含缺陷平板光子晶体十字波导传输特性研究

提出一种在光子晶体十字波导中加入点阵缺陷的特殊结构波导.采用时域有限差分法(FDTD)对该结构的导波特性进行数值模拟,计算结果表明:含缺陷结构的光子晶体波导的透射光谱较不含缺陷结构光子晶体波导的透射光谱带宽变得更窄,此结构具有窄带滤波作用.当改变波导中缺陷结构折射率取值时,该波导透射光的中心频率随缺陷介质折射率的增大而线性减小.改变中心缺陷介质柱直径时,该波导透射光波中心频率随介质柱直径的增大也呈逐渐减小趋势.这种光子晶体十字波导可作为一种窄带滤波器、分光器和可调式选频器等器件,具有一定的应用前景.     

光子晶体波导中的孤子传输及其延迟特性研究

研究了正方晶格和三角晶格空气背景硅介质柱光子晶体线缺陷波导导模左带隙边缘处的亮孤子脉冲传播特性及其慢光延迟特性. 采用平面波展开法仿真分析了波导相邻两行介质柱大小r₁和r₂以及波导宽度D对孤子脉冲传输所需峰值功率P₀和延迟时间T_s的影响,总结了其变化规律. 通过调整波导结构得到了正方晶格和三角晶格优化波导结构,优化后,正方晶格结构波导P₀减小了81.17%,T_s增加了66.32%;三角晶格结构波导P₀减小了73.7%,T_s增加了67.63%,实现了孤子传输性能的大幅度优化. 关键词： 光子晶体波导 光孤子 峰值功率 延迟时间     

二维正方光子晶体波导中的慢光传输

采用平面波展开法分析了正方晶格光子晶体线缺陷波导中的慢光传输.数值模拟结果表明,介质柱光子晶体线缺陷波导中,缺陷模的群速度与填充比、缺陷处介质柱半径及与波导相邻介质柱半径有关,其中缺陷处介质柱半径对缺陷模的群速度影响最大;随着填充比和缺陷处介质柱半径的增大及与波导相邻介质柱半径的减小,缺陷模的群速度逐渐减小且最小减至0.015c.进而对比研究空气柱光子晶体线缺陷波导,发现缺陷模的群速度随着缺陷处空气柱半径的减小而减小,最小减至0.008c.     

耦合腔光子晶体慢光波导结构

在单线缺陷结构中引入两个附加的相邻介质柱,构成一种新型的光子晶体耦合腔波导结构.通过平面波展开法对波导结构的的慢光特性进行了仿真分析,研究了平移线缺陷上下两侧介质柱,以及改变腔体的长度对器件色散特性和群速度的影响.结果表明:与平移缺陷上下两侧介质柱相比,通过改变腔体的长度,不仅可将光群速度低到0.03c(c为真空下的光速),而且器件的有效波长范围接近20nm.利用时域有限差分法得到波导结构的传输场分布图,研究波长的选取对入射激励光在光子晶体耦合腔波导中传输场的影响,发现结构参量优化后的光子晶体耦合腔波导仍然具有良好的传输特性.     

非对称金属包覆左手介质平板波导的微扰分析

基于微扰方法分析非对称金属包覆左手介质平板波导的特性,给出了该波导的复有效折射率的一级近似解,并对波导的传输特性和损耗特性进行了数值模拟.结果表明:非对称金属包覆左手介质平板波导没有零阶模式;TE1模传播系数随波导厚度的增加迅速减小,损耗系数随波导厚度的增加快速增加,达到最大值后又迅速减小;非对称金属包覆左手介质平板波导传输特性相对于左手介质三层平板波导传输特性发生了很大改变,TM1模,TM2和 TE2模式以及更高阶模式在截止厚度附近出现双值现象;特别在二阶及高阶模式中,TE模式相对于TM模式具有高传输低损耗的特性,而以右手介质为芯层的金属包覆波导不具有这一特性.     

