Size characteristics of surface plasmons and their manifestation in scattering properties of metal particles

The change of the scattering properties of sodium, gold and silver spherical particles with size is discussed in the context of surface multipolar plasmon resonances. The presented surface plasmon size characteristics are abstracted from the quantity which is observed and deliver multipolar plasmon resonance frequencies and plasmon damping rates in the form of a continuous function of particle radius. The performed analysis of the plasmon dispersion relation is analogous to the problem of surface plasmon localized at a semi-infinite, flat metal/dielectric interface.Correlation between the multipolar plasmon resonance parameters, and the spectroscopic optical properties of conductive nanoparticles appearing as peaks in the measurable light intensities is analyzed. We discuss the fact, that such peaks arise from interference of all the electromagnetic fields contributing to the measured intensity, and not solely to the fields due to surface plasmon multipolar modes.We describe the results of light scattering experiment in orthogonal polarization geometries with use of spontaneously growing sodium droplets. The polarization geometry of the experiment allows for distinct separation of resonant contribution of dipole and quadrupole plasmon TM mode contributions to the measured intensities as a function of size.Predictions concerning size characteristics for dipole and quadrupole plasmons are compared with the results of light scattering experiments using spherical sodium droplets (our results) and gold and silver particles in suspension [other authors: Sönnichsen C, Franzl T, Wilk T, von Plessen G, Feldmann J. Plasmon resonances in large noble-metal clusters. New J Phys 2002; 4:93.1–8; Haiss W, Thanh NTK, Aveyard J, Fernig DG. Determination of size and concentration of gold nanoparticles from UV–vis spectra. Anal Chem 2007; 79:4215–21; Njoki PN, Lim I-IS, Mott D, Park H-Y, Khan B, Mishra S, et al. Size correlation of optical and spectroscopic properties for gold nanoparticles. J Phys Chem C 2007; 111:14664–9; Mock JJ, Barbic M, Smith DR, Schultz DA, Schultz S. Shape effects in plasmon resonance of individual colloidal silver nanoparticles. J Chem Phys 2002; 116:6755–9].     

Nanophotonics of isolated spherical particles

The problem of extreme focusing of an optical beam into the spatial region with wavelength dimensions is considered with the use of the special features of radiation interaction with isolated spherical particles. Results of numerical computations of the optical field intensity at the surface of silver particles of different radii upon exposure to laser radiation with different wavelengths are presented. It is demonstrated that the relative intensity of the plasmon optical field on the nanoparticle surface increases and the field focusing region decreases with increasing particle radius. Results of numerical computations illustrating the influence of the shell of composite nanoparticles comprising a dielectric core and a metal shell on the optical field intensity in the vicinity of the particle are presented. The problem of local optical foci of a transparent microparticle (photonic nanojets) is investigated. It is established that variation of the micron particle size, its optical properties, and laser radiation parameters allows the amplitude and spatial characteristics of the photonic nanojet region to be controlled efficiently.     

各向异性特异材料波导中表面等离子体的共振性质

使用时域有限差分法,研究了各向异性特异材料(AMM)作为包层的AMM/介质/AMM波导中表面等离子体的共振性质.色散关系表明,当特异材料为负磁导率的always-cutoff型时,AMM/介质/AMM波导支持TE极化的表面等离子体,表面等离子体的波长随着中间介质层的厚度和特异材料磁等离子体频率的减小而变短.在有限长度AMM/介质/AMM波导中,由于两端界面的反射,表面等离子体模在波导中形成Fabry-Perot共振,而实现亚波长的表面等离子体微腔.在共振频率,电场强度在微腔的中部达到最大值,而磁场分别在两端界面处达到最大,电磁能强局域在中间介质层中,这一性质将在可调的具有强局域特性的亚波长微腔及腔量子电动力学中具有潜在的应用.     

光学微操纵过程的轴平面显微成像技术

光学俘获技术利用光与物质相互作用产生的光势阱效应来实现对微粒的操控,已经成功应用于生物医学、材料科学等交叉领域.在对微粒进行三维俘获时,传统的宽场光学显微技术只能观测到某一平面内微粒的横向运动,对微粒沿轴向运动的观测受到很大限制.本文将轴平面显微成像技术引入光学微粒操控研究中,利用45?倾斜的反射镜把微粒的轴向运动信息转换到横向平面进行观测,与传统宽场显微成像技术相结合,实现了对二氧化硅小球俘获过程横向和轴向运动的同步观测.该成像方法无需扫描和数据重构,具有实时快速等优点,在新型光束光镊、厚样品三维观测和成像等领域具有潜在的应用价值.     

The Nonlinear Optical Response of Hashin-Shtrikman Geometric Composite Materials

The nonlinear optical response of composite materials consisting of metal grains with insulator shells at finite frequency is investigated. It is found that the surface plasmon peak is stronger and the position of surface plasmon peak is "red-shifted" with the shell being thinner. We also found that the nonlinear optical response is related to the shape of the particles and the dielectric property of the shells.     

