Modulation of Splitting Beam Angle with Metal--Nonlinear Optical Material--Metal (M-NL-M) Array Structure

We demonstrate active manipulating plasmonic signals with metal--nonlinear optical material--metal (M-NL-M) arrays consisting of slits with variant widths. The parameters of the M-NL-M array structure are derived by theoretical analysis of dispersion relationship. The splitting angle can be modulated by the incident light intensity, and verified by a nonlinear two-dimensional finite difference time domain method. The physical principle of this phenomenon is analysed from the phase of surface plasmon polaritons and Fabry--Pérot (F-P) resonance in slits     

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。     

Characterization of the radiation from single-walled zig-zag carbon nanotubes at terahertz range

This paper investigates the radiation characteristics of metal single-walled zig-zag carbon nanotubes as a dipole antenna at terahertz wave range. The current distribution, input impedance and mutual impedance are calculated for various geometrical parameters of vertically-aligned carbon nanotubes. The numerical results demonstrate the properties of the antenna depending strongly on the geometrical parameters such as the radius, the lengths of carbon nantobues, and the spacing between nanotubes. It is found that the zig-zag carbon nanotubes exhibit very high input impedance and the mutual impedances for antenna array applications. These unique high impedance properties are different from the conventional metal thin wire antenna. The far-field patterns and gain of antenna array are also calculated. The maximum gain of array of 100-element array is up to 20.0~dB, which is larger than the gain of 0.598~dB of single dipole antenna at distance d = 0.5\lambda .     

Correction of ultrasonic array images to improve reflector sizing and location in inhomogeneous materials using a ray-tracing model

The use of ultrasonic arrays has increased dramatically within recent years due to their ability to perform multiple types of inspection and to produce images of the structure through post-processing of received signals. Phased arrays offer many advantages over conventional transducers in the inspection of materials that are inhomogeneous with spatially varying anisotropic properties. In this paper, the arrays are focused on austenitic steel welds as a representative inhomogeneous material. The method of ray-tracing through a previously developed model of an inhomogeneous weld is shown, with particular emphasis on the difficulties presented by material inhomogeneity. The delay laws for the structure are computed and are used to perform synthetic focusing at the post-processing stage of signal data acquired by the array. It is demonstrated for a simulated austenitic weld that by taking material inhomogeneity and anisotropy into account, superior reflector location (and hence, superior sizing) results when compared to cases where these are ignored. The image is thus said to have been corrected. Typical images are produced from both analytical data in the frequency domain and data from finite element simulations in the time domain in a variety of wave modes, including cases with mode conversion and reflections.     

微带阵列天线的时域散射特性

本文用时域有限差分方法(FDTD)研究了微带阵列天线的时域散射场,分析了入射脉冲极化方向不同以及入射方向不同时的散射场的分布情况;讨论了时域散射场的时域波形以及频谱与微带阵列天线结构的关系;用散射场分离算法讨论了地板对于微带贴片阵列散射场的影响.研究发现,有限大地板的微带阵列天线的散射场主要是由地板的边沿电流产生的,同时微带贴片阵列的谐振频率与入射脉冲的极化方向有关,因此不同的极化方向对应于不同的散射频谱. 关键词： 时域散射 微带天线 FDTD     

Localization of Surface Plasmon Waves in Hybrid Photodetectors with Subwavelength Metallic Arrays

The spectral characteristics of the hole photocurrent in plasmon photodetectors based on Ge/Si heterostructures with Ge quantum dots combined with regular arrays of subwavelength apertures of various shapes in a gold film on the semiconductor surface are investigated. Dispersion relations characterizing the propagation of surface plasmon waves along the metal–semiconductor interface are determined from the dependences of the photocurrent on the angle of incidence of light. It is established that the plasmonic enhancement of the photocurrent in rectangular aperture array is suppressed as compared to that in circular and square aperture arrays. It is found that, in hybrid structures with rectangular apertures, there exists a range of wave vectors where the energy of surface plasmons is independent of the wave vector of incident radiation. The results are explained by the excitation of dipole modes localized at rectangular apertures with a large aspect ratio by light waves.     

