Design and properties analysis of multi-layer dielectric used in pulse compressor gratings

Used in chirped-pulse amplification system and based on multi-layer thin film stack, pulse compressor gratings (PCG) are etched by ion-beam and holographic techniques. Diffraction efficiency and laser-induced damage threshold rely on the structural parameters of gratings. On the other hand, they depend greatly on the design of multi-layer. A theoretic design is given for dielectric multi-layer, which is exposed at 413.1 nm and used at 1053 nm. The influences of coating design on optical characters are described in detail. The analysis shows that a coating stack of H3L (H2L)9H0.5L2.01H meets the specifications of PCG well. And there is good agreement of transmission between experimental and the theoretic design.     

Dielectric loss of quartz crystals electrodiffused with either Na+ or Li+

Dielectric relaxation techniques were used in order to better understand the behavior of defects that affect the frequency stability of α-quartz crystals. Crystals were studied that have been swept (electrodiffused) either with Na⁺ or with Li⁺. The Na-swept samples showed two well-known loss peaks (called α and β) due to the Al-Na pair, corresponding to the Na⁺ ion, respectively, in nn and nnn interstitial sites about a substitutional Al³⁺ ion. Precise activation energies and preexponentials were obtained by comparison with analogous anelastic loss peaks. It was also found that the α:β equilibrium is frozen-in below 300 K and can be varied, by heat treatment. Values of the components of the dipole moment of the α and β pairs were determined, and absolute calibrations obtained for the two peaks. In the case of the Li-swept samples, however, there were no peaks present, from which it was concluded that for the Al-Li pair, the Li⁺ ion sits on the twofold symmetry axis of the AlO₄ tetrahedron.     

Ab initio investigations in magnetic oxides

It is argued that band theory can be used as a valuable tool to investigate and model properties of several classes of oxide systems. This theoretical framework is presented in the first part of the article with details on the density functional theory (DFT) and its approximations leading to the local spin density functional LSDF as well as on the common methods built around it (mainly the augmented spherial wave ASW method used throughout this work). The major part of the article is devoted to case studies meant to illustrate the wealth and limits of the LSDF in addressing the electronic and magnetic structures of a selected variety of transition metal oxides of technological importance. Such systems are iron monoxide “FeO” (mainly the high pressure form), the archetype of half metallic ferro magnets CrO₂, used in magnetic recording media, the simple and double perovskite derived systems: SrFeO₃ and La₂TIrO₆ (T=Mn, Fe, Co) as well as manganese oxides (CaMnO₃, Ca₄Mn₄O₁₀ and Tl₂Mn₂O₇) for which a giant magnetoresistive GMR behaviour was identified in recent years. The computed ground state magnetic configurations and electronic structure results are discussed and confrontations with experimental data are carried out when available. The chemical bonding properties are analysed, and a new conceptual approach is provided.     

Two-dimensional microwave band-gap structures of different dielectric materials

We report the use of low dielectric constant materials to form two-dimensional microwave band-gap structures for achieving high gap-to-midgap ratio. The variable parameters chosen are the lattice spacing and the geometric structure. The selected geometries are square and triangular and the materials chosen are PTFE (ε = 2.1), PVC (ε = 2.38) and glass (ε = 5.5). Using the plane-wave expansion method, proper lattice spacing is selected for each structure and material. The observed experimental results are analyzed with the help of the theoretical prediction     

Wear-out of Al-Ta2O5/SiO2-Si structures under dynamic stress

Wear-out of Al-Ta₂O₅/SiO₂-Si stacked layers under dynamic current stresses was studied. It was found that a detrapping of negative charges occurs between the pulses, similarly to SiO₂ and SiO_xN_y films. Additional consumption of the SiO₂ interfacial layer results in a decrease of the gate voltage in some stages of the stress, depending upon the stress time and current density.     

