Detailed study of the flat bands appeared in two-dimensional magnetic photonic crystals with square symmetry

By virtue of the efficiency of the Dirichlet-to-Neumann map method, the details of the band structure of a two-dimensional magnetic photonic crystal having a square array of parallel circular ferrite rods in air background influenced by an external static magnetic field applied in the rod direction has been investigated. We show that there are two sets of flat bands at the band structure of the system for TM-polarization. These flat bands are created around the magnetic surface plasmon frequency and frequency in which the magnetic permeability has singular value. For the frequency around the magnetic surface plasmon, the modes are highly localized at the interface of the cylindrical ferrite rods and air background and also by approaching the modes to the magnetic surface plasmon frequency the localization length decreases and the number of field's nodes increases considerably. Moreover, we realized that the modes with frequencies lying immediately below the singular value act similar to as resonance cavity modes created in a single metallic cylindrical waveguide.     

Resonant coupling of surface plasmons to radiation modes by use of dielectric gratings

Efficient outcoupling of surface-plasma waves to radiation modes by use of dielectric diffraction gratings on a flat metallic surface is discussed. The dielectric gratings, which have a surface-relief structure with only several tens of nanometers in peak-to-trough height on a flat metal surface, can efficiently extract radiation modes propagating in free space from the surface-plasmon modes. An outcoupling efficiency of 50% is estimated with the rigorous coupled-wave diffraction theory, and it is confirmed by the experiment.     

Fabrication of grating-like polystyrene latex monolayer structure as three-dimensional calibration standards for scanning probe microscope

This paper illuminates the preparation of grating-like polystyrene latex monolayer structure, which can minimize the effects of the size deviation of spheres and the defect transfer on the accuracy as calibration samples for microscopes. The latex films are grown on freshly cleaved mica substrates by vertical deposition method. The concentration dependence of the structure and the topography of latex films is characterized by optical microscope, ultraviolet--visible transmission spectrum and scanning probe microscope. The origination of such a grating-like structure is also discussed.     

Surface shear horizontal waves associated with a periodic array of wires deposited on a substrate

The vibrational and electronic spectra of a semi-infinite crystal with a planar surface are modified by the presence of surface inhomogeneities or roughness such as ridges or grooves, quantum wires or tips. We develop a Green's function formalism to investigate the localized and resonant acoustic modes of shear horizontal polarization associated with the surface of a substrate supporting a single and a periodic array of wires. Each material is assumed to be an isotropic elastic medium. The calculation can be applied to an arbitrary choice of the shape and elastic parameters of the wires. The surface modes are obtained as well-defined peaks of the densities of states (DOS). In this paper, we calculate the variation of the density of states associated with the adsorption of a single wire, and the dispersion curves of the surface modes for a periodic array of wires on the flat surface of a substrate. We discuss their behaviors as a function of the elastic parameters and the relationship between resonant modes of the single wire and dispersion curves of the surface modes for a periodic structure. Received 6 December 2000     

Band structure of two-dimensional square lattice photonic crystals of circular dispersive metamaterial rods

By virtue of the efficiency of the Dirichlet-to-Neumann map method, the details of the band structure of a two-dimensional square lattice photonic crystal composed of dispersive metamaterial circular rods in air background has been studied. We show that there are two flat bands at the band structure of the system for both H-polarization and E-polarization. These flat bands are created around the magnetic resonance frequency, surface plasmon frequency and magnetic surface plasmon frequency. We realized that the modes with frequencies lying above the resonance frequency behave like resonant cavity modes created in a single metallic cylindrical waveguide. While, due to the relatively large and imaginary refractive index of the metamaterial rods at the frequencies lying below the resonance frequency, the modes are localized modes with negligible penetration into the rods. Moreover, the modes are localized at the interface of the cylindrical metamaterial rods and the air background for the frequencies around the surface plasmon frequency and the magnetic surface plasmon frequency.     

Evolution of the optical reflectivity of a monolayer of nanoparticles during its growth on a dielectric thin film

The reflectivity variations of a dielectric thin film during the deposition of a disordered monolayer of metallic nanoparticles are studied. We present experimental results and provide theoretical physical insight into the behavior of the reflectivity signal and its dependence on the dielectric thin-film thickness and structural features of the monolayer of nanoparticles. A closed-form expression is used to describe the reflectivity of a disordered monolayer of particles on a flat substrate within the frame of a coherent-scattering model approach. It is shown that the model reproduces qualitatively the behavior of the reflectivity signal during the experiment. Finally we study the optical response in the limit of small particles for low surface coverage fractions of the monolayer to evidence the main parameters that dictate the evolution of the reflectivity signal during the growth of a monolayer of nanoparticles.     

Hydrodynamic coupling of two brownian spheres to a planar surface

We describe direct imaging measurements of the collective and relative diffusion of two colloidal spheres near a flat plate. The bounding surface modifies the spheres' dynamics, even at separations of tens of radii. This behavior is captured by a stokeslet analysis of fluid flow driven by the spheres' and wall's no-slip boundary conditions. In particular, this analysis reveals surprising asymmetry in the normal modes for pair diffusion near a flat surface.     