同轴转弯波导的设计与实验研究

提出了一种同轴转弯波导。介绍了该同轴转弯波导的基本原理,设计并数值模拟了中心频率为4.0 GHz的同轴转弯波导,并对此同轴转弯波导进行了实验研究。实验结果表明:同轴转弯波导在中心频率4.0 GHz下,传输损耗约为0.17 dB,驻波系数为1.2;在3.8~4.2 GHz的频率范围内传输损耗小于0.2 dB,驻波系数小于1.25。同轴转弯波导内部无介质支撑,且体积小,结构简单,易于实现,适用于高功率微波馈线系统中的同轴波导的转弯和连接。     

阵列波导光栅平坦化的研究

阵列波导光栅的平坦化在实际应用中有很重要的意义.本文系统地研究了阵列波导光栅的平坦化.在输入波导、输出波导、阵列波导输入端与输出端上分别引入了指数型锥形波导.通过改变锥形波导的形状和尺寸来实现平坦化的优化.本文首先从理论上论述了引入指数型锥形波导的输出光谱特性,给出了结构参量的关系表达式,阐明了输入波导处的锥形波导是影响输出光谱平坦化的主要因素,阵列波导和输出波导处的锥形波导对输出光谱的平坦化有一定的影响.其次采用数值模拟的方法模拟了输出光谱,优化了结构参量,总结出了指数型锥形波导对平坦化影响的趋势和规律.模拟结果显示,输出光谱1 dB带宽大于通道间隔的50％,插入损耗从5.2 dB减小到了4.0 dB,串扰小于-30 dB.最后,本文给出了实验结果,插入损耗减小了0.87 dB,串扰减小了3.67 dB,1 dB带宽增加0.1 nm,增加了54.7％.实验结果表明引入指数型锥形波导提高了阵列波导光栅器件的光谱性能.     

阵列波导光栅平坦化的研究

阵列波导光栅的平坦化在实际应用中有很重要的意义.本文系统地研究了阵列波导光栅的平坦化.在输入波导、输出波导、阵列波导输入端与输出端上分别引入了指数型锥形波导.通过改变锥形波导的形状和尺寸来实现平坦化的优化.本文首先从理论上论述了引入指数型锥形波导的输出光谱特性,给出了结构参量的关系表达式,阐明了输入波导处的锥形波导是影响输出光谱平坦化的主要因素,阵列波导和输出波导处的锥形波导对输出光谱的平坦化有一定的影响.其次采用数值模拟的方法模拟了输出光谱,优化了结构参量,总结出了指数型锥形波导对平坦化影响的趋势和规律.模拟结果显示,输出光谱1 dB带宽大于通道间隔的50%,插入损耗从5.2 dB减小到了4.0 dB,串扰小于-30 dB.最后,本文给出了实验结果,插入损耗减小了0.87 dB,串扰减小了3.67 dB,1 dB带宽增加0.1 nm,增加了54.7%.实验结果表明引入指数型锥形波导提高了阵列波导光栅器件的光谱性能.     

跑道型结构光子晶体波导定向耦合器

鉴于波导定向耦合器在集成光路以及光电集成方面的广泛应用,提出了一种基于光子晶体波导间高效耦合的光子晶体定向耦合器。通过主波导和耦合波导间的耦合,可以实现对波长为1 490 nm和1 550 nm电磁波的高效分光。在将器件长度控制在30 μm左右的同时,其总效率高达93.05%。另外,发现主波导和耦合波导间介质柱结构参数对电磁波的耦合周期有着极大的影响。并通过将介质柱沿z方向拉伸0.1a(a为晶格周期),设计了工作波长为1 530 nm和1 540 nm的光子晶体定向耦合器,器件长度仅为60 μm。通过拉伸介质柱的纵向长度,可以大幅减小耦合周期,这对缩小器件体积以及实现更为密集的波分复用有着重要的意义。     

全矢量有限元模型及其在光波导中的应用

为了研究光波导和光子晶体光纤的模式特性和传输特性,从矢量波动方程出发,推导出了各向异性介质中场微分方程复数泛函表达式,利用棱边/节点混合元离散了该泛函,加入了各向异性介质匹配层边界条件,得到关于传播常量的广义特征值方程.以矩形波导为例,对各向异性介质匹配层边界条件的吸收特性进行了研究,得到了基模以及几个高阶模的场分布、色散曲线和损耗曲线.结果表明该方法可靠有效.对正六边形晶格光子晶体光纤进行了分析.数据表明：光纤有效折射率随空气孔直径或波长的增大而减小,但与空气孔圈数无关;光纤限制损耗(confinement loss)随波长增大近似成指数增大,而增加空气孔直径或者空气孔圈数则可使之显著降低.     

