THESIS
2015
xvi, 147 pages : illustrations (some color) ; 30 cm
Abstract
This dissertation discusses the nonlinear optical properties and influence of molecular
environment on the optical properties of several novel organic compounds developed by
collaborators in chemistry.
When z-scan technique is adopted to determine the nonlinear absorptive and refractive
coefficient, femtosecond pulses with a 1 kHz repetition rate are more suitable as the incident source
than the 76 MHz ones, as the latter would introduce a significant thermal effect. Performing the
open aperture z-scan under several irradiances and estimating the two-photon absorption (2PA)
coefficients from fitting would help to avoid the exaggeration of 2PA cross-sections due to other
nonlinear effects such as excited state absorption, resulting in 2PA cross-sections estimated
consistent...[
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This dissertation discusses the nonlinear optical properties and influence of molecular
environment on the optical properties of several novel organic compounds developed by
collaborators in chemistry.
When z-scan technique is adopted to determine the nonlinear absorptive and refractive
coefficient, femtosecond pulses with a 1 kHz repetition rate are more suitable as the incident source
than the 76 MHz ones, as the latter would introduce a significant thermal effect. Performing the
open aperture z-scan under several irradiances and estimating the two-photon absorption (2PA)
coefficients from fitting would help to avoid the exaggeration of 2PA cross-sections due to other
nonlinear effects such as excited state absorption, resulting in 2PA cross-sections estimated
consistent with and comparable to other literature.
The organic fluorophores investigated for nonlinear optical properties all have donor-π-acceptor
structure motifs, whose lowest-energy 2PA bands are identical to those in one-photon absorption.
The molecular 2PA cross-sections of these fluorophores are 50-250GM, increasing with the
addition of donor/acceptor moieties, and influenced by alternation of donor/acceptor groups.
For these fluorophores studied, the molecular environment, such as whether the fluorophores are
dispersed or aggregated, what solvent is used, and the formation of crystals, can all influence
optical properties, in aspects like absorption and emission spectra, fluorescence quantum
efficiencies and lifetimes, as well as nonlinear optical properties. The aggregation enhancement in
two-photon excited fluorescence can reach around two-orders of magnitudes. The 2PA cross-sections
studied can be enhanced by the formation of aggregations, slightly influenced by solvent
polarity, and reduced by the formation of a twisted intramolecular charge-transfer state for dipolar
molecules with large charge separation. The changes in molecular conformations and molecular
stacking modes in crystal polymorphs both influence the optical properties, via intra- and interchain
conjugation as well as intra- and inter-molecular interactions.
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