Role of shape in middle-infrared transmission enhancement through periodically perforated metal films

We report experimental results on the role of aperture shape in middle-infrared enhanced transmission through a metal film perforated with a periodic aperture array. The transmission spectrum is highly dependent on the aperture shape. When the aperture shape changes from circular to cross-dipole or the cross-dipole arm length increases, the transmission spectrum exhibits a large redshift. The enhanced transmission is proposed to be an interplay of surface plasmon polaritions at the metal surface and localized surface plasmons (LSPs) inside the apertures. The shift of the transmission bands is proposed to be due to the redshift of the LSP resonance spectrum of the individual apertures. The role of shape in enhanced transmission in the cascaded structure is also studied.     

Resonantly enhanced transmission of light through subwavelength apertures with dielectric filling

Using both analytical and numerical methods to study transmission of light through dielectric-filled subwavelength apertures in a real metal, we have found that a propagating mode can in principle exist inside a waveguide of arbitrary small size if a particular relationship between the dielectric constants of the cladding and filling materials at the incident frequency is satisfied. Practical transmission through a subwavelength aperture of finite depth can be enhanced when the depth is such that Fabry-Pérot-like resonances are excited. For 810 nm light incident on a silicon-filled 50-nm-diameter aperture in a 200-nm-thick gold film, we found that a normalized near-field intensity ratio of 1.6 at the exit can be achieved. This resonantly enhanced transmission phenomenon may be advantageously applicable to near-field scanning optical microscopy and single-molecule spectroscopy.     

Super-transmission of light through subwavelength annular aperture arrays in metallic films: Spectral analysis and near-field optical images in the visible range

This paper presents experimental studies of the enhanced light transmission through metallic films pierced by subwavelength annular apertures. Two different methods (e-beam lithography and focused ion beam) have been used to build the nano-structures. We have experimentally recorded their far-field spectral response in the visible range and the optical near-field above the nano-structures when they are excited at 633 nm. The spectral response exhibits a transmission peak at 700 nm with maximum efficiency around 16%. The near-field exhibits a characteristic two-lobe structure just above the aperture. Finite difference time domain (FDTD) simulations reproduce quite well the experimental results.     

Optical transmission properties and enhanced loss for randomly positioned apertures in a metal film

The transmission of light through randomly distributed rectangular apertures in a gold film on glass substrate is calculated for the first time. The results for randomly distributed apertures, when compared with the transmission resonance through a single aperture, show the new features of a peak reduction, a blue-shift and the appearance of ripples. Only the latter of these features has been reported previously. The loss from the isolated aperture is compared with the loss from the randomly distributed apertures to show how coupling between apertures enhances the loss in the presence of a lossy metal film. PACS 81.07.-b; 71.36.+c; 78.66.Bz; 42.25.Dd     

Fluorescence enhancement from an array of subwavelength metal apertures

We report the resonant excitation of a fluorescing molecular monolayer applied to a periodic array of subwavelength apertures in a metal film. The peak fluorescence occurs under conditions of maximum transmission of the excitation light, which indicates strong enhancement of the incident light within the apertures under resonant conditions. Fluorescence output normalized to the aperture fill fraction is enhanced by nearly 40 times, and the rate of photobleaching is increased by approximately a factor of 2, suggesting an increase in the fluorescence yield.     

基于随机拓扑的复杂腔体电磁辐射统计特性

针对电磁波经孔缝耦合进入多端口微波混沌腔体的电磁辐射问题,推导了孔缝随机耦合模型,以预测单腔体各孔缝处耦合电压的统计特性;并将该模型与电磁拓扑理论结合,提出了一种复合计算方法孔缝随机拓扑模型,用于分析级联腔体的孔缝耦合电压和传输系数等电磁量的统计特性,将该算法的计算结果与网络级联理论的计算结果进行对比,验证了算法的正确性。该模型可成为分析复杂不规则混沌腔体的孔缝耦合问题以及混响室研究的有利工具。     

Plasmonic effects in near-field optical transmission enhancement through a single bowtie-shaped aperture

In this paper, the enhanced optical transmission through a special type of aperture of a bowtie shape is investigated through near-field imaging and finite-difference numerical analysis. Under linear polarizations in two orthogonal directions, the optical near fields of the bowtie aperture and comparable square and rectangular apertures made in gold and chromium thin films are measured and compared. The bowtie aperture is able to provide a nanometer-sized optical spot when the incident light is polarized across the bowtie gap and delivers a considerable amount of light. Localized surface plasmons are clearly observed in the near-field images for both bowtie and rectangular apertures in gold, but invisible in chromium. Finite-difference time-domain calculations reveal that, depending on the polarization of the incident light, the unique optical properties of the bowtie aperture are a result of either the optical waveguide and the coupled surface plasmon polariton modes existing in the bowtie gap or the coupling between the two open arms of the bowtie aperture. PACS 81.07.-b; 07.79.Fc; 71.36.+c; 78.66.Bz; 42.79.Gn; 42.79.Vb     