Optimization of near-field optical field of multi-layer dielectric gratings for pulse compressor

Multi-layer dielectric (MLD) gratings for pulse compressors in high-energy laser systems should provide high diffraction efficiency as well as high laser induced damage thresholds (LIDT). Nonuniform optical near-field distribution is one of the important factors to limit their damage resistant capabilities. Electric field distributions in the gratings and multi-layer film region are analyzed by using Fourier modal method. Optimization of peak electric field in the gratings ridge is performed with a merit function, including both diffraction efficiency and electric field enhancement when the top layer material is HfO₂ and SiO₂, respectively. A set of optimized gratings parameters is obtained for each structure, which reduce the peak electric field within the gratings ridge to being respective 1.39 and 1.84 times the value of incident light respectively. Finally, we also discuss the effects of gratings refractive index, gratings sidewall angle and incident angle on peak electric field in the gratings ridge.     

Statistical thermodynamics and dielectric properties of dipole-quadrupole hard-sphere fluid

Within a hindered rotation model, approximate analytical expressions have been obtained for thermodynamic functions and the Kirkwood factor of the model polar fluid of dipole-quadrupole hard spheres in the nearest neighbor approximation. A peculiarity of the considered system from the viewpoint of the hindered rotation model is the ability to generate a three-dimensional grid of quadrupole-bonded molecules, as indicated by the behavior of quadrupole hard-sphere fluid at large values of the quadrupole moment.     

Ce, Y-TZP陶瓷的内耗峰和介电损耗峰机制

测量了淬火前后8Ce，0．5Y-TZP陶瓷（稳定剂Y2O3的摩尔分数为0．50％）在室温到250℃温度范围内的内耗行为，并对出现的内耗峰机理进行了研究，发现淬火前后的样品在100℃左右均出现的内耗峰与YZr，VOE偶极子的再取向有关，而淬火后样品出现的高温内耗峰起源于马氏体相变。此外，本文还研究了8Ce，0．5Y-TZP和8Ce，0．75Y-TZP陶瓷在室温到400℃温度范围内的介电损耗，计算了介电驰豫的激活能，结果表明与内耗试验中机械驰豫的激活能一致。     

Influence of impurity hydrogen on the structure and properties of bulk Li and pressure effects

The structure and properties of a 16-atom body-centered cubic lithium cell with an interstitial hydrogen atom are studied using a pseudopotential-plane-wave method within the density functional theory at 0 K and high pressures. The host lattice is dramatically distorted by the introduction of H. Although the hydrogen atom is stable at the tetragonal site in perfect bcc host lattice, it favors the octahedral site formed by six non-equivalent Li atoms after full relaxation of the cell, showing P4/mmm symmetry within the pressures ranging from 0 to 6 GPa. The lattice ratio (a/c) changes irregularly with external pressure at about 3 GPa. The hydrogen band lies in the bottom of the valence band, separated by a gap from the metallic bands, illustrating the electronegativity of hydrogen. High reflectivity in the low frequency area induced by the impurity hydrogen is observed when only interband transitions are taken account of. A dip in reflectivity due to parallel band transitions is observed at ∼0.4 eV. Another dip at ∼4.3 eV appears when external pressure increases over 4 GPa.     

Theory of dielectric relaxations due to the interfacial polarization for two-component suspensions of spheres

A theoretical formula of dielectric relaxation in a form of complex relative permittivity is derived for dilute suspensions of spherical particles of two kinds on the basis of the Maxwell-Wagner theory of interfacial polarization. Another theoretical formula is derived further for concentrated suspensions of spheres of two kinds on condition that the formula derived above holds for the infinitesimally increasing process in concentration of the dispersed spheres. Furthermore a theoretical formula is derived for concentrated suspensions of shelled spheres of two kinds as the extension of the formula for concentrated suspensions. By use of the theoretical formulas proposed, values of the permittivities and the conductivities of the two-component suspensions were calculated for some examples with different sets of phase parameters. Results of the numerical calculation demonstrates dielectric relaxation profiles full of variety and characteristic of the suspensions containing two kinds of spheres covered with or without shells.