利用Ag@SiO_2纳米粒子等离子体共振增强发光二极管辐射功率的数值研究

金属纳米粒子利用其局域表面等离子体共振效应(LSPR),可以增强附近荧光分子的自发辐射速率,因而在光学传感、光电器件等领域中具有潜在的应用价值.金属纳米粒子的LSPR与其自身的材料、形状、尺寸以及周围环境介质密切相关,这影响着纳米粒子在具体器件中的应用.本文利用三维时域有限差分法,研究了相同体积的球形、椭球形、立方形与三棱柱形银纳米粒子对薄膜发光二极管辐射功率的影响;计算了不同形状银纳米粒子对偶极子光源辐射功率和薄膜器件光出射强度的增强,并结合LSPR效应讨论了辐射功率变化的物理机理.研究结果表明:银纳米粒子自身形状尖锐程度的增加有利于提高LSPR的共振强度;同时纳米粒子的形状影响了LSPR共振电场与薄膜器件中偶极子辐射电场之间的耦合作用,其中立方形纳米粒子因为能实现最强的耦合作用而对器件的辐射功率增强最大.在此基础上进一步讨论了不同薄膜材料对LSPR共振及光源辐射功率的影响,发现较高的材料折射率有利于增强金属纳米粒子的LSPR与器件的耦合作用,从而改善发光二极管性能.     

菱形金属纳米粒子光学性质的研究

金属纳米粒子的光学性质与金属纳米粒子的组成、形状、尺寸及周围的介电常数有关。利用时域有限差分法研究菱形纳米粒子的尺寸与其消光特性关系, 发现粒子尺寸的大小对其消光谱共振峰有较大影响, 随着粒子尺寸的增大, 消光谱共振峰可分裂成两个或多个共振峰。而且在可见光范围内波长大于480 nm的区域, 表面等离子体共振峰是偶极子激发模式; 而在波长小于480 nm的区域, 共振模式则比较复杂, 有偶极子模式, 也有多极子模式。计算表明, 波长为480~ 600 nm区间与波长大于600 nm的区间, 菱形纳米粒子的尺寸对消光光谱的峰值和谱线宽度影响不同。在波长为480~600 nm之间, 短轴为140 nm时, 其消光效率最高。     

金和银纳米粒子二维周期阵列的光学性质

本文利用离散点偶极子近似方法(DDA)研究了金和银纳米粒子二维周期阵列的光学性质。研究结果表明二维周期阵列的消光性质及其表面等离子共振(SPR)波长受到阵列内粒子组成材料、粒子形状尺寸、阵列周期和阵列排布方式等因素的影响。对于二维正方阵列,当周期较小时(一般小于300 nm),阵列的共振波长主要取决于粒子组成材料和形状尺寸;当周期与阵列单体的共振波长附近时,阵列的消光谱中会出现极窄且锐的SPR共振峰,峰位只与阵列的周期值相关。改变阵列在平行和垂直于入射光偏振方向的周期,可以方便地调节二维长方阵列的共振峰的峰位和峰宽。     

金属纳米颗粒的局域表面等离子体共振与面形状关系研究

采用三维时域有限差分法,数值研究了等高、等体积下不同形状金属纳米颗粒的局域表面等离子体的共振性质及其规律,得到了面形状因子对共振波长的影响关系。研究表明,沿偏振方向的形状变化对共振波长偏移有重要贡献,在空气中,确定金属纳米结构局域表面等离子体共振波长的形状参数L可采用偏振方向上的纵横比和面形状因子描述,L可以拟合为纵横比和面形状因子乘积的线性函数。得到了等高、等体积下不同形状金属纳米颗粒的局域表面等离子体共振的电场能量和共振波长的关系,面形状因子越小,形状越尖锐,电场能量越高,共振波长红移。研究结果为进一步了解金属纳米颗粒的局域表面等离子体共振性质提供有益的参考。     

复杂粒子发射对热核巨共振的影响

讨论了引起统计蒸发模型计算结果与高温热核偶极巨共振实验结果之间存在差别的因素,对比了标准CASCADE程序计算结果与加入复杂粒子发射后的计算结果,计算显示复杂粒子发射对高温热核偶极巨共振的性质有影响.     

