Lu Tiecheng

DEFECTS IN SAPPHIRE:MN INDUCED BY NEUTRON IRRADIATION

Abstract After neutron irradiation, some impurity ions changed their valence states, as well as some new defects such as vacancies, interstitials and the clusters were produced. The thermoluminescence (TL) with electron paramagnetic resonance (EPR), positron annihilation technique (PAT) measurements were used to determine the defects in Sapphire: Mn irradiated by fast neutron of 2 MeV average energy and thermal neutron of 0.023 eV. The results of TL show that before irradiation peaks of 171°C, 259°C, 320°C, 350.2°C and 408°C all belong to the intrinsic crystal, the peak of 196°C is induced by impurity ion of Mn. After neutron irradiation, the peaks all moved to higher temperature, but got different shifts and produced a new peak of 154°C. The shift of peak 196°C is about 30°C. It is explained that Mn ion aggregated with a nearest defect and formed a cluster, which corresponding to EPR results might be Mn 2+ –F cluster and the nearest defect might be F type center. The mechanism of other shifts and another peak are also discussed in the paper.

Terahertz plasmon and surface-plasmon modes in cylindrical metallic nanowires

We present a theoretical study on collective excitation modes associated with plasmon and surface-plasmon oscillations in cylindrical metallic nanowires. Based on a two-subband model, the dynamical dielectric function matrix is derived under the random-phase approximation. An optic-like branch and an acoustic-like branch, which are free of Landau damping, are observed for both plasmon and surface-plasmon modes. Interestingly, for surface-plasmon modes, we find that two branches of the dispersion relation curves converge at a wavevector qz = qmax beyond which no surface-plasmon mode exists. Moreover, we examine the dependence of these excitation modes on sample parameters such as the radius of the nanowires. It is found that in metallic nanowires realized by state-of-the-art nanotechnology the intra- and inter-subband plasmon and surface-plasmon frequencies are in the terahertz bandwidth. The frequency of the optic-like modes decreases with increasing radius of the nanowires, whereas that of the acoustic-like modes is not sensitive to the variation of the radius. This study is pertinent to the application of metallic nanowires as frequency-tunable terahertz plasmonic devices.

Variation of morphology and color of VO2 thin films induced by excimer laser

Abstract The samples of VO 2 thin films are irradiated by excimer laser with a wavelength of 308 nm under different output energy. Precipitation and recrystallization phenomenon on the surface are observed in the irradiated samples with lower and higher irradiation energy of laser respectively. Hence, the obvious variation of morphology is observed in irradiated samples with different irradiation energy. Precipitation process just slightly brings down the intensity of X-ray diffraction (XRD) peaks but recrystallization process can change the intensity and width of XRD peaks of samples. The results of laser Raman spectroscopy (LRS) and scanning electron miscroscopy (SEM) analysis indicate that the size of crystalline grain increases after recrystallization. The other optical properties of samples are also measured before and after irradiation.