Atomic structure of the Si(5 5 12)-2 × 1 surface

Based on the results of scanning tunneling microscopy studies of the reconstructed Si(5 5 12)-2 × 1 surface, its atomic structure has been found. It turns out that Si(5 5 12)-2 × 1 consists of four one-dimensional structures: honeycomb (H) chain, π-bonded H′ (π) chain, dimer-adatom (D/A) row, and tetramer (T) row. Its period is composed of three subunits, i.e., (i) (3 3 7) unit with a D/A row [D(3 3 7)], (ii) (3 3 7) unit with a T row [T(3 3 7)], and (iii) (2 2 5) unit with both a D/A and a T row. Two kinds of adjacent subunits, T(3 3 7)/D(3 3 7) and D(3 3 7)/(2 2 5), are divided by H chains with 2× periodicity due to buckling, while one kind of adjacent subunits, T(3 3 7)/(2 2 5), is divided by a π chain with 1× periodicity. Two chain structures, H and π chains, commute with each other depending upon the external stresses perpendicular to the chain, which is the same for two row structures, D/A and T rows. It can be concluded that the wide and planar reconstruction of Si(5 5 12)-2 × 1 is originates from the stress balance among two commutable chains and two commutable rows.     

Many body effects in the electronic and optical properties of the (1 1 1) surface of diamond

The (1 1 1) face is the cleavage surface of diamond. Understanding its properties is very important for the growing technological interest on the chemistry of diamond surfaces. Within DFT the most stable reconstruction is the Pandey chain model, the atoms on the chain being neither buckled nor dimerised. However this geometry gives rise to a semi metallic band structure in contrast with experimental findings that show the presence of a gap ranging from 0.5 to 2 eV. Here we show that the same equilibrium geometry and thus the same metallic band structure is found relaxing the surface using screened exchange (sX) or Hartree-Fock (HF) functionals. We will discuss in detail how breaking the equivalence of the atoms on the chain affects the band structure and we will show that a buckling would yield a semiconducting surface, but is energetically unfavorable. A semiconducting character can be restored, within the equilibrium geometry, if quasiparticle corrections are carefully included within an iterative GW scheme. The result of the theoretical reflectance anisotropy spectra (RAS) at a DFT-RPA level are also presented and discussed. As expected, a strong anisotropy signal is found at low energies due to transitions between surface states inside the fundamental gap.     

NO dissociation pathways on Rh(1 0 0), (1 1 0), and (1 1 1) surfaces: A comparative density functional theory study

The chemisorption and dissociation pathways of NO on the Rh(1 0 0), (1 1 0), and (1 1 1) surfaces are studied by the plane-wave density functional theory (DFT) with CASTEP program. In addition, the electronic and geometrical effects that affect the NO dissociation reactions have been investigated in detail. The calculation results are presented as following: The effective activation energies of the best NO dissociation pathways on the Rh(1 0 0), the Rh(1 1 0), and the Rh(1 1 1) are 0.63, 0.66 and 1.77 eV, respectively. The activity of the Rh planes for NO dissociation is in the order of Rh(1 0 0) ≈ Rh(1 1 0) > Rh(1 1 1). The low dissociation barrier for Rh(1 0 0) and Rh(1 1 0) is associated with the existence of a lying-down NO structure which acts as a precursor for dissociation. By Mulliken population analysis and structure analysis, both electronic and geometrical effects are found to affect the NO dissociation reactions, but the geometrical effect exceed the electronic. The energy decomposition scheme has been used to provide further insight into the NO dissociation reactions. Based on the calculations, the interaction energy between N and O in the transition state on the Rh(1 1 1) is found much larger than that on the Rh(1 0 0) and the Rh(1 1 0). The major differences of should originate from the variation of the bonding competition effect.     

为2007年亚奥赛物理实验命题的体会

笔者被聘为2007年亚奥赛物理实验命题,经过半年多的努力终于将科研成果——激光测温合理地转换为一个奥赛实验试题——干涉法测量玻璃热膨胀系数和折射率温度系数,本文主要介绍了命题过程及体会.     

应用键轨道连接矩阵方法预测烷烃折光率.