α粒子结构下的p-~(12)C非弹性散射

从12C原子核的α粒子结构观点出发,计算了共振区内能量为T p=200,400,600,700MeV,p-12C的2 (4.44MeV)和3-(9.64MeV)非弹性散射微分截面。其中将12C原子核跃迁形状因子分解成核内α粒子的跃迁形状因子和α粒子本身的形状因子的乘积。利用已知的原子核跃迁密度,得出12C原子核α粒子跃迁密度分布的解析式。理论结果与实验较好地符合。     

耦合分数阶布朗马达在非对称势中的输运

讨论了分数阶Frenkel-Kontorova模型的物理意义, 并应用该模型刻画了耦合粒子链在记忆性介质中的输运现象, 研究了各参数对粒子链运动状态的影响. 数值仿真结果表明: 系统的记忆性对粒子链的运动有显著影响, 尤其出现了在非记忆性情况下所不具有的反向流. 同时发现粒子链的平均流速会随耦合强度、分数阶的阶数变化而产生广义共振; 此外, 平均流速还会随噪声强度的变化出现广义随机共振现象. 关键词： 分数阶Frenkel-Kontorova模型 记忆性介质 随机共振 定向输运反向流     

银纳米三棱柱二聚体的光学性质研究

本文利用离散偶极子近似方法研究了银纳米三棱柱二聚体的光学性质, 考察了粒子相对位置、粒子间隔和入射光偏振方向对消光谱和电场的影响。入射光偏振对二聚体的消光谱和表面电场均有较大的影响。纳米粒子间隔较小时, 粒子间存在较强的耦合; 粒子间隔较大时, 二聚体的消光谱更接近两个相互独立的粒子情形。另外二聚体的表面电场分布和平均电场强度还受到粒子相对位置和间隔的影响, 随着粒子间隔的增大, 表面平均电场值振荡上升达到最大值后减小。     

卷云冰晶粒子散射相函数的展开系数研究

冰晶粒子散射相函数的多项式展开可以应用于卷云条件下大气辐射传输模式的计算。本文利用广义球函数对表面严重粗糙的冰晶粒子相函数进行展开,并对表面严重粗糙的实心棱柱模型、实心棱柱聚合物模型和一般种类混合模型3种冰晶粒子的展开系数特性进行分析研究,数值计算结果表明了广义球函数对表面严重粗糙的冰晶粒子多项式展开的有效性。     

金属粒子阵列共振的偏振特性

当金属纳米粒子形成规则分布且阵列周期与单粒子的共振波长近似匹配时, 会形成一种特殊的阵列共振, 这种共振比单粒子的局域表面等离子体共振具有更窄的共振线宽和更高的共振强度. 基于修正的长波近似方法, 讨论了矩形阵列的消光截面与阵列因子和单粒子的极化率之间的关系; 并详细研究了在不同偏振的入射光照射下, 阵列因子随着电偶极子方向的改变而产生的变化, 以及这一效应对阵列共振和消光截面所产生的影响. 结果表明, 大型的方阵是偏振无关的; 在矩形阵列中, 沿着阵列两个轴向的相邻粒子之间的耦合形成了阵列因子的两个极值, 并且分别对应了散射截面的最小值.     

Absorption and Scattering of Light by Silver and Gold Nanodisks and Nanoprisms

We study the influence of the shape of silver and gold nanoparticles on the spectra of absorption and scattering of the optical-range electromagnetic radiation. We perform numerical calculations of the cross sections of absorption and scattering of light on the basis of the FDTD method for the particles having the shape of a disk and a triangular prism. We establish that the positions of plasmon resonance peaks and the spectral distributions of intensities differ significantly for nanoparticles of different shapes even when the characteristic sizes of the particles are equal. The calculations carried out thus demonstrate that, along with the size and material of the plasmonic particle, its shape is yet another factor that essentially determines its optical features. The results of the study can be utilized in the development of the spectroscopic method of determining the shape of the metal nanoparticles and the quantitative estimate of their characteristic sizes.     

