outperform contemporary computers in some computational tasks. The fundamental building blocks of a quantum computer are quantum logical gates and quantum bits (qubits). Previous research has shown that the optical approach to quantum computing is promising. However, linear optical quantum computing (LOQC) schemes require a huge amount of resource, which makes large scale LOQC impractical, and hence there have been renewed interests in nonlinear optical quantum computing schemes, where less resource is required. The performance of these quantum gates depends on the properties of the nonlinear media. However, requirements on some of the properties for high performance quantum gates are not fully known. This thesis intends to bridge this gap of knowledge and examines the necessary conditions on several types optical nonlinearities that are common in two-qubit quantum gates schemes. These types of nonlinearities are, namely two-photon absorption, $\chi^{(2)}$ nonlinearity and $\chi^{(3)}$ cross-Kerr nonlinearity. The two-photon absorption based quantum Zeno gate is modeled in this thesis. It is shown that for practical absorbers, the photon loss significantly lowers the quantum fidelity of the Zeno gate. Nevertheless, this thesis proposes to use the Zeno gate for fusing optical cluster states. With the best theoretical estimate of single photon loss in the absorbers, the Zeno gate can outperform linear optical schemes. This thesis also proposes to embed the Zeno gate in the teleportation-type of two-qubit gate, namely GC-Zeno gate, such that the success rate of the gate can be traded off for higher gate fidelity. The effect of some mode matching error and detector inefficiency on the GC-Zeno gate are also considered here. It is shown that the photon loss requirement as well as the mode matching requirement are both stringent for having a fault tolerant GC-Zeno gate. This thesis models some of the properties of a $\chi^{(3)}$ optical medium and explores how they affect the fidelity of the cross-Kerr nonlinearity based quantum gate. This thesis shows that for a cross-Kerr medium with fast time response but negligible wave dispersion, the medium would induce spectral entanglement between the input photons and this significantly lowers the fidelity of the quantum gate. Nevertheless, when the dispersion has a stronger effect than the time response, and if phase noise is negligible, it is possible to achieve a quantum gate with high fidelity. However, the noise is actually significant, and this thesis suggests that spectral filtering can be applied to prohibit the occurrence of the noise. The requirements on employing optical $\chi^{(2)}$ nonlinearity for quantum computing are also examined. This study models the spectral effects of a $\chi^{(2)}$ medium on its efficiency. It is shown in this thesis that since the Hamiltonian of the medium does not commute at different times, the unitary operation should be modeled by a Dyson series, which leads to undesired spectral entanglement that lowers the efficiency of the medium. However, in the case of periodical poling, the unitary operation can be modeled by a Taylor series, where under some phase matching conditions, the medium can have a high efficiency. Furthermore, this thesis proposes a Bell measurement scheme and a quantum gate scheme based on $\chi^{(2)}$ nonlinearity that can always outperform linear optics even when the nonlinearity strength is weak. In the case of sufficiently strong nonlinearity, a quantum gate with high success rate can be achieved. In summary, this thesis models some of the properties of two-photon absorbers, $\chi^{(2)}$ nonlinearity and $\chi^{(3)}$ nonlinearity, and shows that it is possible to achieve the conditions required for high performance quantum gates, however these conditions are experimentally challenging to meet.