Surface plasmon radiation forces

We report the first experimental observation of momentum transfer from a surface plasmon to a single dielectric sphere. Using a photonic force microscope, we measure the plasmon radiation forces on different polystyrene beads as a function of their distance from the metal surface. We show that the force magnitude at resonance is strongly enhanced compared to a nonresonant illumination. Measurements performed as a function of the probe particle size indicate that optical manipulation by plasmon fields has a strong potential for optical sorting.     

Manipulation of subwavelength optical fields and resonant field enhancements of a silver-shell nanocylinder pair and chain waveguides with different core–shell patterns

Near field optical properties and surface plasmon resonances on a pair of silver-shell nanocylinder and nanochain waveguides with different core–shell patterns which interact with incident plane wave along chain axis are numerically investigated by using the finite element method. Simulation results show that the peak wavelengths and resonant field enhancements are highly tunable by using the nanoshell particles instead of solid ones, revealing a critical relationship among the wavelengths and illuminated direction of incident light, interparticle spacing, radii, and medium of dielectric holes and the patterns of chain waveguides. Besides, nanochain waveguides with different patterns of core–shell that are operated on resonant multipolar modes can provide higher propagation intensities and the transmission ability can be increased by decreasing the size of nanocylinders along the chain axis.     

Recent Advances of Light Propagation in Surface Plasmon Enhanced Quantum Dot Devices

Surface plasmons are of particular interest recently as their performance is approaching the enhancement of light emission efficiencies, after synthesized close to the vicinity of solid state materials, i.e., semiconductor structure. As other scientific works have been proposed to improve the light-emitting efficiency, such as the use of resonant cavities, photon recycling, and thin-light emitting layers with periodic surface texturing, surface plasmon possesses a promising way to the light enhancement, due to the energy coupling effect between the emitted photons from the semiconductor and the metallic nanoparticles fabricated by nanotechnology. The usual pathway of plasmon enhanced light emitting devices is the use of Ag/Au nanoparticles coating the surface of semiconductor quantum dot (QD) or quantum well (QW) structures. However, apart from efforts to extract as much light as possible from single-driven surface plasmon-QD/QW, it is possible to enhance the light emission rate with double optical-excitations. This approach is based on the quantum interference between the external lasers and the localized quantum light, and promised to stimulate the development of plasmon-enhanced optical sensors. In this review, we describe the quantum properties of light propagation in hybrid nanoparticle and semiconductor materials, i.e., quantum dot or nanomechanical resonator coupled to Ag/Au nanoparticles, driven by two optical fields. Distinct with single excitation, plasmon-assisted complex driven by two optical fields, exhibit specific quantum interference characteristics that can be used as sensitive all-optical devices, such as the slow light switch, nonlinear optical Kerr modulator, and ultra-sensitive mass sensing. We summarize the recent advances of light propagation in surface plasmon-enhanced quantum dot devices, driven by two optical fields, which would stimulate the development of novel optical materials, deeper theoretical insights, innovative new devices, and plasmonic applications with potential for significant technological and societal impact.     

Synthesis and Optical Properties of Anisotropic Metal Nanoparticles

In this paper we overview our recent studies of anisotropic noble metal (e.g. gold and silver) nanoparticles, in which a combination of theory and experiment has been used to elucidate the extinction spectra of the particles, as well as information related to their surface enhanced Raman spectroscopy. We used wet-chemical methods to generate several structurally well-defined nanostructures other than solid spheres, including silver nanodisks and triangular nanoprisms, and gold nanoshells and multipods. When solid spheres are transformed into one of these shapes, the surface plasmon resonances in these particles are strongly affected, typically red-shifting and even splitting into distinctive dipole and quadrupole plasmon modes. In parallel, we have developed computational electrodynamics methods based on the discrete dipole approximation (DDA) method to determine the origins of these intriguing optical features. This has resulted in considerable insight concerning the variation of plasmon wavelength with nanoparticle size, shape and dielectric environment, as well as the use of these particles for optical sensing applications.     

Improved SPR technique for determination of the thickness and optical constants of thin metal films

A novel method with single-wavelength light is developed to determine the optical constants and the thickness of a thin metal film. It bases on a new geometry that consists of a coupling prism, a transparent coating layer directly deposited onto the prism base, a thin metal film to be detected, and the air. The attenuated total reflection technique is employed in our configuration to excite two different kinds of surface plasmon waves simultaneously. As a result, the reflection curve shows two obvious surface plasmon resonant dips. Using the Chen's et al. (Opt. Soc. Am. 71 189, 1981) method, we can obtain two sets of () and thickness of the thin metal film through one resonant dip (surface plasmon), then the real value can be determined through the other resonant dip (modified long-range surface plasmon). Compared to conventional double wavelength method and the double-medium technique (Yang et al. Appl. Opt. 27 11, 1988), the present single-scan method avoids both the ambiguity of different conditions caused by two-scan technique, and the dispersion problem with different light wavelength.     

