Gao Xuexi

Size-dependent optical properties and carriers dynamics in CdSe/ZnS quantum dots

Size-dependence of optical properties and energy relaxation in CdSe/ZnS quantum dots (QDs) were investigated by two-colour femtosecond (fs) pump-probe (400/800 nm) and picosecond time-resolved photoluminescence (ps TRPL) experiments. Pump-probe measurement results show that there are two components for the excited carriers relaxation, the fast one with a time constant of several ps arises from the Auger-type recombination, which shows almost particle size-independence. The slow relaxation component with a time constant of several decades of ns can be clearly determined with ps TRPL spectroscopy in which the slow relaxation process shows strong particle size-dependence. The decay time constants increase from 21 to 34 ns with the decrease of particle size from 3.2 to 2.1 nm. The room-temperature decay lifetime is due to the thermal mixing of bright and dark excitons, and the size-dependence of slow relaxation process can be explained very well in terms of simple three-level model.

Second Harmonic Generation Properties of Centrosymmetric Bisphthalocyaninato Thulium in Langmuir-Blodgett Films

The second order nonlinear optical properties of sandwich thulium bisphthalocyanine (TmPc2) molecule Langmuir-Blodgett (LB) films were investigated using the second harmonic generation (SHG) method. The dependence of the second harmonic intensity on the incident angle of the fundamental beam was measured and the mechanisms of nonlinearity were discussed briefly. The experimental results indicated that the second harmonic signal intensity was very strong and its maximum was obtained at an incident angle of 45°. The second order nonlinear optical susceptibility χ (2) was about 1.152×10 - 8 esu and the hyperpolarizability β was about 1.905×10 - 30 esu. By measuring the polarization properties of the second harmonic signal for the LB film and comparing this with the theoretical analysis, we found that the origin of the second harmonic generation was attributed to the electric quadrupole mechanism for the TmPc2 molecule.