First-principles study of the formation mechanisms of nitrogen molecule in annealed ZnO

Recently, N₂ molecule was reported to induce localized states in the band gap and trap two holes in ZnO. In this Letter, the detailed mechanism for the formation of N₂ molecule in high temperature annealing process in ZnO was investigated based on density-functional theory. By analyzing the interactions between N-related defects, we found that the nitrogen molecule would form by the binding of two interstitial nitrogen atoms. Interstitial oxygen facilitated the formation of N₂ by kicking out N_O to interstitial site. The formation of nitrogen molecule in ZnO would cause low doping efficiency and degeneration of the p-type in annealing process. Our results could explain the recently reported formation of N₂ molecule in high temperature annealing process in N-doped ZnO. Appropriate annealing conditions were suggested in order to get p-type ZnO.     

Hierarchical Mn<Subscript>1.5</Subscript>Co<Subscript>1.5</Subscript>O<Subscript>4</Subscript> microspheres constructed from one-dimensional nanorods as high-performance anode material for lithium-ion battery

Mn_1.5Co_1.5O₄ hierarchical microspheres have been successfully synthesized via a solvothermal method and an annealing procedure. Mn_1.5Co_1.5O₄ exhibits advanced cycling performance, and it retains a reversible capacity of 633 mA h g^?1 at a current density of 400 mA g^?1 with a coulombic efficiency of 99.0% after 220 cycles. Its remarkable performance is attributed to the hierarchical structure assembled with nanorods, which increases the contact area between each nanorod and electrolyte. More significantly, the open space between neighboring nanorods and the pores on the surface of nanorods can improve Li⁺ ion diffusion rate. Furthermore, the nanorods have rapid one-dimensional Li⁺ diffusion channels, which not only possess a large specific surface area for high activity but accommodate the volume change during lithiation–delithiation processes. Therefore, Mn_1.5Co_1.5O₄ hierarchical microspheres can act as a promising alternative anode material for lithium-ion battery.     

Long lasting phosphorescent properties of Ti doped ZrO2

A novel long-lasting afterglow phosphor ZrO₂:Ti is prepared by the conventional solid-stated method and their luminescent properties are investigated. A bluish white strong and broad emissive band, which is attributed to originate from the recombination of electrons trapped by F⁺ centers and the holes created in Valence band, is observed under 254 nm UV irradiation. The identical color long afterglow, which lasts about 1 h, is found in the ZrO₂:Ti phosphor after removing the 254 nm UV light. The mechanism of the long lasting phosphorescence (LLP) has been discussed based on the thermoluminescence (TL) results. The replacement of Zr by Ti produces more anion vacancies, resulting in the enhanced photoluminescence (PL) and LLP of ZrO₂:Ti sample. These results indicate that ZrO₂:Ti phosphor has potential promising applications.     

Dynamically adjusting plasmon-induced transparency and slow light based on graphene meta-surface by bright–dark mode coupling

A graphene-based meta-surface structure consisting of middle strip and side rings is proposed, which can achieve plasmon-induced transparency (PIT) effects by bright–dark mode coupling. By properly adjusting geometry parameters of the meta-surface and gradually varying Fermi energy level of graphene, the PIT window can be actively tuned in a wide range of transmission spectra, allowing for dynamically tunable group delay of the incident light. In this letter, by changing geometry parameters, we find that the value of group delay of light becomes greater when the coupling strength becomes weaker. In other words, the less the transmittance of the PIT window is, the slower the speed of light is.     

Micro-scale Fabry–Perot interferometer with high spectral resolution and tunable transmission frequency via chiral waveguide-emitter coupling

A micro-scale Fabry–Perot interferometer with high spectral resolution and tunable transmission frequency is proposed. In this scheme, two partially reflecting mirrors with a separation of several wavelengths is fabricated in a waveguide, and a two-level emitter is located between the mirrors and coupled to the waveguide with chiral interaction. We analytically show that the single emitter plays the role of a strongly dispersive medium and the full width at half maximum (FWHM) of the transmission fringes around the resonance frequency of the emitter can be narrowed by 5 orders of magnitude. The proposed micro-scale interferometer can have the same spectral resolution as meter-scale traditional interferometers. We also show that the central frequency of the narrowed transmission fringe can be tuned by adjusting the asymmetry of the emitter-waveguide coupling. Our scheme has potential applications in the fields of integrated optical circuit and quantum information processing.     