角反射器阵列作为伪相位共轭器件的保真度分析

 研究了共光路共模块自适应光学系统中用角反射器阵列构成伪相位共轭器件,角反射器阵列形成伪相位共轭波的能力,具体计算了不同单元数的角反射器阵列形成具有Zernike多项式展开的各阶像差的波面的相位共轭的保真度。角反射器阵列的保真度决定于其单元数和阵列结构,对于不同类型的像差保真度也是不同的。最后对各种影响因素如衍射、二面角误差和面形误差等进行了分析。     

Array gain for a cylindrical array with baffle scatter effects

Cylindrical arrays used in sonar for passive underwater surveillance often have sensors surrounding a cylindrical metal baffle. In some operational sonars, the phones in each stave (i.e., each line of phones aligned with the cylinder axis) are hardwired together so that the array is equivalent to a baffled circular array of directional elements, where each element corresponds to a line array of omnidirectional phones steered to broadside. In this paper a model is introduced for computing the array gain of such an array at high frequencies, which incorporates baffle scatter using infinite, rigid cylinder scattering theory, and with ambient noise described by an angular spectral density function. In practice the phones are often offset from the baffle surface, and the acoustic field sampled by the staves is distorted at high frequencies due to interference between the incident and scattered fields. Examples are given to illustrate the resulting array gain degradation, using three noise distributions that are frequently used in sonar performance modeling: three-dimensional isotropic, two-dimensional isotropic, and surface dipole noise.     

THz波段亚波长半导体球形阵列光学特性的数值模拟

利用T-matrix方法对太赫兹波段亚波长半导体球形阵列进行了数值模拟并在数值模拟结果的基础上讨论了其光学特性。在太赫兹波段可以通过掺杂等手段调节半导体的表面等离子体特性。以半导体InSb为例并采用Drude模型,对单个亚波长球及两个或多个亚波长球组成的阵列进行了数值模拟,主要以归一化消光截面为参数,讨论了不同阵元半径、不同球形单元间距、不同单元数目及入射波不同极化方向对阵列特性的影响。     

Gain enhancement of an electrically small antenna array using metamaterials

This paper presents the enhancement of radiated power ratio of a two-dipole array of infinitesimal dipole elements, separated by an infinitesimal distance. It is shown that a properly constructed metamaterial shell configuration with optimized material parameters surrounding the dipole can cancel both the self- and the mutual reactances of the array system. Simultaneously, a very high radiated power ratio is realized for the array system even in the presence of strong mutual coupling between the array elements. This potentially leads to the development of highly efficient ultra-compact antenna arrays. In addition, by using a material of non-unity permittivity surrounding the array system, uniform radiated power ratio characteristics can be achieved with the variation in the metamaterial shell thickness. This removes the major drawback in the practical design of the metamaterial shell proposed earlier for the radiated power ratio enhancement of a single dipole. Very good matching characteristics are obtained for the antenna configuration demonstrating almost complete cancelation of the reactive power of the array system. The frequency dispersive behavior of the antenna configuration is investigated using the lossless and lossy Drude models.     

Discrete Dipole Approximation Aided Design Method for Nanostructure Arrays

A discrete dipole approximation （DDA） aided design method is proposed to determine the parameters of nanostructure arrays. The relationship between the thickness, period and extinction efficiency of nanostructure arrays for the given shape can be calculated using the DDA. Based on the calculated curves, the main parameters of the nanostructure arrays such as thickness and period can be determined. Using this aided method, a rhombic sliver nanostructure array is designed with the determinant parameters of thickness （40 nm） and period （440 nm）. We further fabricate the rhombic sliver nanostructure arrays and testify the character of the extinction spectra. The obtained extinction spectra is within the visible range and the full width at half maximum is 99nm, as is expected.     

