No description available.

The goal of this dissertation is to study the photophysics of suspensions of colloidal C₆₀ particles to determine if their nonlinear optical (NLO) response is superior in any way to benchmark NLO materials such as molecular solutions of C₆₀ and carbon black suspensions (CBS). C₆₀ in molecular form is known to exhibit strong reverse saturable absorption (RSA) and it is posited that colloidal particles composed of many C₆₀ molecules would maintain some degree of RSA behavior upon association, although some quenching is to be expected. CBS is known to have an NLO response that is dominated by nonlinear scattering resulting from a phase change due to heating of the carbon black particles by absorbed energy. Colloidal C₆₀ particles that are many nanometers in diameter are similar to CBS, so it is posited that they would also have a nonlinear scattering mechanism contributing to their NLO response. Three samples of C₆₀ colloids are characterized by several techniques, along with two carbon black suspensions and one molecular solution of C₆₀. Transmission electron microscopy is used to determine morphology. Femtosecond pump-probe spectroscopy is used to determine the absorption spectrum and the relaxation kinetics of the first excited singlet state. Nanosecond laser flash photolysis is used to determine the absorption spectrum and the relaxation kinetics of the first excited triplet state. Z-scan is used to determine triplet-triplet absorption cross-sections. An experiment is performed to determine the percentage of the input energy that is transmitted, scattered, or absorbed by each sample. Computer modeling is performed to compare the experimental results to theory. Results show that all materials that exhibit nonlinear scattering have a constant extinction coefficient in the nonlinear regime, implying a characteristic size for the scattering centers that is independent of input energy. Quenching processes in C₆₀ colloids are found to be morphology dependent, with more crystalline structures resulting in stronger quenching and less RSA. C₆₀ colloids with stronger RSA are found to result in less nonlinear scattering than strongly quenched colloids. Highly crystalline C₆₀ colloids were shown to have a stronger NLO response than the benchmark materials at medium to high energies.

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.