Quantum information science has shown that computers which exploit the quantum nature of particles, namely quantum computers, can outperform contemporary computers in some computational tasks. The fundamental building blocks of a quantum computer are quantum logical gates and quantum bits (qubits). Previous research has shown that the optical approach to quantum computing is promising. However, linear optical quantum computing (LOQC) schemes require a huge amount of resource, which makes large scale LOQC impractical, and hence there have been renewed interests in nonlinear optical quantum computing schemes, where less resource is required. The performance of these quantum gates depends on the properties of the nonlinear media. However, requirements on some of the properties for high performance quantum gates are not fully known. This thesis intends to bridge this gap of knowledge and examines the necessary conditions on several types optical nonlinearities that are common in two-qubit quantum gates schemes. These types of nonlinearities are, namely two-photon absorption, $\chi^{(2)}$ nonlinearity and $\chi^{(3)}$ cross-Kerr nonlinearity. The two-photon absorption based quantum Zeno gate is modeled in this thesis. It is shown that for practical absorbers, the photon loss significantly lowers the quantum fidelity of the Zeno gate. Nevertheless, this thesis proposes to use the Zeno gate for fusing optical cluster states. With the best theoretical estimate of single photon loss in the absorbers, the Zeno gate can outperform linear optical schemes. This thesis also proposes to embed the Zeno gate in the teleportation-type of two-qubit gate, namely GC-Zeno gate, such that the success rate of the gate can be traded off for higher gate fidelity. The effect of some mode matching error and detector inefficiency on the GC-Zeno gate are also considered here. It is shown that the photon loss requirement as well as the mode matching requirement are both stringent for having a fault tolerant GC-Zeno gate. This thesis models some of the properties of a $\chi^{(3)}$ optical medium and explores how they affect the fidelity of the cross-Kerr nonlinearity based quantum gate. This thesis shows that for a cross-Kerr medium with fast time response but negligible wave dispersion, the medium would induce spectral entanglement between the input photons and this significantly lowers the fidelity of the quantum gate. Nevertheless, when the dispersion has a stronger effect than the time response, and if phase noise is negligible, it is possible to achieve a quantum gate with high fidelity. However, the noise is actually significant, and this thesis suggests that spectral filtering can be applied to prohibit the occurrence of the noise. The requirements on employing optical $\chi^{(2)}$ nonlinearity for quantum computing are also examined. This study models the spectral effects of a $\chi^{(2)}$ medium on its efficiency. It is shown in this thesis that since the Hamiltonian of the medium does not commute at different times, the unitary operation should be modeled by a Dyson series, which leads to undesired spectral entanglement that lowers the efficiency of the medium. However, in the case of periodical poling, the unitary operation can be modeled by a Taylor series, where under some phase matching conditions, the medium can have a high efficiency. Furthermore, this thesis proposes a Bell measurement scheme and a quantum gate scheme based on $\chi^{(2)}$ nonlinearity that can always outperform linear optics even when the nonlinearity strength is weak. In the case of sufficiently strong nonlinearity, a quantum gate with high success rate can be achieved. In summary, this thesis models some of the properties of two-photon absorbers, $\chi^{(2)}$ nonlinearity and $\chi^{(3)}$ nonlinearity, and shows that it is possible to achieve the conditions required for high performance quantum gates, however these conditions are experimentally challenging to meet.

Neste trabalho usamos pulsos de femtossegundos na fabricação de estruturas poliméricas em escala microscópica, através da técnica de fotopolimerização via absorção de dois fótons. Graças ao confinamento espacial da polimerização, resultante do processo de absorção de dois fótons, este método permite a fabricação de microestruturas tridimensionais complexas, com alta resolução, visando diversas aplicações tecnológicas, de fotônica até biologia. Inicialmente, desenvolvemos a técnica de fotopolimerização via absorção de dois fótons, desde a implantação da montagem óptica até a confecção dos sistemas de movimentação e controle do posicionamento do feixe laser. Através da fabricação e caracterização de microestruturas, produzidas em resinas acrílicas, o sistema foi aperfeiçoado permitindo a produção de microestruturas da pordem de 30um com razoável resolução espacial. Uma vez que a maior parte as microestruturas reportadas na literatura são elementos passivos, ou seja, suas propriedades ópticas não podem ser controladas por meios externos, numa segunda etapa deste projeto produzimos microestruturas opticamente ativas. Neste caso, a microfabricação foi feita em resinas acrílicas dopadas Rodamina B, exibindo, portanto, fluorescência quando excitadas com luz de comprimento de onda em torno de 540nm. Finalmente, visando a produção eficiente de estruturas em escala milimétrica para aplicações biológicas, implementamos também um sistema de fotopolomerização via absorção de um fóton. / In this work we used femtosecond pulses to fabricate polymeric structures at microscopic scale, by using the two-photon photopolymerization technique. Due to the spatial confinement of the polymerization, provided by the two-photon absorption, this method allows for the fabrication of complex three-dimensional microstructures, with high resolution, aiming to several technological applications, from photonics to biology. Initially, we developed the two-photon polimerization technique, from the optical setup to the mechanical systems to control the movement and the positioning of the laser beam. Through the fabrication and characterization os microestrutures, produced in acrylic resin, the apparatus was improved, allowing the fabriation of 30-um microstructures with reasonable spatial resolution. Since most the report in the literature are passive elements that is, their optical properties cannot be altered by any external means, in a second stage of this project we fabricated optical active microstructures. In this case, the microfabrication was carried out in acrylic resins doped with Rodamine B, exhibiting, consenquently, fluorescence when excited with light at 540nm. Finally, in order to eficiently produce milimetric structures for biological applications, we also implemented a one-photon polimerization setup.