微孔阵列铜表面二次电子发射系数抑制研究

针对微孔阵列对铜表面二次电子发射系数(SEY)的抑制效应进行实验研究以提高电真空器件性能。首先利用Casino软件模拟了入射能量分别为0.5 keV和3 keV的电子束垂直入射到方形微孔阵列表面的SEY,分析了方孔阵列的深宽比和孔隙率对本征二次电子发射系数(ISEY)、背散射二次电子发射系数(BSEY)及总二次电子发射系数(TSEY)的影响。然后采用半导体光刻工艺在铜箔表面制备具有不同形貌参数的圆孔阵列,采用激光扫描显微镜进行形貌分析和几何结构参数提取,采用二次电子测试平台进行TSEY测试。仿真结果表明:微孔阵列的深宽比、孔隙率越大,其SEY抑制特性越明显;随着微孔阵列深宽比逐渐增大,SEY逐渐趋于饱和;入射电子束能量较低时,微孔阵列对SEY抑制效应比入射能量较高时更为明显。实验结果表明:微孔阵列能有效抑制铜表面SEY,实测结果与仿真结果规律一致,为微孔阵列结构用于铜表面SEY抑制提供了依据。     

Effect of shape and interstice on surface enhanced Raman scattering (SERS) of molecules adsorbed on gold nanoparticles in the near‐dipole and quadrupole regions

Surface enhanced Raman scattering (SERS) of adsorbed molecule on colloidal gold nanoparticles of different shapes, namely nanospheres (NSs), nanorods (NRs), and nanoprisms (NPs) as well as the three NPs arrays of different interstice prepared by NS lithography, are studied with incident wavenumbers in the near‐dipole and near‐quadrpole regions of the nanoparticles. In the colloidal gold nanoparticles, the SERS enhancement is the largest for the sharp tip followed by the truncated tip NPs, then the NRs and least enhancement for the NSs. This decreasing order of enhancement occurs although the incident wavenumber was near the dipole resonance of NSs and the quadrupole resonance for the NPs. These varied enhancements are explained in part as due to the binding energies of the nanocrystal facets, but the larger contribution results from the plasmon electromagnetic fields. A parallel finite difference time domain (FDTD) calculations were carried out, which corporate the experimental results and show agreement with ratios of the SERS enhancement for the different shapes. The normalized SERS intensity for NPs of different interstice distances show a sharp rise with the decrease of the interstice distances because of interparticle dipolar and quadrupolar coupling as evidenced also by FDTD calculations. Furthermore, these calculations show that the enhancement is polarization independent for an incident wavelength near quadrupole resonance but polarization dependent for an incident wavelength near the plasmon dipole transition. In the last case, the enhancement is larger by an order of magnitude for a polarization parallel to the NPs bisector than for polarization normal to the bisector with no hot spots for the relatively large interstice dimensions used. Copyright © 2012 John Wiley & Sons, Ltd.     

考虑互耦的半波振子线阵辐射和散射方向图综合

对于半波振子阵列天线,可用感应电动势法算得其阻抗矩阵,依据等效电路原理计入单元间的互耦计算辐射场.将其推广至考虑互耦的散射场计算,基于此辐射场和散射场的计算公式,提出一种考虑互耦同时综合半波振子阵列天线辐射和散射方向图的新方法.运用粒子群优化算法,通过优化振子的间距,同时综合指定辐射和散射方向图.运用该方法,通过对中心工作频率处的辐射和散射特性以及在威胁角度给定频带内的散射特性优化,有效降低半波振子阵在中心工作频率处的辐射和散射方向图的副瓣电平,并减小其在威胁角度给定频带内的雷达散射截面.计算结果与FEKO仿真结果吻合良好,验证方法的正确性.     

Nanoantenna arrays for infrared detection with single-metal nanothermocouples

Antenna-coupled nanothermocouples (ACNTC) for infrared detection have been widely studied. It has been shown that dipole antennas receive incident infrared radiation, and radiation-induced antenna currents heat the hot junction of the nanothermocouple, thus producing an electrical potential by the Seebeck effect. We have already demonstrated small thermopiles constructed from the series connection of ACNTCs. Here we study the infrared response of large-scale (N > 500) nanoantenna arrays constructed from ACNTCs, where the antennas are spaced over a range of 25–300% of the incident wavelength. COMSOL simulations show temperature oscillations, and both simulations and experiments show corresponding open-circuit voltage oscillations as a function of antenna spacing. When the distance between the antennas is less than 2λ, constructive and deconstructive interference leads to an enhancement or attenuation of the antenna currents. Our simulations and experimental results are in excellent agreement, and show that the open-circuit voltage response of the array depends on the inter-column distance of the array and the separation between the hot and cold junctions. Furthermore, we report polarization- and array-size-dependent measurements to confirm that the responses of the arrays are the result of the heating of the hot junction by the radiation-induced antenna currents.     

