Polarization-independent unidirectional light transmission by an annular photonic crystal prism

Unidirectional transmission of light irrespective of its polarization by a two-dimensional annular photonic crystal in the form of a right prism is numerically demonstrated. Band structure of the crystal obtained through the plane-wave expansion method reveals a directional band gap along a principal axis, leading to prohibition of wave transmission in the reverse direction. In the forward direction, however, transmission of waves is facilitated by circumventing the directional band gap due to altered surface orientation. Polarization-independent unidirectional light transmission is demonstrated through finite-difference time-domain simulations. Unidirectional operation is enhanced and the polarization independence is established through the introduction of an anti-reflection coating layer, which increases the forward transmittances for both polarizations up to 0.44, such that a contrast ratio of 0.96 is attained at a free-space wavelength of 1.55 μm. Although polarization independence deteriorates, unidirectionality is preserved between 1.45 μm and 1.60 μm, provided that the angle of incidence remains between ?5° and +5°. Device performance is also influenced by the transverse source size, where leakage in the reverse direction may be suffered if the source width is beyond a critical value.     

Generating ultra wide low-frequency gap for transverse wave isolation via inertial amplification effects

Low-frequency transverse wave propagation plays a significant role in the out-of-plane vibration control. To efficiently attenuate the propagation of transverse waves at low-frequency range, this letter proposed a new type phononic beam by attaching inertial amplification mechanisms on it. The wave propagation of the beam with enhanced effective inertia is analyzed using the transfer matrix method. It is demonstrated that the low-frequency gap within inertial amplification effects can possess much wider bandwidth than using the local resonance method, thus is more suitable for designing applications to suppress transverse wave propagation.     

准方波脉冲形成网络的理论分析与设计

针对微波驱动源、固态调制器等对宽平顶脉冲源的应用需求,开展了基于脉冲形成网络输出准方波的理论研究与分析设计。首先采用Prony算法获得准方波波形的解析表达式,然后基于最佳一致逼近的优化控制思路,列出极点控制方程,采用数值方法求解非线性方程组,获得优化波形参数。在此基础上采用阻抗函数匹配的算法求解出脉冲形成网络的元件参数初始值,根据工程设计的实际情况舍弃部分参数并对个别参数进行优化后,获得最终的元器件参数。通过理论分析与数值模拟相结合的方法,系统地给出了获取长脉宽、低纹波准方波脉冲形成网络的设计方法以及设计准则。该方法可用于设计任意阶低纹波系数的准方波脉冲源,也可用于设计其他输出波形要求的脉冲源。     

变形及电场作用对石墨烯电学特性影响的第一性原理计算

利用基于密度泛函理论的第一性原理方法,系统研究了变形、电场及共同作用对石墨烯电学特性影响的电子机理.研究表明,本征石墨烯的能隙及态密度值在费米能级处均为0,呈现出半金属特性;在一定的变形量下对石墨烯施加剪切、拉伸、扭转及弯曲变形作用,发现剪切和扭转变形对打开石墨烯能隙的作用明显;对本征石墨烯施加不同方向的电场,可知010电场方向对打开石墨烯能隙的作用效果最强.这是因为该电场方向下石墨烯C-C原子间的布居数正值较大,成键键能较高,而负值数值较小,反键键能较低;线性增加电场强度,石墨烯的能隙呈线性增长势;变形及电场共同作用下,外加电场提高了变形对打开石墨烯能隙的作用效果,但不及两种外场叠加的作用效果.     

Correlation between electronic structure and energy band in Eu-doped CuInTe_2 semiconductor compound with chalcopyrite structure

The Eu-doped Cu(In, Eu)Te_2 semiconductors with chalcopyrite structures are promising materials for their applications in the absorption layer for thin-film solar cells due to their wider band-gaps and better optical properties than those of CuInTe_2. In this paper, the Eu-doped CuInTe_2(CuIn_(1-x)Eu_xTe_2, x = 0, 0.1, 0.2, 0.3) are studied systemically based on the empirical electron theory(EET). The studies cover crystal structures, bonding regularities, cohesive energies, energy levels,and valence electron structures. The theoretical values fit the experimental results very well. The physical mechanism of a broadened band-gap induced by Eu doping into CuInTe_2 is the transitions between different hybridization energy levels induced by electron hopping between s and d orbitals and the transformations from the lattice electrons to valence electrons for Cu and In ions. The research results reveal that the photovoltaic effect induces the increase of lattice electrons of In and causes the electric resistivity to decrease. The Eu doping into CuInTe_2 mainly influences the transition between different hybridization energy levels for Cu atoms, which shows that the 3d electron numbers of Cu atoms change before and after Eu doping. In single phase CuIn_(1-x)Eu_xTe_2, the number of valence electrons changes regularly with increasing Eu content,and the calculated band gap E_g also increases, which implies that the optical properties of Eu-doped CuIn_(1-x)Eu_xTe_2 are improved.     

一种模拟热导率的非平衡分子动力学方法

提出一种计算热导率的非平衡分子动力学(NEMD)方法,通过构造均匀内热源获得抛物线形温度分布,并基于Fourier导热定律计算热导率,与Müller-Plathe发展的反扰动非平衡分子动力学(RNEMD)方法相比,不仅具有能量动量守恒和收敛性好的优点,还克服了常规NEMD方法中热冷源区域存在局域热力学非平衡的问题,并有模拟系统温差影响小的特点.对液态氩的热导率进行模拟并与RNEMD方法的模拟结果进行对比.     

可变焦列阵柱面透镜均匀线聚焦系统

用两个列阵柱面透镜与非球面透镜组成变焦线聚焦系统，能将入射激光束会聚成辐照均匀、长度连续可调的焦线，它可用于Ｘ光激光实验研究．文中详细分析了焦线上光场分布的情况，给出了物理光学分析的结果，并与巳有技术作了比较．     

平面波展开法计算二维磁振子晶体带结构

磁振子晶体是光子晶体或声子晶体在磁性材料领域内的替代品,是近来的一个研究热点. 本文提出了磁振子晶体领域内的一种平面波展开法,其较传统的平面波展开法能节约一半以上的计算时间. 采用此方法,数值计算了由Fe/EuO二种铁磁材料构成的二维磁振子晶体带结构. 数值计算结果表明,在一定的体积填充率下,有自旋波带隙的出现;影响磁振子晶体带隙结构形成的主要因素是有效场中的交换作用场,其他作用场的影响相对很小. 关键词： 磁振子晶体 带隙 平面波展开法