应用键轨道连接矩阵方法提取结构参数PX1CC和PX1CH， 并用于建立预测烷烃折光率的QSPR模型．该模型不仅预测精度较高(其误差仅为0.0048)，而且模型中采用的参数表达了直接与分子的折光能力相关的结构信息(即烷烃分子中电子被极化的能力)．此参数提取简单，既具备拓扑指数简单、易算，物理意义明确．     

Modal attenuation in metal-clad graded-index slab optical waveguides

The paper presents the influence of refractive profile shape on the distribution of modal attenuation in planar metal-clad optical waveguide structures. A calculation method has been presented in which we applied the Snyder–Love expression on modal attenuation coefficients and matrix method 4 × 4 for the analysis of planar waveguides. The obtained results are almost identical with the exact results for refractive profiles: the parabolic, exponential, Gaussian and linear ones. It has been demonstrated that the applied calculation method can be used in waveguides of any profiles of the refractive index.     

+1/-1相位掩模板和532nm激光下低损耗As_2S_3硫系光纤布喇格光栅的制备

采用改进型Sagnac干涉光栅写入系统,利用532nm准带隙光曝光源和带+1/-1衍射级的相位掩模板,在两种不同直径的低损耗As_2S_3硫系玻璃光纤上刻写布喇格光栅,并研究曝光期间光栅的动态特性.实验表明,As_2S_3光纤布喇格光栅透射峰值随光纤直径的减小而增强;在曝光过程中,布喇格波长先是较快地向短波长方向移动,随着曝光时间的延长,布喇格波长缓慢地向长波长方向回复.曝光时间为800~1 000s时,在包层直径为140μm的As_2S_3光纤上获得质量良好的布喇格光栅光谱,其透射峰值可达-2.6dB,带宽为0.37nm.对As_2S_3硫系光纤纤芯的光敏性分析结果表明,折射率调制幅度和平均折射率变化随曝光时间分别可达到10-4和10-3数量级.     

单端面长周期光栅透射模式测量技术

为了实现长周期光栅透射谱测量模式的远距离监测,设计了单端面镀反射膜的测量装置系统,对单端面镀银膜长周期光栅的传感原理做了分析,并从实验的角度分别对单端面镀银膜模式系统和直接透射模式系统的长周期光栅在不同折射率的环境介质中的响应进行了研究,比较了它们的异同。首先,采用2×2单模光纤耦合器分别连接光谱分析仪、光源、长周期光栅。然后,在包含长周期光栅的光纤的另一个端面制备反射银膜。最后,通过测量一系列不同折射率的环境介质,比较了直接透射模式与单端面镀银膜模式下的长周期光栅的响应光谱。实验结果表明:采用波长解调表达时,对于同一种环境介质,两种模式下长周期光栅的响应光谱的谐振波长基本相同;采用功率/峰值解调表达时,随着甘油浓度从水变为80%的甘油溶液,直接透射模式下的光损耗从-6.05 d B变为-9.22 d B,单端面镀银膜模式下的光损耗从-8.03 d B变为-11.33d B。与直接透射模式相比,单端面镀银膜的长周期光栅光谱中的相对光损耗明显增加,谐振峰更尖锐,更有利于谐振波长和谐振峰光损耗值的识别。本研究设计的单端面镀银膜的长周期光栅测量系统不仅保留了长周期光栅透射谱的感应模式,而且使长周期光栅在对环境介质的测量中操作更加灵活方便,尤其是在远距离、恶劣环境或深层液体的折射率测量中具有独特的优势。     

一种基于结构性改变的光子晶体光纤光栅理论研究

对一种新型基于结构性改变的光子晶体光纤光栅原理进行了研究.采用多极法分析了结构性改变对折射率的影响,得到有效折射率与包层气孔塌缩之间的关系,建立了结构性改变光子晶体光纤光栅模型.利用耦合模理论对所成光栅性能进行了分析,重点研究了包层空气孔层数、空气孔占空比、气孔塌缩程度对光栅谐振波长、带宽的影响.研究结果表明,光子晶体...     

梯度折射率非局域介质中空间孤子振荡行为研究

应用等效粒子近似方法研究了光学空间孤子在带有局域和非局域非线性横向非均匀介质中的传输动力学行为.发现孤子在传播过程中横向的周期性振荡.折射率调制幅度和波导的归一化宽度决定了振荡周期的大小.介质的非局域对振荡振幅有着较小的影响.模拟了孤子传输过程,所得数值结果与理论分析符合很好.此外,模拟传输还发现多孤子束缚态能够在这个模型中稳定传播.这种振荡特性或许可以应用于光学路由器、转换器、开关等.