Three-dimensional focus engineering using dipole array radiation pattern

A systematic three-dimensional focus engineering method based on the dipole-array radiation pattern is proposed. High numerical aperture focusing of generalized cylindrical vector beam is interpreted as the reversing of the radiation from electrically small dipole arrays. Required pupil plane illumination can be found for the desired focal field with specific characteristics. Magnetic-dipole array radiation is utilized to create ultra-long (~ 8λ) diffraction limited three-dimensional optical tube with maximal uniformity. Combined with a recently reported optical needle field generation with the help of electric dipole array radiation, ultra-long (~ 8λ) three-dimensional flattop focus that has top hat profiles in both transversal and longitudinal directions can be realized with the help of radiations pattern from co-located magnetic and electric dipole arrays.     

Double-resonant extremely asymmetrical scattering of electromagnetic waves in non-uniform periodic arrays

A new type of Bragg scattering – double-resonant extremely asymmetrical scattering (DEAS) of optical waves in oblique, non-uniform, periodic Bragg arrays is analysed theoretically and numerically. Steady-state DEAS is demonstrated to occur in the extremely asymmetrical geometry where the scattered wave propagates parallel to the front array boundary. The non-uniform array is represented by two joint uniform, strip-like, periodic arrays with different phases (and amplitudes) of the grating. DEAS is characterised by a unique combination of two simultaneous resonances with respect to frequency and phase variation at the interface between the joint arrays. As a result, a strong resonant increase in the scattered wave amplitude compared with the amplitude of the incident wave is predicted and investigated theoretically. The amplitude of the incident wave inside the array is also shown to increase resonantly in the middle of the array where the step-like variation in the phase of the grating takes place. The effect of different widths of the joint arrays, and magnitudes of the grating amplitudes on DEAS is analysed. Physical explanations of this type of scattering, based on the diffractional divergence of the scattered wave from one of the joint arrays into another, are presented.     

Equilibrium states and dynamics of the dipole moment of square arrays of dipoles

The dipole lattices in the form of 3 × 3–8 × 8 dipole square arrays have been considered. It has been shown that, after turning off the external field orienting dipoles along the edges of the array, two types of equilibrium configurations of dipole moments can exist depending on the size of the system, namely, the state symmetric with respect to the diagonal of the array and the state with the total dipole moment directed along the edges of the array. It has been found that there are differences in these types of configurations with respect to the alternating-field-induced oscillation modes of the total dipole moment of the system. The dependence of the oscillation modes on the direction of linear polarization of the alternating field has been investigated.     

Microtrap on a concave grating reflector for atom trapping

We propose a novel scheme of optical confinement for atoms by using a concave grating reflector.The two-dimension grating structure with a concave surface shape exhibits strong focusing ability under radially polarized illumination.Especially,the light intensity at the focal point is about 100 times higher than that of the incident light.Such a focusing optical field reflected from the curved grating structure can provide a deep potential to trap cold atoms.We discuss the feasibility of the structure serving as an optical dipole trap.Our results are as follows.(i) Van der Waals attraction potential to the surface of the structure has a low effect on trapped atoms,(ⅱ) The maximum trapping potential is ~1.14 mK in the optical trap,which is high enough to trap cold ~(87)Rb atoms from a standard magneto-optical trap with a temperature of 120 μK,and the maximum photon scattering rate is lower than 1/s.(ⅲ) Such a microtrap array can also manipulate and control cold molecules,or microscopic particles.     

Parallel laser microfabrication of terahertz metamaterials and its polarization-dependent transmission property

Large-area split ring resonator (SRR) array was fabricated by femtosecond laser micro-lens array lithography at a fast speed. Transmission spectra of the SRR arrays at different incident terahertz wave polarization states were characterized by terahertz time domain spectroscopy. A polarization-dependent transmission property of the SRR array was observed. Polarization-dependent loss (PDL) spectrum was characterized to investigate the polarization properties of the terahertz metamaterials. The PDL characterization can eliminate the substrate effect and provide a flexible platform to study the characteristics of free-standing terahertz metamaterials.