二维旋转直柱光子晶体的频率带隙结构分析

设计了一种二维方形旋转正四边形直柱光子晶体,利用平面波展开方法计算了其光子频率带结构,发现在低频和高频区域,该类光子晶体的光子频率禁带明显增大.计算了空气中Al材料的旋转四边形直柱光子晶体的带结构和态密度,当填充比等于0.5时存在绝对带隙,旋转角度为45°时绝对带隙最大,旋转角度为0时,光子频率禁带位于高频区域.利用FDTD方法检验了计算结果,并分析了旋转角度为45°时,正四边形直柱光子晶体的波导特性以及TM模的电场分布.     

一种基于局域共振的低频超宽带隙瓣状声学超材料

针对低频声波的衰减问题,设计了一种基于局域共振(Local Resonance,LR)机理的带瓣型结构的声学超材料,并运用COMSOL有限元软件MULTIPHYSICS 4.3计算分析了所设计的结构元胞带隙特性及其振动特性,研究表明,所设计的声学超材料比无瓣型声学超材料带隙更宽,并且仅用单层结构同种元胞组合就可以在一定频率下形成较宽的多个完全带隙,而不需要通过不同种元胞组合或是多层结构的复合而达到一定带隙特性。在此基础上,进一步对所设计的声学超材料的几何尺寸进行了优化。     

Full photonic band gap properties of plasma photonic crystals with triangular structure

In this study, we analyse full photonic band gap (PBG) properties of two-dimensional plasma photonic crystals (PCs) with triangular lattice, composed of anisotropic tellurium rods with different geometrical shapes immersed in plasma background. Using the finite-difference time-domain method, we discuss the maximization of the full PBG width as a function of tellurium rods parameters with different shapes and orientations. The numerical results show that our proposed structures represent significant large full PBGs in comparison to previously studied plasma PCs.     

Development of NLO tunable band gap organic devices for optoelectronic applications

This paper, presents a preliminary guide to tune the band gap of photonic crystals for optoelectronic applications. This study includes the theoretical calculations to determine the optical constants of a material, synthesis of a new photonic crystal, and a technique for photonic band gap tuning which is minimally required for developing the optoelectronic device. It is observed from the optical studies that the band gap can be tuned by the replacement of the functional group in the compound. It is confirmed that this new crystal has the maximum transparency in the entire visible region and hence it should exhibit a non-linear optical property. The presence of moieties with intermolecular hydrogen bonding in the lattice of such compounds is responsible for the second harmonic generation (SHG).     

Studies on the optical band gap and cluster size of the polyaniline thin films irradiated with swift heavy Si ions

Polyaniline thin films prepared by RF plasma polymerisation were irradiated with 92 MeV Si ions for various fluences of 1×10¹¹, 1×10¹² and 1×10¹³ ions/cm². FTIR and UV-vis-NIR measurements were carried out on the pristine and Si ion irradiated polyaniline thin films for structural evaluation and optical band gap determination. The effect of swift heavy ions on the structural and optical properties of plasma-polymerised aniline thin film is investigated. Their properties are compared with that of the pristine sample. The FTIR spectrum indicates that the structure of the irradiated sample is altered. The optical studies show that the band gap of irradiated thin film has been considerably modified. This has been attributed to the rearrangement in the ring structure and the formation of CC terminals. This results in extended conjugated structure causing reduction in optical band gap.     

Optical band gap and optical constants in amorphous Se96−xTe4Agx thin films

The optical constants (absorption coefficient (α), refractive index (n), extinction coefficient (k), real and imaginary part of dielectric constant) have been studied for a-Se_96−xTe₄Ag_x (where x=0, 4, 8, 12) thin films as a function of photon energy in the wavelength range (500-1000 nm). It has been found that the optical band gap increases while n and k decreases on incorporation of Ag in Se-Te system. The value of α and k increases, while the value of n decreases with incident photon energy. The results are interpreted in terms of the change in concentration of localized states due to the shift in fermi level. A correlation between the optical band gap and electronegativity of the alloys indicates that the optical band gap increases with the decrease of electronegativity.     

