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Previous issue date: 2012-07-30 / CNPQ / O principal objetivo deste trabalho foi investigar fenômenos ópticos ultrarrápidos em sistemas
nanoestruturados empregando diferentes técnicas espectroscópicas não lineares, tanto no
domínio do tempo quanto no domínio da frequência. Para fornecer uma base adequada que
permita entender os experimentos feitos nessa tese, os princípios físicos das espectroscopias
ópticas não lineares são apresentados. Inicialmente é apresentada uma descrição da função
resposta não linear no domínio do tempo. A evolução temporal da polarização óptica, que gera o
sinal espectroscópico, é descrita em detalhes usando uma teoria de perturbação diagramática.
Técnicas ópticas não lineares são apresentadas, tais como eco de fótons, bombeamento-e-sonda
e hole burning, assim como o comportamento dinâmico de um material pode ser interpretado a
partir do sinal gerado. A técnica de mistura degenerada de quatro ondas com luz incoerente foi
usada para investigar, pela primeira vez, o defasamento ultrarrápido de éxcitons em uma
vitrocerâmica contendo nanocristais de niobato de sódio. O tempo de defasamento medido (T2 =
20 fs) indica qu
empregada para investigar processos de transferência de carga em colóides com nanopartículas
de TiO2 e rodamina 6G. O comportamento do sinal de depleção transiente é comparado com o
observado para a rodamina livre suspensa em etanol. A análise dos resultados permitiu atribuir o
comportamento de depleção à transferência de carga de estados excitados termalizados das
moléculas de corante para a banda de condução do semicondutor e a transferência no sentido
inverso do semicondutor para as moléculas. / The main objective of this work was the investigation of ultrafast optical phenomena in selected
nanostructured systems employing different nonlinear spectroscopic techniques, either in the
time or the frequency domain. To provide an appropriate background to understand the
performed experiments the principles of nonlinear optical spectroscopies are presented. Initially
a description of the nonlinear optical response function in the time domain is given. The time
evolution of the optical polarization, that gives rise to the spectroscopic signal, is described in
detail using a diagrammatic perturbation theory. Nonlinear optical techniques are discussed such
as photon echoes, pump-and-probe and hole-burning, as well as how the dynamical behavior of
a material can be interpreted from the generated signals. The degenerate four-wave mixing
technique with incoherent light was used to investigate for the first time the ultrafast dephasing
of excitons in a glass-ceramic containing sodium niobate nanocrystals. The short dephasing time
measured (T2 = 20 fs) indicates that different dephasing channels contribute for the excitonic
dephasing, namely: electron-electron scattering, electron-phonon coupling and fast trapping of
electrons in defects on the nanocrystals interface. Low-temperature luminescence experiments
were also performed to measure excitonic and trap states lifetimes. The persistent spectral holeburning
technique was applied to measure localized surface plasmons dephasing times in
colloidal silver nanoparticles capped with different stabilizing molecules. The dependence of T2
with three different stabilizers was demonstrated and theoretically analyzed. The results show
that the dephasing times are shorter than the theoretically calculated T2 using the bulk dielectric
functions of the metal. This discrepancy is attributed to changes in the electronic density of
states at the nanoparticles interface caused by the presence of the stabilizers. Ab-initio
calculations based on the Density Functional Theory were performed to further understand the
interaction between the nanoparticles and stabilizing agents. The femtosecond transient
absorption technique was employed to study the ultrafast dynamics of in-gap states in a glassceramics
containing sodium niobate nanocrystals. Two main temporal components were found
for the excited state absorption signal: a fast component, with decay time of ≈ 1 ps, and a slower
component which is attributed to deep trap states. This slower component is responsible for the
excited state absorption contribution in optical limiting experiments previously reported in the
literature. The dynamics of the optical limiting in this sample was also studied, in the
millisecond range, exciting the sample with a train of femtosecond pulses. The optical limiting
behavior reflects the dynamics of population in the excited and trap states and this dynamics
was modeled using rate equations for the electronic states’ populations. Finally, the pump-andprobe
transient absorption technique was employed to investigate charge-transfer processes in
colloids with rhodamine 6G and TiO2 nanoparticles. The transient bleaching signal behavior is
compared with the one observed for unlinked rhodamine 6G dissolved in ethanol. The analysis
of the results allowed the attribution of the bleaching behavior to charge-transfer from
thermalized excited states of the dye molecules to the semiconductor conduction band and to the
back charge-transfer from the semiconductor to the molecules.
| Identifer | oai:union.ndltd.org:IBICT/oai:repositorio.ufpe.br:123456789/18659 |
| Date | 30 July 2012 |
| Creators | ALMEIDA, Euclides Cesar Lins |
| Contributors | http://lattes.cnpq.br/7109489698613515, ARAÚJO, Cid Bartolomeu de |
| Publisher | Universidade Federal de Pernambuco, Programa de Pos Graduacao em Fisica, UFPE, Brasil |
| Source Sets | IBICT Brazilian ETDs |
| Language | Breton |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, info:eu-repo/semantics/doctoralThesis |
| Source | reponame:Repositório Institucional da UFPE, instname:Universidade Federal de Pernambuco, instacron:UFPE |
| Rights | Attribution-NonCommercial-NoDerivs 3.0 Brazil, http://creativecommons.org/licenses/by-nc-nd/3.0/br/, info:eu-repo/semantics/openAccess |
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