Yantao Xu

Syntheses and structure of nanocrystalline gallium nitride obtained from ammonothermal method using lithium metal as mineralizator

A double layer CeO2/Nb2O5 thin film by magnetron sputtering was prepared. The property and microstructure of the thin film were carefully characterized by x-ray photoelectron spectroscopy (XPS), transmission electron microscopy, and high resolution transmission electron microscopy, XPS measurement clearly demonstrated that the Nb2O5 layer underlying CeO2 as a electrical-conductive layer could greatly improve the reduction ability of CeO2 thin film compared with that of a single layer CeO2 thin film: Fitting to the XPS experimental data and deconvoluting the spectra showed the increase of Ce3+ concentration in the double layer CeO2/Nb2O5 thin film. The microstructure of the double layer thin film was composed of three distinguishable layers, which are a polycrystalline CeO2 layer, an amorphous Nb2O5 layer, and an intermediate amorphous layer with regard to the silicon substrate. The interface between CeO2 and Nb2O5 was rough and indistinct. A detailed analysis was given from the viewpoint of interdiffusion at the CeO2/Nb2O5 interface

Luminescence in the fluoride-containing phosphate-based glasses: A possible origin of their high resistance to nanosecond pulse laser-induced damage

Fusion power offers the prospect of an almost inexhaustible source of energy for future generations. It was reported that fusion fuel gains exceeding unity on the National Ignition Facility (NIF) were achieved, but so far great deal of scientific and engineering challenges have to be overcome for realizing fusion power generation. There is a bottleneck for color-separation gratings in NIF and other similar inertial confinement fusion (ICF) lasers. Here we show a series of high performance phosphate-based glasses that can transmit the third harmonic frequency (3ω) laser light with high efficiency meanwhile filter the fundamental (1ω) and the second harmonic frequency (2ω) laser lights through direct absorption, and especially they exhibit excellent damage threshold induced by nanosecond pulse laser compared with that of the fused silica used in NIF. Yellowish-orange fluorescence emits during the laser-material interaction process, and it can be tailored through regulating the glass structure. Study on its structural origin suggests that the fluorescence emission is a key factor that conduces to the high laser-induced damage resistance of these glasses. The results also indicated the feasibility of utilizing these high performance glasses in novel color separation optics, allowing novel design for the final optics assembly in ICF lasers.

Synthesis, Raman scattering, and infrared spectra of a new condensed form of GaN nanophase material

A new form of transparent condensed nanophase material of GaN was synthesized directly by ammono-thermal synthetic route. Nano-sized effects and thermal stability of that material were investigated through Raman scattering and infrared spectra. Compared with bulk GaN, we observed the Raman low-energy-shift of the phonon frequency of E-2(high) and the transverse optical mode [E-1(TO)], the infrared high-energy-shift of omega(T), and the variation of relative intensity I-E2/E1(TO). These characteristics can be attributed to the existence Of the interface effects and the vacancy of N in the GaN nanophase material. This material has a high thermal stability even at 900 degrees C as indicated through infrared and Raman spectral investigation of annealed samples of as-synthesized nanophase material.

400 mW narrow-linewidth Tm-doped silica fiber laser output near 1750 nm with volume Bragg grating.

A type of Tm-doped silica fiber laser with narrow-linewidth and output wavelength near 1750 nm was firstly presented, by using a 1550 nm Er-doped fiber laser pump source and a volume Bragg grating (VBG). By means of a 15 cm Tm-doped fiber, a 400 mW continuous wave (cw) at ~1750 nm with FWHW of ~54 pm was generated, corresponding to an average slope efficiency up to 23.5% with respect to absorbed pump power. The influence of gain fiber length on the spectrum wavelength has been investigated in detail. All experimental results demonstrate that this fiber laser will be an effective and promising pump source for mid-IR laser output.

Fabrication of nano-sized AlGaN alloy by dry milling and thermal annealing

Al0.61Ga0.39N alloy nano-powders were obtained by a thermal annealing process from the Al+GaN system in NH3 flow. The activation process of nano-sized Al and GaN powders during dry milling in N-2 plays a key role in the formation of the AlGaN alloy. A too high fraction of aluminium leads to AM, while a too low fraction results in insufficient activation of the Al powder. The GaN localized mode and the AIN localized mode of the E-2 phonon frequency, and the one-mode-type behavior of the A,(LO) and A,(TO) phonon frequencies were observed in the Raman spectra of the alloy

Ultra-short wavelength operation in Thulium-doped silica fiber laser with bidirectional pumping

An ultra-short wavelength operation of Tm-doped all fiber laser based on fiber Bragg gratings (FBGs) was developed. A bi-directional pump configuration for the ultra-short wavelength operation was designed and investigated for the first time. the laser yielded 3.15W of continuous-wave output at 1706.75nm with a narrow-linewidth of ~50pm and a maximum slope efficiency of 42.1%. The dependencies of the slope efficiencies and pump threshold of the laser versus the length of active fiber and reflectivity of the output mirror (FBG) were investigated in detail. An experimental comparative study between two Thulium-doped fiber lasers (TDFLs) with two different pumping configuration(forward unidirectional pumping and bidirectional pumping) was presented. It is indisputable that the development of 1.7μm silicate fiber lasers with Watt-level output power open up a number of heart-stirring and tempting application windows.

High Verdet constants and diamagnetic responses of GeS 2 -In 2 S 3 -PbI 2 chalcogenide glasses for integrated optics applications

Chalcogenide glasses as kind of diamagnetic magneto-optical materials have promising applications in the field of integrated optics and optical communication systems due to their excellent properties, such as easy to be processed into waveguide and temperature independence of the Verdet constants. For clarifying the influence factors following the compositional variation on Faraday effect and finding a glass with a large Verdet constant, novel pseudo-ternary chalcogenide glass system, GeS2 - In2S3 - PbI2, was prepared and investigated. The composition, wavelength and temperature dependences on the Verdet constants were systematically investigated at the wavelengths of 635, 808, 980 and 1319 nm. PbI2 was confirmed to have positive contribution to the Verdet constant and the Becquerel rule was proved to be an effective guidance for predicting the Verdet constant in chalcogenide glasses. The 60GeS2·15In2S3·25PbI2 glass was found to possess the largest Verdet constant (V = 0.215 min·G-1·cm-1, @808nm), which is great larger than that of commercial diamagnetic glasses. These glasses also possess good glass-forming ability and VIS-IR transmittance, therefore be a good candidate for next-generation integrated optical isolator and other magneto-optical devices.

Mid-infrared emissions of Pr{sup 3+}-doped GeS{sub 2}–Ga{sub 2}S{sub 3}–CdI{sub 2} chalcohalide glasses

For elucidation of the glass compositions influence on the spectroscopic properties in the chalcohalide system and the discovery of a new material for applications in mid-infrared fiber-lasers, a serial Pr3+-doped (100-x)(0.8GeS(2).0.2Ga(2)S(3))xCdI(2) (x = 5, 10, 15 and 20) chalcohalide glasses were prepared. similar to 4.6 mu m mid-infrared fluorescence emission from Pr3+ in the sulfide glass is successfully observed at room temperature excited by a 2.01 mu m Tm3+:YAG ceramic laser system, and the effective line-width of fluorescence band is 106-227 nm. Intense compositional dependence of mid-infrared emissions is found. The radiative rates of Pr3+ ions in these glasses were calculated by using the Judd-Ofelt theory