窄带隙聚吡咯甲烷衍生物的制备与光学性能

由于高分子发光材料具有柔韧性好、合成简单、可大面积制备和使用寿命长等优点,成为近年来显示和照明领域的研究热点。本文以N-甲基吡咯和苯甲醛类为单体,采用缩聚法制备了3种可溶性、窄带隙、分子主链为部分共轭的聚吡咯甲烷衍生物,并利用红外光谱、紫外-可见光谱、荧光光谱、X射线衍射谱和热失重分析等方法对其结构、光致发光性质和热稳定性能等进行了测试分析,比较了大分子链上苯环对位取代基对材料发光性能的影响。研究结果表明,3种衍生物的热稳定性较好且具有部分结晶性,其光学禁带宽度均小于1.6 eV,属于窄带隙聚合物;在355 nm左右激发光下,3种衍生物均会产生约420 nm的发射光,为蓝紫色发光材料,且大分子链上苯环的对位取代基对该类材料的发光行为基本没有影响;在循环伏安曲线中,3种衍生物均存在较为明显的氧化还原峰。聚吡咯甲烷衍生物在光电器件材料方面有着潜在的应用价值。     

Genetic optimization of two-dimensional photonic crystals for large absolute band gaps under light line

A binary genetic algorithm with floating crossover and mutation probabilities is used to design two-dimensional photonic crystals for large absolute band gaps under a light line. The unit cell is composed of a small number of round rods and is arranged in a square lattice. The photonic band structure of each chromosome is calculated by the plane-wave expansion method. Starting from randomly generated photonic crystals, the genetic algorithm finally yielded a photonic crystal with an absolute common band gap of 0.0618(2πc/a) at the mid-frequency of 0.4084(2πc/a).     

Two- and three-dimensional models for analysis of optical absorption in tungsten disulphide single crystals

The optical energy gaps of WS₂ single crystal were determined from the analysis of the absorption spectrum near the fundamental absorption edge at room temperature using light parallel to c-axis incident normally on the basal plane. On the basis of two- and three-dimensional models it was found that both direct and indirect band transitions took place in WS₂ bdand the indirect transition was of the allowed type. The optical energy gaps corresponding to both transitions were determined and the phonon energies associated with the indirect transitions estimated. The implications of the results have been discussed.     

Variation of crystallite size of Al1-x In x N for different values of x and band gap

By using wide angle X-ray scattering (WAXS) technique, we have characterized the epitaxial Al_1-xIn_xN films (thickness, 150 nm) with 0 <x < 1 grown by plasma source molecular beam epitaxy on sapphire (0001) at the low substrate temperature of 375°C, by computing the crystallite size in these samples. We observe that the crystallite size decreases with increase in the concentration of indium and that the band gap in these samples has a direct correlation with the crystallite size.     

Effect of thermal annealing on some electrical properties and optical band gap of vacuum evaporated Se65Ga30In5 thin films

Electrical properties and optical band gap of amorphous Se₆₅Ga₃₀In₅ thin films, which were thermally evaporated onto chemically cleaned glass substrates, have been studied before and after thermal annealing at temperatures above the glass transition temperature and below the crystallization temperature. The I-V characteristics, which were recorded in the temperature range (200-300 K), were obtained at different voltages and exhibit an ohmic and non-ohmic behavior at low (0-5 V) and high (5-18 V) voltages, respectively, for annealed and as-prepared films. Analysis of the experimental data in the high voltage range confirms the presence of space charge limited conduction (SCLC) for annealed and as-prepared films. The dependence of DC conductivity on temperature in the low voltage region shows two types of conduction channels: The first is in the range 270-300 K and the other at the lower temperature range (200-270 K). The conduction in the first region is due to thermally activated process, while in the other is due variable range hopping (VRH) of charge carriers in the band tails of the localized states. After annealing, the conductivity has been found to increase but the activation energy decreases. This is attributed to rupturing of Se-In weak bonds and formation of Se-Ga strong bonds. This process changes the concentration of defects in the films which in turn decreases the density of states N(E_F) as predicted by Mott's VRH model. Analysis of the absorption coefficient of annealed and as-prepared films, in the wavelength range 300-700 nm, reveals the presence of parabolic densities of states at the edges of both valence and conduction bands in the studied films. The optical band gap (E_g) was obtained through the use of Tauc's relation and is found to decrease with annealing temperature.     