Similarities and differences for light-induced surface plasmons in one- and two-dimensional symmetrical metallic nanostructures

Two types of double-sided nanostructure, one possessing a slit aperture with parallel grooves and the other possessing a circular aperture with concentric grooves, were fabricated to examine the similarities and differences of their diffraction behavior in one-dimensional (1-D) and two-dimensional (2-D) nanostructures. Based on the projection-slice theory, we conjecture that the surface plasmons in these two different nano-scale grooves possess similar modes. A localized surface plasmon (LSP) was used to examine the transmission characteristics induced by the apertures. The transmission characteristics of the slitted nanostructure and the circular nanostructure aperture were then measured. We coupled the transmission spectra measured from these two apertures with a 1-D parallel groove transmission curve simulated by a 1-D rigorous coupled wave analysis. Measured spectra results show reasonable agreement with the simulated data. We propose that the apparent blueshift observed in the peak frequency of a 2-D nanostructure is due to the difference in the shape of the aperture and the spot transmission characteristics of 1-D and 2-D systems as induced by a LSP.     

Enhanced optical transmission through an array of compound unit with multiple concentric or eccentric annular apertures

In this paper, transmission properties through an array of compound unit with multiple concentric or eccentric annular apertures are numerically studied. It is demonstrated that the transmission of the compound structure is not a simple superposition of that of each annular aperture array but exhibit substantial changes in both the position and intensity of resonant peaks. It is also found that the gap plasmon resonance exhibit split peaks when the symmetry is broken, which can be well explained from the view of charge oscillation. These properties provide a method to achieve enhanced transmission in multi-frequencies simultaneously and are of significant applicable value to the design of tunable optical devices.     

速调管双间隙双耦合口输出结构

基于等效电路理论,提出了一种双间隙双耦合口输出结构的设计方法及计算双耦合口输出腔外观品质因数的方法。利用三维电磁场软件,设计了满足阻抗频率特性要求的双间隙单耦合口输出结构及前置腔和输出腔分别具有相同参数的双间隙双耦合口结构。然后,将两者的阻抗频率特性进行比较,通过微调整该输出腔与耦合槽的尺寸,可以得到与单耦合口输出结构一致的阻抗频率特性曲线。     

Enhanced light transmission through a single subwavelength aperture

The optical transmission through a subwavelength aperture in a metal film is strongly enhanced when the incident light is resonant with surface plasmons at the corrugated metal surface surrounding the aperture. Conversely, the aperture acts as a novel probe of the surface plasmons, yielding useful insights for optimizing the transmission enhancement. For the optimal corrugation geometry, a set of concentric circular grooves, three times more light is transmitted through the central subwavelength aperture than directly impinges upon it. This effect is useful in the fabrication of near-field optical devices with extremely high optical throughput.     

有孔双层屏蔽腔体的宽频带屏蔽效能

 用扩展为可分析腔内高次模的传输线方法研究了有孔双层屏蔽腔体的屏蔽效能,该方法可以考虑腔内较宽的频带范围。计算了双层屏蔽腔内电场的屏蔽系数,与单层屏蔽腔内屏蔽系数的比较表明,采用双层屏蔽使得腔体的屏蔽效能大为提高。分析了双层屏蔽腔体孔缝耦合的共振效应、腔内的谐振。结果表明：满足共振效应成立条件时双层屏蔽腔内也发生共振现象,屏蔽效能在共振频率附近明显降低;在腔内的谐振点屏蔽系数出现极小值,此时屏蔽效果较差;在0.1~4.5 GHz的范围内,屏蔽系数随着频率的增加总体上呈下降趋势。     

The impact of local resonance on the enhanced transmission and dispersion of surface resonances

We investigate the enhanced transmission through the square array of metallic coaxial annular apertures (MCAAs) in microwave frequency by varying the inner diameter of the apertures. The impact of local resonance is thoroughly embodied in the even-mode surface resonances, while the odd-mode surface resonances, invisible in measurements, are only scaled by periodicity. For MCAAs with smaller inner diameters, the transmission extrema are measured at specific incident angles matching the folded degenerate resonant states. The results are well understood in terms of the dispersion of surface resonances.     

湍流特征光学测试对比

在10℃～230℃温差下,对大气相干长度r0分别采用夏克-哈特曼的到达角起伏法、差分像运动法、波面法三种测量法和四象限探测器进行了测试和对比;对折射率结构常量Cn2及闪烁功率谱分别采用夏克-哈特曼和光电倍增管进行对比.实验结果表明:对于r0,在强湍流时四象限探测器比夏克-哈特曼的稳定性明显降低,且对夏克-哈特曼三种方法,差分像运动法可克服设备抖动等问题,但引入了方向上不一致的问题,波面法可有效避免该问题;对于Cn2,夏克-哈特曼比光电倍增管测量更稳定,拟合相关系数高达0.96;对于闪烁功率谱,由于噪音影响,在200℃时夏克-哈特曼比光电倍增管测得的最大频率高15Hz;最后,通过对夏克-哈特曼子孔径的闪烁功率谱分析得出,若同一子孔径入射光强不在CCD响应的线性区间时无法准确测量闪烁功率谱,否则可通过不同子孔径可完成湍流均匀性的测量.这将为湍流池提供最优的测试方法及理论依据.     

