Ultrafast Optical Excitation Of Several Optical Functional Materials And Photodynamic Process

The blue light-emitting diode (LED) is a rapidly expanding research field in recent years. In this thesis, we focus on two blue fluorescent materials, organic and inorganic, which could be the promising candidates for blue LED application. Using femtosecond (fs) pulses, we studied the photoluminescence (PL) emission from these materials under nonlinear optical excitation, and used the fs pump-probe and time-resolved photoluminescence (TRPL) techniques to investigate the mechanisms of these processes. We also studied the energy transfer processes in light harvesting complexes (LH2) from Rb.Sphaeroides by using femtosecond pump-probe technique.The main research and the innovative results are given below.1. Organic material: Para-sexiphenyl (PSP) nano-needlesThe PSP nanoneedle film is a long-range ordered structure consisted of oriented nano-needles, which are parallel to each other in a limited area and turn to a tilted orientation in adjacent areas. We studied the PL emission of PSP nano-needles under linear and nonlinear optical excitation. Excited by the femtosecond pulses, two-photon absorption (TPA) induced PL emission spectrum shows several peaks corresponding to the vibronic transitions in PSP molecules. Polarization-dependent PL measurements reveal that the TPA induced PL in the highly-ordered structures, similar with the linear excitation case, is strongly dependent on the excitation polarization. The measurement of PL emission with different polarizations and at different detection directions reveals that the PL emission from these highly-ordered structures has strong dependence on the polarization and emission direction. We also studied the population evolution in this nano-needles film under 400 nm excitation. The ultrafast dynamics show a 2 ps relaxation process of the excitation state and a 25 ps transition from the singlet state to the ground state.2. Inorganic material: Zinc oxide (ZnO)The multi-photon absorption induced ultraviolet stimulated emission were observed from both ZnO bulk crystals and ZnO nanorods under the pumping of the femtosecond pulses at wavelength of 800nm. For ZnO bulk crystals, the PL emission peak shows a red-shift with the increasing pumping, while the ultrafast dynamic shows a lifetime of about 4 ps, so the optical gain from ZnO crystal is attributed to the electron hole plasma (EHP). For ZnO nanorods, the formation time of the stimulated emission peak was determined to be longer than 5 ps, and the mechanism of the stimulated emission is attributed to the exciton-exciton scattering (EE) process. With the increase of the excitation flux, the PL peak shows an obvious red-shift, and the formation time decreases, so the EHP becomes the main optical gain mechanisms under high excitation flux.The research shows that two materials are promising candidates in the blue LED field. The organic material PSP may have potential applications in polarized LED.3. We also studied the energy transfer processes in light harvesting complexes (LH2) from Rb.Sphaeroides by using femtosecond pump-probe technique. At excitation wavelength around 835 nm, a sharp photobleaching signal was observed which was assigned to the contribution of the two-exciton state, further confirmed by the power dependence measurement. Rate equations with eight-level scheme were used to simulate the population evolution in LH2 and the transient dynamics under femtosecond pulse excitation. The photo-oxidation effect of LH2 was investigated by using the absorption spectroscopic and fs pump-probe techniques. The experimental results reveal that in comparison with the unoxidized BChl-B850 molecules, BChl-B850 radical cation in oxided B850 could act as an efficient relaxation channel to rapidly trap the excitation energy, but the B800→B850 energy transfer process is almost unaffected in the oxidation process.