Linearization techniques for band structure calculations in absorbing photonic crystals

Band structure calculations for photonic crystals require the numerical solution of eigenvalue problems. In this paper, we consider crystals composed of lossy materials with frequency‐dependent permittivities. Often, these frequency dependencies are modeled by rational functions, such as the Lorentz model, in which case the eigenvalue problems are rational in the eigenvalue parameter. After spatial discretization using an interior penalty discontinuous Galerkin method, we employ a recently developed linearization technique to deal with the resulting rational matrix eigenvalue problems. In particular, the efficient implementation of Krylov subspace methods for solving the linearized eigenvalue problems is investigated in detail. Numerical experiments demonstrate that our new approach is considerably cheaper in terms of memory and computing time requirements compared with the naive approach of turning the rational eigenvalue problem into a polynomial eigenvalue problem and applying standard linearization techniques. Copyright © 2011 John Wiley & Sons, Ltd.     

Work function and valence band structure of tin-doped indium oxide thin films for OLEDs

Transparent conducting ITO thin film has been widely used as anode material in OLEDs due to its good optical transparency, low electrical resistivity, ease of patterning, high work function and efficient hole injection properties. The interface between ITO and organic layer in OLED device is thus important and can influence the electrical and luminescent properties. In this report, ITO substrates were treated with 20% H₃PO₄ solution. The corresponding changes in crystalline morphology were studied by X-ray diffraction. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) were performed at ∼10⁻⁹ Torr to study the work function and the valence band structure of ITO substrates. It was found that work function became slightly lower after the treatment, probably caused by the formation of metal complex compounds and metal hydroxides. The binding energy of In 3d_5/2 shifted from 444.6 to 445.3 eV. This shifting was referred to the formation of In-OH bonding. It would be easier to provide electron by In-OH bonding than by In-O-In or Sn-O-Sn when photons reached ITO surface. The interface between ITO and CuPc was improved through polar surface and less aggregation. In addition, the OLED devices exhibited improved performance in both external quantum efficiency and luminescence efficiency.     

Structural and optical properties of phosphate-zinc-nickel oxide glasses for narrow band pass absorption filters

Novel two phosphate glass systems with compositions 50P₂O₅–30ZnO–20NiO and 50P₂O₅–30ZnO–20[NiCl_2–6H₂O] were prepared using a conventional melt-quench technique. The absorbance and transmittance were measured using a spectrophotometer in the spectral range between 190 and 1100 nm. The data demonstrates that the system acts as a narrow bandpass optical absorption filter, with a transmission band in the UV region of 311–376 nm, which is centred at 350 nm and has a full width half maximum (FWHM) of almost 34 nm, whereas, the red region is the UV region of 617–684 nm, and centred at 650 nm and has a full width half maximum (FWHM) of nearly 32 nm. However, the refractive index (n), optical band gap (E_opt), non-linear refractive index n₂, third order non-linear susceptibility χ⁽³⁾ and non-linear absorption coefficient β were also calculated. It was apparent that the non-linear refractive index, third order non-linear susceptibility and non-linear absorption coefficient increases by decreasing the optical energy gap. Finally, we investigated the structure of the prepared glasses by using Raman and FTIR spectra. We found that the local network structure based mainly on Q¹ and Q² tetrahedron units connected by P–O–P linkages.     

Synthesis of poly(2,5-Thienylene Vinylene) and its derivatives: Low band gap materials for photovoltaics

Poly-(2,5-Thienylene Vinylene) (PTV) derivatives have been synthesised via the “dithiocarbamate precursor route” in good yield and satisfactory molecular weight. Structural characterisations of the conjugated PTV polymers reveal an optical band gap around 1.8 eV. Organic field effect transistors and organic based photovoltaic devices were made with the plain PTV, via a precursor approach. More specifically, solar cells were produced using a blend of the precursor PTV polymer and PCBM (1-(3-methoxycarbonyl) propyl-1-phenyl [6,6] C₆₁) at various ratios, converted in situ in thin film. A promising power efficiency of 0.76% was achieved.     

