Xi-Shan Li

Low-loss planar and stripe waveguides in Nd3+-doped silicate glass produced by oxygen-ion implantation

We report on the fabrication and characterization of low-loss planar and stripe waveguides in a Nd3+-doped glass by 6MeV oxygen-ion implantation at a dose of 1×1015ions∕cm2. The dark mode spectroscopy of the planar waveguide was measured using a prism coupling arrangement. The refractive index profile of the planar waveguide was reconstructed from a code based on the reflectivity calculation method. The results indicate that a refractive index enhanced region as well as an optical barrier have been created after the ion beam processing. The near-field mode profiles of the stripe waveguide were obtained by an end-fire coupling arrangement, by which three quasitransverse electric modes were observed. After annealing, the propagation losses of the planar and stripe waveguides were reduced to be ∼0.5 and ∼1.8dB∕cm, respectively.

Ion-implanted waveguides in Nd3+-doped silicate glass and Er3+/Yb3+ co-doped phosphate glass

The data are presented on the waveguides formation in the Nd 3þ -doped silicate glass and Er 3þ /Yb 3þ co-doped phosphate glass by the implantations of He þ or Si þ ions, respectively. The prism-coupling method is used to measure the effective refractive indices of the waveguide dark modes. The refractive index profiles of the waveguides are reconstructed by using the reflectivity calculation method (RCM) and a comparison of such profiles among the different waveguides has been made. The reasons for the formation of the present waveguides are analyzed in a primary way. # 2002 Elsevier Science B.V. All rights reserved.

The significant effect of the thickness of Ni film on the performance of the Ni/Au Ohmic contact to p-GaN

The significant effect of the thickness of Ni film on the performance of the Ohmic contact of Ni/Au to p-GaN is studied. The Ni/Au metal films with thickness of 15/50 nm on p-GaN led to better electrical characteristics, showing a lower specific contact resistivity after annealing in the presence of oxygen. Both the formation of a NiO layer and the evolution of metal structure on the sample surface and at the interface with p-GaN were checked by transmission electron microscopy and energy-dispersive x-ray spectroscopy. The experimental results indicate that a too thin Ni film cannot form enough NiO to decrease the barrier height and get Ohmic contact to p-GaN, while a too thick Ni film will transform into too thick NiO cover on the sample surface and thus will also deteriorate the electrical conductivity of sample.

Monomode, nonleaky planar waveguides in a Nd3+-doped silicate glass produced by silicon ion implantation at low doses

Monomode, nonleaky planar waveguides in a Nd3+-doped silicate glass were fabricated by 3.0 MeV Si+ ion implantation at doses ranging from 5×1013 to 6×1014 ions/cm2 at room temperature. The enhancement of the refractive index confined the light propagating in the waveguide without tunneling effects in contrast to the traditional barrier-type waveguide. Annealing at moderate temperatures (about 200 °C) in air was performed, which reduced the propagation loss from 20–30 dB/cm to a minimum of 0.5 dB/cm.

The effect of Zr addition on the formation and structural properties of 3:29 compounds in the Fe-Nd-Ti-Zr system

A new series of 3:29 compounds in the Fe–Nd–Ti–Zr quaternary system were synthesized successfully. The formation and structural properties were studied by means of powder x-ray and powder neutron diffraction. The results reveal that only a small amount of Zr content can exist in the 3:29-type compounds in the Fe–Nd–Ti–Zr quaternary system. Higher Zr content induces the formation of rhombohedral Th2Zn17-type phase. In the structures of 3:29-type compounds, the Zr content partially substitutes for Nd and enters the 2a and 4i crystallographic sites exclusively. Upon Zr substitution, the lattice parameters a, b, c and the unit cell volumes V of (Nd,Zr)3(Fe,Ti)29 compounds decrease monotonically and their intrinsic magnetic properties, including the Curie temperature and saturation magnetization, lessen as well.

Superconducting fluctuation effect in CaFe0.88Co0.12AsF.

Out-of-plane angular dependent torque measurements were performed on CaFe0.88Co0.12AsF single crystals. Superconducting fluctuations, featured by magnetic field enhanced and exponential temperature dependent diamagnetism, are observed above the superconducting transition temperature T c, which is similar to that of cuprate superconductors, but less pronounced. In addition, the ratio of T c versus superfluid density follows well the Uemura line of high-T c cuprates, which suggests the exotic nature of the superconductivity in CaFe0.88Co0.12AsF.

Angular dependent torque measurements on CaFe0.88Co0.12AsF

Out-of-plane angular dependent torque measurements were performed on CaFe0.88Co0.12AsF (Ca1 1 1 1) single crystals. In the normal state, the torque data shows [Formula: see text] angular dependence and H (2) magnetic field dependence, as a result of paramagnetism. In the mixed state, the torque signal is a combination of the vortex torque and paramagnetic torque, and the former allows the determination of the anisotropy parameter γ. At T   =  11.5 K, γ (11.5 K ≃ 0.5 T c)  =  19.1, which is similar to the result of SmFeAsO0.8F0.2, [Formula: see text] at [Formula: see text]. So the 11 1 1 is more anisotropic compared to 11 and 122 families of iron-based superconductors. This may suggest that the electronic coupling between layers in 1 1 1 1 is less effective than in 11 and 122 families.

Structure analysis of Nd3−xYxFe23−yCo6Moy (x=0.36–3.0;y=1.1,0.9) compounds

A series of compounds Nd3−xYxFe23−yCo6Moy (x=0.36–3.0) has been synthesized and studied by x-ray powder diffraction and neutron diffraction. Rietveld analyses of the diffraction data indicate that all the compounds crystallize in A2∕m. The lattice parameters a, b, and c and unit-cell volume V decreased monotonically with the increase of Y content. Neutron-diffraction results show that Mo atoms occupy three sites, two 4i sites and one 4g site, while Co atoms are found to occupy all the Fe sites not shared with the Mo atoms. However, the most preferential sites for Co are Fe1(2c), Fe8(8j), and Fe11(4e). It is also observed that those sites preferred by Mo have the long average bond length, while sites preferred by Co have short average bond lengths.