Waveguided spoof surface plasmons with deep-subwavelength lateral confinement

We present a new type of waveguide scheme for terahertz circuitry based on the concept of spoof surface plasmons. This structure is composed of a one-dimensional array of L-shaped metallic elements horizontally attached to a metal surface. The dispersion relation of the surface electromagnetic modes supported by this system presents a very weak dependence with the lateral dimension and the modes are very deep-subwavelength confined with a long-enough propagation length.     

Numerical study of guided modes in arrays of metallic nanowires

We numerically investigate the band structure and guided modes within arrays of metallic nanowires. We show that bandgaps appear for a range of array geometries and that these can be used to guide light in these structures. Values of attenuation as low as 1.7 dB/cm are predicted for arrays of silver wires at communications wavelengths. This is more than 100 times smaller than the attenuation of the surface plasmon polariton modes on a single silver nanowire.     

Microwave transmission of a compound metal grating

An array of subwavelength slits in a metallic substrate supports a series of Fabry-Perot-like resonances, where each harmonic results in a transmission peak. Addition of extra slits per period yields a compound grating with a structure factor associated with the basis. In this study each repeat period is comprised of a central slit flanked by a pair of narrower slits. It supports three resonances for every Fabry-Perot-like solution. New and useful insight into this phenomenon is gained by describing each of the modes in terms of the band structure of diffractively coupled surface waves.     

Tuning of plasmonic behaviours in coupled metallic nanotube arrays

We theoretically investigate the influence of the shape of nanoholes on plasmonic behaviours in coupled elliptical metallic nanotube arrays by the finite-difference time-domain (FDTD) method.We study the structure in two cases:one for the array aligned along the minor axis and the other for the array aligned along the major axis.It is found that the optical properties and plasmonic effects can be tuned by the effective surface charges as a result of the variation in the minor axis length.Based on the localized nature of electric field distributions,we also clearly show that the presence of localized plasmon resonant modes originates from multipolar plasmon polaritons and a large magnitude of opposing surface charges build up in the gap between adjacent nanotubes.     

Transmission of light through metallic Z-shaped slit array with inner metallic bar

The transmission of a normally incident wave through a Z-shaped channel metallic slit array with metallic bar inside has been investigated by using finite-difference time-domain method. It is obtained that transmission spectra are nearly the same of the slit array with straight channel as that with Z-shaped channel in the condition the material of the slit array the same as that of the inner bar. If the Au bar is replaced by Al of the Au slit array, both resonance modes red shift obviously, especially for the structure with bent channel. Along with the width of the inner bar increasing, the localized waveguide resonance mode red shifts regularly with a tiny decrease of the peak value of all the kinds of composed structure introduced here, and the surface plasmon resonance mode red shifts regularly accompanied by peak value changing uniformly for the structures with only one type of metal. However, the surface plasmon resonance behaves different obviously, its center peak first moves to a larger wavelength fast, then red shifts slowly, for the Au-film Al-bar structure, but that moves in a very small wavelength range for the Al-film Au-bar one. The results obtained here are helpful to design subwavelength optical devices.     

Plasmonic interactions between a metallic nanotube array and a thin metallic film

Using the finite difference time domain (FDTD) method, we numerically simulate the plasmonic interactions and the optical properties of a metallic nanotube array near a thin metallic film. We show that the energies and intensities of the plasmon resonances depend strongly on the aspect ratio (the ratio of the inner to outer radius) of the nanotube, the separation between the center of the nanotube and the upper surface of the metallic film and the thickness of the film. In the thin film limit, the high-energy localized tube-related plasmons can induce their images on both sides of the metallic film, so the degeneracy state splits into two modes. Based on the nature of the field distribution, we also show clearly the plasmon resonant characteristics that the electromagnetic fields decay away from the surface of the nanotubes and both sides of the metallic film.     

Fabrication and magnetic behaviour of 2D ordered Fe/SiO2 nanodots array

We have demonstrated a simple and universal morphology-controlled growth of 2D ordered Fe/SiO₂ magnetic nanodots array, which was based on 2D colloidal monolayer template composed of polystyrene (PS) spheres and one-step sol-gel spin-coating technique. The Fe/SiO₂ nanodots have a well-ordered structure arranged in a hexagonal pattern. The dots have the shape of quasi-pyramidal tetrahedron, which reside in the interstitial region between three PS spheres and the substrate. Magnetic measurements reveal that the nanodots array exhibits the in-plane easy magnetization direction. Compared with the unpatterned Fe/SiO₂ thin film, the dots array has lower saturated field, higher remanence and coercivity. The present method is applicable to 2D ordered nanodots array of other magnetic materials.     

