Kong Xianggui

Photoluminescence and optically pumped ultraviolet lasing from nanocrystalline ZnO thin films prepared by thermal oxidation of high-quality ZnS thin films

We present a simple and useful method for preparing high-quality nanocrystalline ZnO thin films, i.e. the thermal oxidation of high-quality ZnS films prepared by the low-pressure metal-organic chemical vapour deposition technique. The x-ray diffraction measurements reveal that the nanocrystalline ZnO has a hexagonal wurtzite structure. Raman spectra show that the longitudinal optical phonon with the E1-mode appears at 578 cm-1. The multiple phonon scattering process is also observed, indicating the formation of a high-quality nanocrystalline ZnO thin film. The photoluminescence spectrum has a single emission peak at 3.264 eV from the free-exciton mission, under the condition of low excitation power at room temperature. However, when excitation intensities exceed the threshold of 150 kW/cm2, a new and narrow peak emerges at lower energies, which are attributed to exciton-exciton collisions, and is called the P line. The intensity of this peak increases superlinearly with the pumping power over a threshold value. This supplies strong evidence of stimulated emission. The multiple longitudinal cavity modes observed in the stimulated emission spectrum indicate the successful realization of optically pumped lasing from nanocrystalline ZnO films at room temperature.

Femtosecond Laser Induced Optical Waveguides and Micro-mirrors Inside Glasses

Optical waveguides and micro-mirrors have been successfully induced inside fused silica glass and k9 glass, respectively, by focusing a 800 nm femtosecond (fs) pulsed laser with a repetition rate of 1 kHz. The change of refractive index was determined to be 0.001-0.008 in the fused silica glass and 0.006 in the k9 glass. The refractive index change is dependent on both the dose of irradiation and the power density of the fs pulsed laser. Photoluminescence was observed in the irradiated region, and was attributed to the defects induced by fs laser irradiation. We discuss the relationship between the optical property and the luminescent property of the irradiated region.

Crystal characterization of GaP epilayer grown on GaAs by atmospheric pressure metalorganic vapor phase epitaxy

The crystal perfection in GaP film grown on GaAs by atmospheric pressure metalorganic vapor phase epitaxy has been studied by the use of double crystal X-ray diffraction, backscattering spectrometry and Raman scattering techniques. By means of the morphology and full-width at half maximum of X-ray diffraction peak for the GaP epilayers, the growth temperature and V/III ratio were optimized. In the temperature range from 720 to 800°C and with the V/III ratio range from 10 to 80, it was concluded that the optimum growth temperature was 800°C with a V/III ratio of approximately 15. The result of backscattering spectrometry revealed that the minimum yield for the GaP epilayer grown under nearly optimized growth conditions exceeded that for a perfect crystal. In addition, the residual strain of GaP epilayer was calculated by using a biaxial stress model and Raman scattering measurement.

Structural Changes in Nitrogen-Doped a C:H Films

The influence of nitrogen on the microstructure of a-C:H film is investigated by infrared, photoluminescence (PL), and transmittance-reflectance spectra. It is indicated that nitrogen incorporation into a-C:H network will weaken some C-H bonds, which is confirmed by differential scanning calorimeter experimental results. It is also demonstrated that nitrogen entering into a-C:H film can raise luminescence intensity. On the other hand, nitrogenation will reduce PL thermal stability.