Using simple particle shapes to model the Stokes scattering matrix of ensembles of wavelength-sized particles with complex shapes: possibilities and limitations

We investigate to what extent the full Stokes scattering matrix of an ensemble of wavelength-sized particles with complex shapes can be modeled by employing an ensemble of simple model shapes, such as spheres, spheroids, and circular cylinders. We also examine to what extent such a simple-shape particle model can be used to retrieve meaningful shape information about the complex-shaped particle ensemble. More specifically, we compute the Stokes scattering matrix for ensembles of randomly oriented particles having several polyhedral prism geometries of different sizes and shape parameters. These ensembles serve as proxies for size-shape mixtures of particles containing several different shapes of higher geometrical complexity than the simple-shaped model particles we employ. We find that the phase function of the complex-shaped particle ensemble can be accurately modeled with a size distribution of volume-equivalent spheres. The diagonal elements of the scattering matrix are accurately reproduced with a size-shape mixture of spheroids. A model based on circular cylinders accurately fits the full scattering matrix including the off-diagonal elements. However, the modeling results provide us with only a rough estimate of the effective shape parameter of the complex-shaped particle ensemble to be modeled. They do not allow us to infer detailed information about the shape distribution of the complex-shaped particle ensemble.     

Diffusion of Anisotropic Colloidal Microparticles Fabricated Using Two-Photon Lithography

Polymeric particles with complex shapes are required for biomedical therapies, colloidal self-assembly, and micro-robotics. It has been challenging to synthesize particles beyond simple shapes (e.g., spheres, cubes) with high structural accuracy using existing methods. Here, a method for fabricating polymeric microparticles of complex 3D shapes is reported using two-photon lithography, and dispersing the particles in an aqueous solution on a glass substrate. The fabrication of polyhedrons (e.g., tetrahedron, pyramid), polypods (e.g., tetrapod, hexapod), and other shapes of 5–10 µm in size is demonstrated. Confocal microscopy is used to track the motion of the sphere, tetrahedron, tetrapod, and screw-shaped particles near the substrate, and determine their translational diffusion coefficients. HYDRO++ is used to simulate the motion of the particles far from the substrate. The influence of particle size and substrate effects on diffusion in the spherical particles is determined and finds that the non-spherical particles have increased hindrance at the substrate compared to the spherical particles.     

Monte Carlo simulation and theory of proton NMR transverse relaxation induced by aggregation of magnetic particles used as MRI contrast agents

Superparamagnetic particles are widely used in MRI as R2 contrast agents. In this last decade, different studies have focused on aggregation of superparamagnetic particles for important applications such as multimodal agents. A complete study--via simulations--of the influence of aggregation on the MR efficiency of these particles at high magnetic field is presented here. First, an empirical expression is proposed for R2 in the presence of uniformly distributed nanoparticles, taking into account two regimes at once (motional averaging and slow motion regimes). Three cluster shapes are simulated: Sphere, shell and line. An analytical model is proposed to understand water transverse relaxation induced by spherical and shell aggregates. Simulations lead to the conclusion that, in the motional averaging regime, the most efficient aggregate contrast agent is the densest sphere or shell.     

Facile Fabrication of Anisotropic Colloidal Particles with Controlled Shapes and Shape Dependence of Their Elastic Properties

A facile technique with only one step for fabrication of anisotropic colloidal particles at the air/water interface is demonstrated. Anisotropic colloidal particles with controlled shapes can be easily obtained by tuning the incubation time in solvent vapor at room temperature. The formation of separate anisotropic particles is attributed to the lateral stretch on the particles by the interfacial forces and repulsion among the neighboring particles by the generation of the polymer solution flows. To further explain the proposed formation mechanism of the colloidal particles with shape anisotropy, an in situ experiment is designed for direct observation of the arrangement change of the colloidal particles. This fabrication technique is general and applicable to polymer colloidal particles with various initial sizes ranging from microscale to nanoscale. Moreover, the elastic properties of the anisotropic colloidal particles are measured which exhibit a prominent change with different shapes and the change trend of the elastic moduli is similar for particles with different original sizes. This work provides a versatile approach for fabrication of anisotropic colloidal particles with tunable shapes and sizes and establishes the interplay between particle shape and elastic property, which is much valuable for further research on the effect of particle parameters on drug delivery process.     