Spectral resonance of nanoscale bowtie apertures in visible wavelength

We report spectroscopic measurements of transmitted field through bowtie-shaped nanoscale apertures in visible wavelength region. Resonance in these apertures and its relation with the aperture geometry are investigated. The near-field spectral response is also investigated using finite difference time domain (FDTD) computation and compared with the spectroscopic measurements. The dependences of the peak wavelength and peak amplitude on the geometry of the bowtie aperture are illustrated. Design rules are proposed to optimize the bowtie aperture for producing a sub-wavelength, high transmission field. PACS 81.07.-b; 07.79.Fc; 71.36.+c; 78.66Bz; 42.79.Gn; 42.79.Vb     

不同形状孔缝微波耦合的实验研究

 探讨了椭圆、圆环、方环、十字交叉形、圆形、正方形、正三角形及双矩形孔缝在2～18GHz频率范围内的耦合系数，并与矩形孔缝的耦合特性进行了比较。结果表明: 各种不同形状孔缝微波耦合的共振特性与其纵横比有关, 纵横比较大的孔缝耦合共振现象比较明显，纵横比较小的孔缝耦合特性趋于高通；各种孔缝的共振频率与孔缝长度有密切关系。     

Role of interference between localized and propagating surface waves on the extraordinary optical transmission through a subwavelength-aperture array

We report in this Letter that when radiation is incident on a metal surface perforated with an array of ring-shaped subwavelength apertures, the phase difference between the propagating surface Bloch wave and the localized surface wave can be tailored by the geometrical parameters of the array so as to affect the shape of the transmission spectrum. Above the resonant frequency of the aperture, interference between the two kinds of surface waves leads to a minimum in the transmission spectrum, whereas below it, the interference leads to a maximum. We suggest that this feature provides flexibility in engineering surface-wave-based all-optical devices.     

Low frequency scattering through an aperture in a rigid screen—Some numerical results

The penetration of a plane wave through an aperture in a rigid screen is investigated. The first three terms of the low frequency expansion are evaluated numerically for both simply connected and annular apertures. In the numerical method use is made of a division of the aperture into arbitrary triangles. The singular behaviour at the edges is incorporated in the shape functions, which yields good accuracy and fast convergence. Special attention has been given to the thin ring which can serve as a model for, e.g., imperfectly shielded windows.     

高功率微波对双层屏蔽腔体的孔缝耦合特性

 用多模传输线模型对高功率微波与双层屏蔽腔体的孔缝耦合特性进行了研究,此方法可以考虑高功率微波孔缝耦合进入腔内时的较宽的频率范围。用这种模拟方法获得了双层屏蔽腔体微波耦合的一些规律性结果：双层屏蔽可以使得腔体内的耦合电场比单层屏蔽时有显著的减弱,这与FDTD方法的结论是一致的;双层屏蔽外腔体中的一些谐振会影响到内腔体的耦合系数,外腔体中的场模式经由内孔缝会影响内腔体中的场模式;不论是单层屏蔽还是双层屏蔽,保持每层孔缝总面积不变时,随着孔阵中孔缝数量的增加,进入腔体内的耦合电场也逐步地减弱,这与单层屏蔽时的结论一致;通风总面积不变的情况下,孔缝数量越多,屏蔽效能越好。     

空间SiC反射镜背部中心支撑特性

基于特定的轻量化形式和支撑结构,采用有限元方法研究了口径为500mm的空间SiC反射镜的背部中心支撑特性.通过分析镜体结构参量对反射镜性能的影响,确定了最佳的支撑孔直径与反射镜口径的比例为0.23,指出对不同口径的反射镜需通过优化确定最佳的背部形状,当重力沿径向作用时,增大支撑深度有利于提高面形准确度.支撑结构分析结果表明,柔性连接件底部螺栓圆半径是影响温变载荷工况下的面形准确度和反射镜组件的一阶固有频率的关键因素,要确定最佳的底部螺栓圆半径需综合考虑面形准确度和结构基频两方面的指标要求;背部中心支撑的反射镜面形准确度受外界装配应力的影响较小,且对柔性连接件切槽深度的变化不敏感;支撑长度主要影响结构的动态刚度,减小支撑长度能提高反射镜组件的一阶固有频率.最后确定了空间SiC反射镜背部中心支撑的最大适用口径为750mm,对口径小于750mm的SiC反射镜在结构允许的前提下采用背部中心支撑均能满足设计要求.