胶体晶体自组装排列进展

自组装排列胶体晶体是发展光子晶体等亚微米周期有序结构及新型光电子器件十分重要的环节.高电荷密度单分散胶体球在较弱的离子强度和稀浓度下会自发排列形成紧密堆积的周期性结构(ccp),常常是面心立方(fcc),科学家们以此为基础发展了多种结晶化胶体粒子的方法,包括重力场沉积、电泳沉积、胶体外延技术、垂直沉积、流通池、物理束缚排列及其他的许多方法.目前排列的胶体粒子基本为球形,材料也多为SiO2、PS、PMMA,此外一些复合粒子,主要为核壳粒子的排列这里也稍作介绍,这些方法及其变通的使用可以形成类蛋白石及反蛋白石结构,最终实现光子带隙及其它多种用途。     

Archimedes 32,4,3,4结构光子晶体中与偏振无关的自准直分束器

通过等频图分析并结合时域有限差分法法模拟, 在Archimedes 3²,4,3,4结构排列的二维光子晶体中同一频率下同时实现了电磁波两种偏振态的自准直. 研究发现,在该结构光子晶体中引入缺陷,当线缺陷宽度改变时, TE和TM两种偏振态光束的分束效果将会随之变化.由此通过控制缺陷宽度,分别实现了两种偏振态光束的50% ∶50%分束以及90°大角度光折弯,分束和折弯的效率都较为理想,为未来设计基于光子晶体的新型光子学器件提供了一种新的思路.     

RTCM算法在一维光子晶体缺陷模中的应用

利用RTCM算法研究一维光子晶体的缺陷模.研究了TE波和TM波入射时的情况,通过改变杂质层的光学厚度以及杂质层的折射率从而得出一些有重要指导意义的缺陷模特性.同时对有缺陷的一维光子晶体在窄带滤波器中的应用做了一定程度的探讨.结果表明:正入射时,TE波和TM波的透射率几乎相同,随着杂质层光学厚度的增加,透射峰数目增加,这有助于制作多道窄带滤波器.因此,有缺陷的一维光子晶体可以制作波分复用中的多道滤波器.     

Fluctuation effects in disordered Peierls systems

We review the density of states (DOS) and related quantities of quasi one‐dimensional disordered Peierls systems in which fluctuation effects of a backscattering potential play a crucial role. The low‐energy behavior of non‐interacting fermions which are subject to a static random backscattering potential will be described by the strictly one‐dimensional fluctuating gap model (FGM). Recently, the FGM has also been used to explain the pseudogap phenomenon in high‐T_c superconductors. We develop a non‐perturbative method which allows for a simultaneous calculation of the DOS and inverse localization length for an arbitrary given disorder potential by solving a simple initial value problem. In the white noise limit, we recover all known results by solving a Fokker‐Planck equation. For the physically interesting case of finite correlation lengths, we use analytical and numerical methods to show that a complex order parameter leads to a suppression of the DOS, i.e. a pseudogap, and that for a real order parameter this pseudogap is overshadowed by a singularity in the DOS. We will also consider the case of classical phase fluctuations which applies to low temperatures where amplitude fluctuations are frozen out. For this regime we present analytic results for the DOS, the inverse localization length, the specific heat, and the Pauli susceptibility.     

Electronic Structure and X‐Ray Photoelectron Spectroscopy of Wurtzite GaxAl1‐xN Solid Alloy

Band energy structure of Ga_xAl_1‐xN solid alloy (x=0.15; 0.65) is calculated. All one‐electron methods of band energy calculations (self‐consistent norm‐conserving pseudo‐potential (NCPP), linear muffin tin orbital method (LMTO) and linear combination atomic orbital (LCAO)) methods give considerable disagreement with the experimental data. We demonstrate that better agreement with experiment is achieved when the NCPP wavefunctions are orthogonalised to the core‐like states. Simultaneously additional geometry optimization improve the agreement with the optical data. We have carried out theoretical and experimental investigation of the band density of states for two representatives of the investigated wurtzite Ga_xAl_1‐xN solid alloys (with x=0.15 and 0.65).     

Effects of electron–electron interaction on the band structure in polydiacetylenes

The ground‐state band structure of polydiacetylenes is theoretically studied with the extensional Su–Schriffer–Heeger model supplemented by electron–electron interactions. The results show the following. First, the interval of valence bands (conduction bands) increases because of the electron–electron interactions. Second, the effect of the on‐site Coulomb energy (U) is different from that of the nearest neighbor Coulomb repulsion (V); the competition between U and V shows that U makes the bandwidth narrower and the gap broader, whereas V makes the bandwidth broader and the gap narrower. There is a critical value of U/V. Third, the whole band width (E_w) decreases when the U/V ratio is less than 1.0 and increases when the U/V ratio is greater than 1.0 at V = 2.0 eV. Thus, the ground‐state band structure is sensitive to the U/V ratio. The results also show that electron–electron interactions can play an important role in the band structure of polydiacetylenes. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1656–1661, 2000     

The N‐limit of spectral gap of a class of birth–death Markov chains

We extend Zeifman's method for bounding the spectral gap and obtain the asymptotical behaviour, as N→∞, of the spectral gap of a class of birth–death Markov chains known as random walks on a complete graph of size N. Copyright © 2000 John Wiley & Sons, Ltd.     

初始啁啾对飞秒脉冲在光子晶体光纤中超连续谱产生的影响

采用分步傅里叶方法模拟了初始啁啾对光子晶体光纤中超连续谱产生的影响.根据光纤长度，将光子晶体光纤中脉冲的演化分成初始展宽、剧烈展宽和饱和展宽三个阶段.通过讨论啁啾脉冲和无初始啁啾脉冲在各阶段演化的区别，发现啁啾只在初始展宽和剧烈展宽阶段对光谱有影响，当β₂C<0时啁啾有利于光谱的展宽，当β₂C>0时则刚好相反，在饱和展宽阶段啁啾不再对光谱产生影响.要想利用啁啾脉冲来获得较宽的光谱，必须选择合适的光纤长度，使输出的脉冲处于剧烈展宽阶段.这为利用啁啾脉冲在光子晶体光 关键词： 超连续谱 光子晶体光纤 分步傅里叶法 啁啾     

Electric field patterns in finite two-dimensional wire photonic lattices

Electric field patterns in finite two-dimensional wire photonics lattices are investigated by means of electromagnetic simulations. These studies reveal a strong analogy with the envelope wavefunction approach extensively used for semiconductor superlattices. Several propagating eigenmodes in the pass-band of the periodic structure as well as evanescent modes in the stop-bands are clearly identified. They are interpreted on the basis of the matching of the wavelength of electromagnetic field to the lattice dimensions with an envelope wavefunction resulting of a Fabry–Pérot effect in the cavity formed by the periodic structure. In terms of transmission coefficient, the calculated frequency dependence are in agreement with those measured in finite metallic rod arrays between 50 and 110 GHz.     

Method of calculating band shape for molecular electronic spectra

A method for approximating the band shape of molecular electronic transitions based on a single geometric configuration is described. The band shape is modeled using an empirical parameter to estimate the width at half-height for each transition. In addition to generating a shape for allowed transitions, a procedure is developed for approximating the oscillator strength for the symmetry forbidden bands. The results obtained using these two approximations are then compared with experimental spectra and to the results obtained from stochastic methods for simple organic molecules, such as benzene, naphthalene, and the diazobenzenes. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 781–796, 1998