Ultrafast Optical Nonlinearity And Photo-Physical Mechanism Of Functional Organic Conjugated Small Molecules

Due to their low cost, structure flexibility, large nonlinear polarizability and fast response speed, organic conjugated molecules have received considerable attention in all-optical switching, optical limiter, two-photon fluorescent imaging, optical data storage, optical micro-fabrication and photo-dynamics therapy. The macroscopic nonlinear optical(NLO) properties of materials are related to their microstructure and nonlinear polarizability of excited-states. It is of importance to investigate the NLO properties and excited-state dynamics of organic conjugated molecules under ultrafast laser excitation not only for developing a better knowledge of the interaction between laser and materials but also for providing theoritial and experimental instruction for designing novel NLO devices in the future.In this dissertation, the ultrafast nonlinear absorption and photo-physical mechanism of several organic conjugated small molecules are investigated using Z-scan measurements and time-resolved pump-probe techniques. The effects of organic cation, halogen anion, the possession percentage of main nonlinear group occupied frontier molecular orbitals, peripheral functional substituents and intramolecular energy transfer process on the nonlinear absorption and excited-state dynamics are analyzed. The main works contain the following parts:The ultrafast NLO response of organic thiadiazines are systematically investigated via Z-scan, optical limiting and femtosecond transient absorption spectra. The influence of halogen anion and organic cation on the nonlinear absorption response is analyzed. Organic cation plays an important role in nonlinear absorption, while halogen anion makes minor contribution in thiadiazines. The single excited-state absorption cross section and lifetime of four thiadiazine materials are extracted from the experimental data based on a simplied three energy level model.The broadband nonlinear responses of pyrene-based isomer are systematically investigated by using combination of quantum chemistry theory, broadband Z-scan, femtosecond degenerate pump-probe technique and transient absorption measurement. We calculate the possession percentage of each component occupied frontier molecular orbitals and analyze the nonlinear absorption mechanism of pyrene-based isomer. It is observed that the higher possession percentage of pyrene occupied frontier molecular orbitals, the larger NLO response. The NLO absorptive parameters of isomer at different wavelengths are extracted through numerical simulation from Z-scan curves under different energies.The effect of different peripheral functional substituents on the excited-state dynamics of pyrene derivatives and chalcone derivatives are investigated through femtosecond transient absorption measurement. We develop a photo-physical model including singlet and triplet excited-states calculated by quantum chemistry theory and analyze the mechanism of transient absorption entire reversal behavior within visible regions between two chalcone molecules. In addition, the modulation of different peripheral functional substituents on singlet excited-state lifetime of pyrene derivatives are studied by combination of transient absorption measurement and frontier molecular theory. The photo-physical parameters of these materials are extract by multi-exponential fitting kinetic traces.The NLO response and ultrafast dynamics of Ni-contained porphyrin-phthalycanine conjugates are investigated by picosecond and femtosecond Z-scan and femtosecond degenerate time-resolved pump-probe measurement. The timescale of energy transfer process is several hundred femtoseconds, which result in reverse saturable absorption(under femtosecond condition) to saturabel absorption(under picosecond condition). Based on energy transfer model and its rate equations, we extract the photo-physical parameters of porphyrin-phthalycanine conjugates.