Cr和Mo掺杂对单层WSe_2能带结构影响的第一性原理研究

采用基于密度泛函理论的第一性原理平面波赝势方法,研究了Cr和Mo掺杂对单层WSe2能带结构的影响。计算结果表明,Mo对单层WSe2的能带结构没有影响,而Cr则影响很大。随着掺杂浓度的增加,带隙宽度逐渐减小,能带由原来的直接带隙变为了间接带隙。Cr掺杂后所产生的应力是导致能带结构发生变化的直接原因。     

Anisotropy of inter-band transitions and band structure of Cs3Zn6B9O21 nonlinear optical crystals

Polarized optical functions near the fundamental absorption edge of novel Cs₃Zn₆B₉O₂₁ nonlinear optical crystals possessing a wide UV transparency down to 190 nm were investigated. The anisotropy of optical functions is not well studied yet which restrains the further strategy of the formation on its base of solid state compounds with desirable properties. The studies were performed using a band structure calculations as well as the experimental measurements of fundamental polarized UV absorption edge and X-ray photoelectron spectra of the studied crystals. The experimental data were used for evaluation of scissor factors of the band structure. The results of the calculations were compared with the XPS and polarized absorption optical spectra of the investigated crystals. The introduction of the scissor factor was performed in order to obtain a sufficient agreement with experiment. The observed anisotropy of absorption may be explained by the anisotropy of carrier effective masses.     

基于辛数学方法的一维声子晶体禁带计算

将一维声子晶体的原胞简化为有限多个自由度的弹簧振子结构,在辛对偶变量体系下探讨晶格振动,引入辛数学方法确定波矢与本证值的色散关系。通过本证值计数法计算特征频率,从而得到禁带区间。与传统集中质量法相比,该算法的计算结果与之吻合很好,且提高了计算精度和计算效率,更重要的是在低频处收敛性更好。     

（001）应变对正交相Ca2P0.25Si0.75能带结构及光学性质的影响

为了研究(001)应变对正交相Ca2P0.25Si0.75能带结构及光学性质的影响,采用第一性原理贋势平面波方法对(001)应变下正交相Ca2P0.25Si0.75的能带结构及光学性质进行了模拟计算.计算结果表明:晶格(001)面发生100%~116%张应变时,带隙随着应变增加而减小;在晶格发生88%~100%压应变时,带隙随着张应变的增加而增加;84%~88%压应变时,带隙随着压应变的增加而减小.当施加应变后光学性质发生显著的变化:随着压应变的增加,静态介电常数、折射率逐渐减小,张应变则增大.施加压应变反射向高能方向偏移,施加张应变反射向低能方向偏移,但施加应变对反射区域的影响不显著.施压应变吸收谱、光电导率的变化与介电函数和折射率相反.综上所述,(001)应变改变了Ca2P0.25Si0.75的电子结构和光学常数,是调节Ca2P0.25Si0.75光电传输性能的有效手段.     

ZnO对碲铌基重金属氧化物玻璃的结构和光学性能的影响

TeO2 Nb2O5系统重金属氧化物玻璃是一种新型的重金属氧化物玻璃材料,具有优良的非线性光学性能。本文研究了ZnO对碲铌基玻璃的结构、非线性光学性能和光学透过率的影响,研究表明:碲铌基玻璃的非线性光学效应源于电子云畸变效应,掺杂少量ZnO有利于玻璃非线性光学性能的提高,增加ZnO至2.5%(质量分数)时用四波混频法测得的玻璃三阶非线性极化率χ(3)可达9.2×10-13esu,然后随ZnO含量增至10%(质量分数)时χ(3)减至6.0×10-13esu。ZnO含量在10%～15%(质量分数)时χ(3)则无明显变化。同时,加入15%(质量分数)的ZnO有利于玻璃透过率的提高。Raman光谱研究表明,掺入过多的ZnO(>5%(质量分数))时,网络间隙中的Nb5+可被Zn2+取代而进入网络,同时提供更多的桥氧,从而使玻璃网络中的[TeO4]4-基团向[TeO3]2-转化。