随机取向双层椭球粒子偏振散射特性研究

基于T矩阵方法,给出了随机取向、轴对称、含核椭球粒子的散射计算方法.散射体的核和外壳均可为非球形粒子,整个粒子具有轴对称性.以含核椭球粒子为模型,计算了含有吸收性内核(黑炭,black carbon)的水凝物气溶胶的散射特性,分析了核的大小、形状对消光系数、散射系数、吸收系数、不对称因子、单次散射反照率以及Muller矩阵等的影响. 关键词： 光散射 T矩阵 Muller矩阵 椭球粒子     

ZnO nanoparticles with controlled shapes and sizes prepared using a simple polyol synthesis

Nanocrystalline zinc oxide (ZnO) particles with controlled shapes and sizes were prepared at 180 ^°C by a simple polyol method. The amount of water and the method of addition played an important role in determining the characteristics of the synthesized particles. Rod-shaped ZnO particles with major axis lengths of ∼114 nm were obtained by heating the precursor solution, while equiaxial particles with average diameters of ∼24 nm were prepared by injecting water into hot precursor solution. Increasing the amount of water added to the precursor solution enlarged the aspect ratio of the rod-shaped particles and increased the particle size of the equiaxial particles due to enhanced hydrolysis and condensation of the Zn ion complex.     

Physics in finance

A short history of observations on the shapes of snow crystals starts in ancient China, summarizes the remarkable but little-known essay by Kepler in 1611, includes contributions by Descartes and Hooke, and ends with the beautiful collections of microphotographs by Bentley and Humphreys (1931) and Nakaya (1954). Modern observations from aircraft and mountain observatories reveal that the dominant crystal habits, hexagonal plates, columns, needles and dendritic stellar forms, and transition from one to another, are strongly correlated with air temperature, with the supersaturation of the vapour having only a secondary influence. There follows a review of laboratory experiments to determine the nature and origin of natural ice-forming nuclei and the properties of artificial nuclei, such as silver iodide, through the study of epitaxial growth of ice crystals on single crystalline substrates. Growth of snow crystals under carefully controlled conditions in laboratory cloud chambers has established that the crystal habit changes sharply six times over the temperature range 0 to—25°C. The author argues that the basic habit is determined in the very early stages of growth by the relative rates of surface diffusion of water molecules across the basal and prism faces. He deduces the average migration distance, x_s, for molecules on the basal face from measurements on the velocity of growth layers and of that on the prism faces from measurements on the axial ratios of small crystals. In both cases, x_s, varies rapidly between 1 and 6 μm with temperature, and the two curves intersect at three temperatures that correspond to transitions between crystal habits. A theory is developed to explain the transition from regular plates to dendritic stellar crystals; the corners sprout when material from the vapour phase arrives at corners more rapidly than it can be carried away by surface diffusion and occurs only when the diameter of the plate exceeds a critical value d_c, ∝ D_s, /D_v, = αx² _p, /D_v, where D_s, D_v, are the coefficients of surface and volume diffusion and x_p, is the surface migration distance on the (prism) edge of the plate. The predictions of the theory are compared with observations on natural snow crystals. Equations are derived for the growth rates of snow crystals as they fall through the atmosphere. The predicted maximum attainable diameters of the various shapes are in good agreement with observations.     

温差条件下组合色散棱镜支撑结构的实验分析

介绍了超光谱成像仪采用的组合色散棱镜的支撑结构形式,讨论了温度变化对成像的影响。根据使用需求选择了支撑座的材料,模拟了5～10℃温差条件下棱镜组件的变形情况。利用有限元法解决材料线膨胀系数不同所引起的棱镜反射面变形,并分析了采用聚四氟乙烯垫片做温度补偿后垫片的厚度及接触面积对石英棱镜面型的影响。实验结果表明,在支撑座和棱镜之间增加聚四氟乙烯垫片后,石英棱镜的面型达到了λ/6的指标要求。本文提出的优化方法为组合色散棱镜支撑结构的设计提供了参考依据。     

Fano resonances in prism-coupled square micropillars

We report Fano resonances in the frustrated total internal reflection (TIR) spectra of a prism-coupled square micropillar. Our angle-selective frustrated TIR technique reveals characteristically asymmetric resonance line shapes, which evolve spectrally over approximately a 2pi phase in the far field within a subdegree range of reflection angles. We theoretically model the asymmetric line shapes by the interference between a high-Q resonance that is evanescently coupled and partially confined by TIR, and a coherent background that is total internally reflected at the prism surface without coupling to the micropillar.