| This dissertation describes experimental and theoretical studies of the nonlinear optical (NLO) absorption mechanisms in chalcogenidodiphenylphosphino-substituted bithiophenes and phosphonato-substituted bithiophenes. The NLO properties of these compounds were studied by measurements of NLO absorption versus input energy and Z-scan experiments. Both of these NLO experiments were performed in the picosecond (ps) and nanosecond (ns) time regimes in the blue spectral region. These experiments demonstrate that these compounds exhibit both high linear transmissions and strong nonlinear absorptions in the blue spectral region. Saturated solutions of the bithiophenes monosubstituted with chalcogenatodiphenylphosphino groups exhibit the highest NLO absorbances of picosecond laser pulses that have been reported to date. Thus these compounds are promising candidates for broadband optical power limiters (OPLs), which are devices that are highly transparent for low intensity light, but strongly absorb high intensity light. It has been demonstrated that properties such as the extent of the NLO absorbance and the maximum wavelength at which the linear absorption occurs, as well as materials properties such as solubilities, can be tuned by varying the structure of groups attached to the phosphorus and the substituents on the bithiophene. These structure-property relationships suggest that materials with even better combinations of linear and nonlinear optical properties, as well as materials solubilities, can be developed. To gain more detailed insight into these structure-property relationships, ns and ps Z-scan measurements of solutions of bithiophenes disubstituted with chalcogenidodiphenylphosphino groups were fit with three energy level models (two-level, three-level and five-level) to ascertain photophysical properties. The five-level model provides far superior fitting to the two-level and three-level models for the compounds. The comparisons with different models have demonstrated that the nonlinear optical properties of the compounds depend on two-photon absorption (TPA), which pumps singlet and triplet excited state absorption (ESA). Photophysical parameters such as TPA cross section, the singlet ESA cross section, the triplet ESA cross section, and the intersystem crossing time have been obtained from the five-level model. These photophysical parameters are strongly dependent on the chemical structure, which raises the possibility of further optimizing this class of compounds in the future by appropriately modifying in the chemical structure. |