Growth of GaAs/AlGaAs hexagonal pillars on GaAs (1 1 1)B surfaces by selective-area MOVPE

We report on the growth of GaAs and GaAs/AlGaAs heterostructured hexagonal pillar structures using selective area (SA) metalorganic vapor phase epitaxy (MOVPE). By performing growth on SiO₂-masked (1 1 1)B GaAs substrates with circular or hexagonal hole openings, extremely uniform array of hexagonal GaAs/AlGaAs pillars consisting {1 1 0} vertical facets with their diameter of order of 100 nm were obtained. Unexpectedly, strong intense light emission was observed for the room temperature photoluminescence measurement of the pillar arrays in triangular lattice, which is promising for the application to the photonic crystals to enhance the light extraction efficiency from the materials with high refractive index. Furthermore, it was also found that hexagonal pillars with size 60 nm and large aspect ratio (>100) by reducing the size of initial hole size of mask, opening a possibility to grow nanowires using epitaxial growth.     

Formation of nano-textured conical microstructures in titanium metal surface by femtosecond laser irradiation

We report for the first time that a regular array of sharp nano-textured conical microstructures are formed on the titanium metal surface by irradiation with ultrafast laser pulses of 130 fs duration, 800 nm wavelength in vacuum (∼1 mbar) or in 100 mbar He. The microstructures are up to 25 μm tall, and taper to about 500 nm diameters at the tip. Irradiation in the presence of SF₆, air or HCl creates a textured surface but does not create sharp conical microstructures. The surfaces of these microstructures exhibit periodic nano-texture of feature size comparable to the wavelength of light consistent with ripple formation. Contrary to pillar formation by femtosecond laser irradiation of silicon where the initial ripples evolve into the pillars and the ripples disappear, the ripples on titanium pillars have a much smaller periodicity than the pillars and remain on the surface of the pillars. The textured surface is pitch black compared to its original silver-grayish color, i.e, it exhibits greatly reduced reflectivity throughout the measured visible spectrum. PACS 52.38.Mf; 79.20.Ds; 81.07.-b; 81.16.-c; 82.53.-k     

Wavelength and coherence effects on the growth mechanism of silicon nanopillars and their use in the modification of spontaneous lifetime emission of BODIPY dye molecules

Silicon nanopillars are grown by an electrochemical anodization of p-type silicon wafers at low current densities in a hydrofluoric acid solution. CW, white light, and various UV pulsed lasers are employed as illumination sources in sample preparation to study wavelength and coherence effects on the growth mechanism of the nanopillars. Coherence is observed to be the foundation of regularity in obtaining conical shapes. The pillar size is found to be almost linearly proportional to the employed illumination wavelength during their growth. BODIPY dye molecules are chemically attached to these silicon nanopillars and the radiative decay rates are investigated by means of a time-resolved fluorescence experiment. The decay rate of the dye molecules embedded in the vicinity of various size pillar tips is significantly affected due to different apex angles of the conical nature. It is demonstrated that the pillar size and the separation between pillars can be adjusted if one uses a coherent light source with an appropriate wavelength during the course of fabrication process. Since change in the decay rate is due to tips of the pillars only, separation of a few micrometers between pillar tips allows one to directly monitor a dye, which is embedded to the tip of a single nanopillar, via a confocal microscopic method for the spontaneous lifetime measurements, without having needed to any extra efforts for an in situ imaging process. It is observed that as the pillar size gets smaller, the inhibition in the spontaneous lifetime of BODIPY is more pronounced. In addition, a more regular pillar structure yields nonvarying decay rates of the dye molecules throughout the silicon sample.     

Tribological behavior of micro/nano-patterned surfaces in contact with AFM colloidal probe

In effort to investigate the influence of the micro/nano-patterning or surface texturing on the nanotribological properties of patterned surfaces, the patterned polydimethylsiloxane (PDMS) surfaces with pillars were fabricated by replica molding technique. The surface morphologies of patterned PDMS surfaces with varying pillar sizes and spacing between pillars were characterized by atomic force microscope (AFM) and scanning electron microscope (SEM). The AFM/FFM was used to acquire the friction force images of micro/nano-patterned surfaces using a colloidal probe. A difference in friction force produced a contrast on the friction force images when the colloidal probe slid over different regions of the patterned polymer surfaces. The average friction force of patterned surface was related to the spacing between the pillars and their size. It decreased with the decreasing of spacing between the pillars and the increasing of pillar size. A reduction in friction force was attributed to the reduced area of contact between patterned surface and colloidal probe. Additionally, the average friction force increased with increasing applied load and sliding velocity.     