Intensive demands have arisen to characterize nonlinear optical properties of materials for applications involving optical limiters, waveguide switches and bistable light switches. The technique of Pulse Delay Modulation is described which can monitor nonlinear changes in transmission with shot noise limited signal-to-noise ratios even in the presence of large background signals. The theoretical foundations of the experiment are presented followed by actual measurements of beam depletion due to second harmonic generation in a LiIO3 crystal and two-photon absorption in the semiconductor ZnSe. Sensitivity to polarization rotation arising from the Kerr Effect in carbon disulfide, saturable absorber relaxation in modelocking dyes and photorefractive effects in ZnSe are demonstrated. The sensitivity of Pulse Delay Modulation is combined with Fabry-Perot enhancement to allow the measurement of two-photon absorption in a 0.46pm thick interference filter spacer layer. Also included is a study of nonlinear optical limiting arising from dielectric breakdown in gases.

Two-photon absorbing (2PA) materials has been widely studied for their highly localized excitation and nonlinear excitation efficiency. Application of 2PA materials includes fluorescence imaging, microfabrication, 3D data storage, photodynamic therapy, etc. Many materials have good 2PA photophysical properties, among which, the fluorenyl structure and its derivatives have attracted attention with their high 2PA cross-section and high fluorescence quantum yield. Herein, several compounds with 2PA properties are discussed. All of these compounds contain one or two fluorenyl core units as part of the conjugated system. The aim of this dissertation is to discuss the application of these compounds according to their photophysical properties. In chapters 2 to 4, compounds were investigated for cell imaging and tissue imaging. In chapter 5, compounds were evaluated for photodynamic therapy effects on cancer cells. Chapters 2 and 3 detail compounds with quinolizinium and pyran as core structures, respectively. Fluorene was introduced into structures as substituents. Quinolizinium structures exhibited a large increase in fluorescence when binding with Bovine Serum Albumin (BSA). Further experiments in cell imaging demonstrated a fluorescence turn-on effect in cell membranes, indicating the possibility for these novel compounds to be promising membrane probes. Pyran structures were conjugated with arginylglycylaspartic acid peptide (RGD) to recognize integrin and introduced in cells and an animal model with tumors. Both probes showed specific targeting of tumor vasculature. Imaging reached penetration as deep as 350 µm in solid tumors and exhibited good resolution. These results suggest the RGD-conjugated pyran structure should be a good candidate probe for live tissue imaging. Chapter 4 applied a fluorene core structure conjugated with RGD as well. Application of this fluorenyl probe compound is in wound healing animal models. Fluorescence was collected from vasculature and fibroblasts up to ≈ 1600 µm within wound tissue in lesions made on the skin of mice. The resolution of images is also high enough to recognize cell types by immunohistochemical staining. This technology can be applied for reliable quantification and illustration of key biological processes taking place during tissue regeneration in the skin. Chapter 5 describes three fluorenyl core structures with photoacid generation properties. One of the structures showed excellent photo-induced toxicity. Cancer cells underwent necrotic cell death due to pH decrease in lysosomes and endosomes, suggesting a new mechanism for photodynamic therapy.

The discovery of any new technology is usually accompanied by a need for new or improved materials which make that technology useful in practical applications. In the case of two-photon absorption (2PA) this has truly been the case. Since its first demonstration in 1961, there has been an ever increasing quest to understand the relationships between two-photon absorption and the structure of two-photon absorbing materials. This quest has been motivated by the many applications for 2PA which have been reported, including fluorescence bioimaging, 3D microfabrication, 3D optical data storage, upconverted lasing, and photodynamic therapy. The work presented in this dissertation represents another step in the effort to better understand the structure/property relationships of 2PA. In this work a new, squaraine-fluorenesquaraine molecule, proposed through a joint effort of quantum and synthetic chemists, was synthesized and its photophysical properties were measured. The measurements included linear and two-photon photophysical properties, as well as solvatochromic behavior. Quantum calculations were done to aid in understanding those photophysical and solvatochromic properties. A single squaraine dye was also synthesized and used as a model compound to assist in understanding this new structure. In Chapter 1 an introduction to 2PA and several of its applications is given. Chapter 2 gives a background of 2PA structure/property relationships that have been reported to date, based on work done with polymethine dyes. Chapter 3 gives a full account of the synthesis, characterization, and detailed quantum chemical analyses of this new squaraine-fluorenesquaraine molecule and the corresponding model compound squaraine dye. Chapter 4 gives some additional work and suggested future directions.

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