Numerical investigations of a multi-walled carbon nanotube-based multi-segmented optical antenna

Motivated by the fabrication potential of multi-walled carbon nanotube structures, we numerically investigated a paired structure consisting of two metallic spheres each grown on one end of a multi-walled nanotube. The paired two-segmented structure is capable to convert free-space radiation into an intense near-field, and, hence, acting as an optical antenna. Vice versa the presence of the two nanotubes enable a current source at the antenna feed to more efficiently energy into the radiation modes, resulting e.g. in correspondingly altered luminescence lifetimes when an excited single molecule is placed in the feed point. Furthermore, the structure represents a mean to localize light on a sub-wavelength scale within different materials, which is interesting in the context of a fabrication technology for integrated nanophotonic components with different material combinations. The optical properties of the nano-antenna are analyzed by means of numerical simulations using the finite element method. Our investigations have revealed that the field enhancement, the resonances, and the radiation patterns can be easily tuned since all these quantities strongly depend on the size of the nanotubes and the metallic spheres, as well as on their material properties The structure we propose here carries a great potential for bio-sensing, for tip-enhanced spectroscopy applications, and for interfacing integrated photonic nano circuits.     

Pure Reflection and Refraction of a Surface Polariton by a Matched Waveguide Structure

We study the fundamental problem of reflection and refraction of a surface polariton as it strikes the interface between two waveguide structures. By making the two waveguide structures match to each other, coupling of surface polaritons to radiating modes is cancelled, and the reflected and transmitted waves consist of only surface modes. The reflection and transmission coefficients are calculated, and negative refraction of surface polaritons is demonstrated. Finite-difference time-domain numerical simulations are also performed to verify the analytical results. As one of the applications of the matched waveguide structure, a square corner reflector for phase-retardation-free reflection of surface polaritons is proposed.     

硅表面抗反射纳米周期阵列结构的纳米压印制备与性能研究

硅表面固有的菲涅耳反射, 使得硅基半导体光电器件(如太阳能电池、红外探测器)表面有30%以上的入射光因反射而损失掉, 严重影响着器件的光电转换效率. 寻找一种方法降低硅基表面的反射率, 进而提高器件的效率成为近年来研究的重点.本文基于纳米压印光刻技术, 在2 英寸单晶硅表面制备出周期530 nm, 高240 nm的二维六角截顶抛面纳米柱阵列结构. 反射率的测试表明, 当入射光角度为8° 时, 有纳米结构的硅片相对于无纳米 结构的硅片来讲, 在400到2500 nm波长范围内的反射率有很明显的降低, 其中, 800到2000 nm波段的反射率都小于10%, 在波长1360 nm附近的反射率由31%降低为零. 结合等效介质理论和严格耦合波理论对结果进行了分析和验证. 关键词： 纳米压印 截顶抛物面阵列 抗反射 等效介质理论     

Solar Cells Based on Laser-Modified Silicon

The fabrication of solar cells based on laser-modified silicon (LMS) has been experimentally investigated. Laser ablation of single-crystal Si in air results in the formation of a quasi-periodic array of microcones. After removing the oxide layer, this array is coated by a transparent conducting tin oxide layer. A comparison of the spectral dependence of the mirror reflection of laser-modified Si with that of the initial flat Si surface shows a 10 to 15% drop in reflectivity. The light-to-electricity conversion efficiency is shown to exceed that of the initial flat Si surface by more than 20%.     

基于人工表面等离激元的双通带频率选择结构设计

本文提出了一种基于人工表面等离激元的频率选择结构的设计方法：将设计的频率选择表面和金属鱼骨结构阵列相结合得到一种新的频率选择结构.文中采用这种方法设计了一种具有陡截止和高透、高抑制性能的双通带频率选择结构.该结构由金属鱼骨结构阵列和上下两层相同的频率选择表面复合而成.通过仿真可得,该结构的两个通带频率范围分别是3.0–4.1和10.5–10.9 GHz,透射率均在-0.5 dB以上.透射率低于-10 dB的频率范围是4.7–9.2和12.1–18 GHz.在12.4–15.5 GHz频率范围内,该结构的透射率甚至低于-20 dB.在通带内,电磁波可以高效地透过该结构;在阻带内,该结构对电磁波的透射具有较好的抑制作用.测试结果表明用这种方法设计出的频率选择结构的实际性能和仿真基本一致.在金属鱼骨结构空隙中填入轻质泡沫后该结构具有一定的力学承载性能,可以实现结构功能一体化的设计.     

Three-dimensional photon confinement in a spherical microcavity with CdTe quantum dots: Raman spectroscopy

We have studied the Raman spectra of a system consisting of a polystyrene latex microsphere coated by CdTe colloidal quantum dots. The cavity-induced enhancement of the Raman scattering allows the observation of Raman spectra from only one CdTe monolayer. Periodic structure with very narrow peaks in the Raman spectra of a single microsphere was detected both in Stokes and anti-Stokes spectral regions, arising from the coupling between the emission from quantum dots and spherical cavity modes.