Stochastic theory for jerky deformation in small crystal volumes with pre-existing dislocations

A statistical theory is proposed to describe the jerky deformation of micron-sized crystal pillars. The probability for a specimen surviving the applied load without generating a strain burst of a given order is analytically expressed in terms of the nucleation rate of that burst. The survival probability can be measured from an ensemble of macroscopically similar deformation experiments to obtain the nucleation rate. From experiments on aluminium pillars, the nucleation rate is found to increase with the pillar size and to decrease with the burst order, indicating that more sources are present in a larger specimen and that the available nucleating sources are progressively exhausted by the occurrence of bursts. The activation volume measured is roughly independent of the pillar size and burst order, indicating a constant mechanism for burst nucleation.     

Terahertz surface plasmon polaritons in textured metal surfaces formed by square arrays of metallic pillars

In this paper, we investigate numerically the characteristics of surface plasmon polaritons (SPPs) sustained by two-dimensional arrays of metallic pillars protruding out of planar metal surfaces at terahertz (THz) frequencies. Various shapes of the pillars are analyzed, and it is shown that the pillar shape only has weak influence on the dispersion of spoof SPPs. However, the loss of spoof SPPs is closely dependent on the pillar shape. It is also shown that spoof SPPs on textured surfaces with pillars can exhibit much better confinement than those on pierced surfaces with holes.     

Top-gathering pillar array of hybrid organic–inorganic material by means of self-organization

Two-dimensional (2D) pillar arrays with submicrometer to micrometer repetitions have been fabricated from hybrid organic–inorganic material by mask lithography or multi-beam interference lithography. The type of array structure depends on structural parameters such as the pillar height, diameter and distance between neighboring pillars. Two kinds of periodic arrays, 2D arrays and ‘top-gathering’ arrays, can be obtained by controlling the structural parameters. In the top-gathering arrays, the pillars are gathered at the top by means of self-organization, and ‘top-gathering’ units composed of four pillars can be formed. PACS 68.35.Gy; 81.20.Fw; 82.50.-m     

一种类分形结构光子晶体的能带

用时域有限差分方法计算了一种类分形结构光子晶体的能带.数值计算结果表明,这种结构的光子晶体在介质柱、空气背景的情况下具有不完全带隙.而且,其能带结构随着级数的增大在整体地趋向于低频的同时,能带结构也趋于稳定.     

Decay of a quantum dot in two-dimensional metallic photonic crystals

In this paper we have developed a theory for the decay of a quantum dot doped in a two-dimensional metallic photonic crystal consisting of two different metallic pillars in an air background medium. This crystal structure forms a full two-dimensional photonic band gap when the appropriate pillar sizes are chosen. The advantage of using two metals is that one can easily control the density of states and optical properties of these photonic crystals by changing the plasma energies of two metals rather than one. Using the Schrödinger equation method and the photonic density of states, we calculated the linewidth broadening and the spectral function of radiation due to spontaneous emission for two-level quantum dots doped in the system. Our results show that by changing the plasma energies one can control spontaneous emission of quantum dots doped in the metallic photonic crystal.     

Size dependence of mechanical properties of gold at the sub-micron scale

The results of both experimental studies and molecular dynamics simulations indicate that crystals exhibit strong size effects at the sub-micron scale. In experimental studies, the size effects are usually explained by strain gradients. By contrast, atomistic simulations suggest that the yield strength depends on the size even without strain gradients and scales with the sample size through a power relationship. Here we address these two different approaches to the size dependence of mechanical properties. Results of uniaxial compression experiments on gold single crystals at the sub-micron scale, without significant stress/strain gradients, are presented. The free-standing single-crystal Au cylinders are created by focused ion beam machining and are subsequently compressed using a nanoindenter fitted with a diamond flat punch. Compressive stresses and strains, as well as pillar stiffnesses, are determined from the test data. The experiments show that the flow stresses of these pillars increase significantly with decreasing pillar diameter, reaching several GPa for the smallest pillars. These high strengths appear to be controlled by dislocation starvation, which is unique to small crystals. PACS 68.60.Bs An erratum to this article is available at .