Anomalous Vibrational Dependence of the Rotational and Hyperfine Parameters in the B(4)Pi-X(4)Sigma(-) Transition of NbO

The laser excitation spectrum of jet-cooled NbO in the region 16 000-18 000 cm(-1) has been recorded at high resolution, giving rotational and hyperfine constants for the levels v=0-3 of the B(4)Pi state and v=1 of the X(4)Sigma(-) state; zero gaps have also been measured at low resolution for some weaker bands involving higher vibrational levels. Taken together with the laser data for the B-X (0,0) band from Adam et al. (J. Chem. Phys. 94, 6240-6262 (1994)) and the Fourier transform emission data for the doublet manifold from Launila et al. (J. Mol. Spectrosc. 186, 131-143 (1997)), the new data give a very complete picture of the vibrational energy level pattern in this region. Strong irregularities in the vibrational dependences of the B(4)Pi rotational and hyperfine constants can be interpreted in terms of spin-orbit interaction between the B(4)Pi state and the f(2)Pi, e(2)Phi, and d(2)Delta states. The interaction is strong enough that all three doublet states can be seen in absorption from the X(4)Sigma(-) ground state, adding to the complexity of the spectrum. The hitherto unknown sigmadeltasigma* (4)Delta state is estimated to lie near 17 500 cm(-1), from the change of sign in the spin-rotation parameter gamma of the B(4)Pi state between v=2 and 3. Copyright 2001 Academic Press.     

Euler calculations with embedded Cartesian grids and small-perturbation boundary conditions

This study examines the use of stationary Cartesian mesh for steady and unsteady flow computations. The surface boundary conditions are imposed by reflected points. A cloud of nodes in the vicinity of the surface is used to get a weighted average of the flow properties via a gridless least-squares technique. If the displacement of the moving surface from the original position is typically small, a small-perturbation boundary condition method can be used. To ensure computational efficiency, multigrid solution is made via a framework of embedded grids for local grid refinement. Computations of airfoil wing and wing-body test cases show the practical usefulness of the embedded Cartesian grids with the small-perturbation boundary conditions approach.     

Kerr property of cubic K0.95Li0.05Ta0.60Nb0.40O3 single crystal

The Kerr (quadratic electro-optic) property of K_0.95Li_0.05Ta_0.60Nb_0.40O₃ was measured by using an automated scanning Mach–Zehnder interferometer. K_0.95Li_0.05Ta_0.60Nb_0.40O₃ has large Kerr effect with R₁₁ = 7.2 × 10⁻¹⁶ m²/V² and R₁₂ = −1.2 × 10⁻¹⁶ m²/V² at 632.8 nm near its phase-transition temperature. The dielectric and Kerr properties as function of temperature were also investigated, thus the quadratic polarization-optic coefficients were calculated. The values of g₁₁ and g₁₂ are 0.083 m⁴/C² and −0.014 m⁴/C², respectively, and do not depend on temperature within the measurement accuracy of 5%.     

Effect of polyvinylpyrrolidone on the structure and laser damage resistance of sol–gel silica anti-reflective films

The effect of polyvinylpyrrolidone (PVP) on the structure and laser-induced damage threshold (LIDT) of sol–gel silica anti-reflective films is investigated. The results of dynamic light scattering, transmission electron microscopy, and small angle X-ray scattering, show that the PVP molecules surrounded the silica sol particles through the strong hydrogen bonds between Si-OH groups and the PVP. As a result, the growth of silica particles was restricted and thus the interface layer between the silica particles and the solvent become thickened with PVP content. Furthermore, the PVP reduced the porosity of the film, so the anti-reflection properties of the film were weakened. A multi-fractal analysis showed that the appropriate addition of PVP, 1 weight percent (wt%), could improve the surface fractal structure of the film, but that higher PVP content resulted in reduced surface uniformity. The addition of PVP lead to improved LIDT.     

干涉型光纤法布里-珀罗传感系统抗噪方法的实验研究

对于干涉型光纤法布里-珀罗(FFP)传感系统,外界环境对解调干涉仪的扰动是其主要噪声源之一.在光纤布拉格光栅构成的FFP传感系统中,对比分析了采用参考传感器和参考光源两种抗噪方法.实验结果表明,对于单频和宽带干扰,两种方法均可提高信噪比到40 dB.而且由于采用窄线宽光纤激光器作为参考光源的噪声水平较低,其差分探测结果具有更好的抗噪效果.