Optical formation and manipulation of particle and cell patterns using a tapered optical fiber

A method for optical formation and controllable manipulation of particle and cell patterns using a tapered optical fiber is demonstrated. With a laser beam at 980‐nm wavelength launched into the fiber, different sized silica particles were formed into particle patterns (both one‐dimensional chains and two‐dimensional arrays) with different particle numbers by optical binding. The formed particle patterns can be controllably manipulated in three dimensions. Using yeast cells as an example, it was demonstrated that the method is applicable for the formation of biological cell patterns, without damage to the yeast cell viability. This method provides a new facile way for biophotonic and biological researches with particles and cells in a highly organized manner.     

Broadening of resonances in yttrium nanoparticle optical spectra

The dielectric function of yttrium in the range between 0.2 μm and 2 μm is composed of a harmonic oscillator contribution due to a discrete interband transition and the contribution of free electrons. Hence, it is possible to discuss surface plasmon polaritons as well as other electronic resonances in the optical extinction spectra of yttrium nanoparticles. For the latter, we discuss the broadening of the resonance caused by the aggregation of particles. When particles are lumped into aggregates, the color of the particle system also changes. Aggregation also affects the surface plasmon resonance in yttrium nanoparticles in a way comparable to silver or gold nanoparticle aggregates. Comparison is made with the first experimental results on yttriumnanoparticles, showing that aggregation is the dominant effect for the broad resonance in the measured extinction spectra. Received: 2 July 2001 / Revised version: 10 September 2001 / Published online: 15 October 2001     

Optical resonance of metal-coated nanoshell

Metal-coated nanoshell,the nanoparticle consisting of a nanometre-scale dielectric core coated with a thin metallic shell,exhibits three distinct optical resonant forms,the sphere cavity resonance(SCR),plasmon resonance(PR),and concentric dielectric sphere resonance(CDSR),The SCR,PR and CDSR of the metal-coated nanoshell reveal a geometric tunability controlled by the core radius and by the ratio of the core radius to the total radius,Classical electrodynamics and Mie scattering theory are used to treat the resonant forms and the transition state between the resonant forms.Based on previous experimental research,we present a group of resonant equations for all the resonant forms.which depend on the geometric structure of the metal-coated nanoshell.     

Numerical analysis of waveguide-enhanced optical bistability

We present the results of a simulation study of a multilayer structure for which strongly enhanced (resonant) electric fields are produced at the surface of, and in the interior of, a planar waveguide which is evanescently coupled to an incoming beam in a prism geometry. The resulting field enhancements can far exceed those associated with the well-known surface plasmon resonance in a typical silver film excited in the conventional attenuated total reflection geometry. Simulations are performed for optical bistability for the case of a typical YAG laser beam coupled to a zinc oxide (ZnO) waveguide, demonstrating that the effect should occur for reasonable values of the parameters involved.     

等离激元增强的石墨烯光吸收

石墨烯中等离激元具有特殊的光电性质,其和入射光的强烈耦合可以引起光吸收的增强.本文基于时域有限差分法和多体自洽场理论研究了等离激元对处于光学谐振腔中的石墨烯光吸收的影响.由于石墨烯中等离激元与入射光动量和能量不匹配而不能直接相互作用,因此石墨烯上施加了金属光栅结构.研究发现光栅结构能够对入射光进行动量补偿并且能够引起其下石墨烯中的电场强度产生很大程度增强,从而导致在该石墨烯结构中太赫兹等离激元和入射光发生强烈耦合而产生太赫兹等离极化激元,同时引起石墨烯光吸收的增强.希望本文能够加深对石墨烯光电特性的理解以及可以为基于石墨烯的太赫兹光电装置提供一定的理论依据.     

Lattice defects in small metallic particles and their influence on size effects

Several systems of isolated silver particles have been produced by different methods and have been analysed by electron microscopy. Their optical absorption spectra in the region of the spherical plasma resonance were recorded and compared with calculated spectra. The experimental plasmon band width was found to differ markedly from the predictions of size effect theories in several particle systems, while in other systems correspondence was observed. These discrepancies are attributed to different concentrations of lattice defects in the particles. Electron-lattice defect relaxation frequencies determined from the optical spectra differ by more than one order of magnitude in differently prepared particles. It is further shown that extended defects such as grain boundaries in regular particles may cause size dependencies of the dielectric constant of the particle material, in addition to those induced by the particle surface.     

超越SNOM探针通光孔径尺寸的金属纳米间隙超分辨测量

采用电磁场有限元方法,数值模拟了孔径型扫描近场光学显微镜(aperture Scanning Near-field Optical Microscopy,a-SNOM)在照明模式下的工作过程.针对金偶极天线结构,改变天线长度和纳米间隙尺寸,计算了a-SNOM探针孔径的远场辐射速率随探针端面中心坐标变化的扫描曲线,实现了超越a-SNOM探针通光孔径尺寸的天线金属纳米间隙的超分辨测量,对于100nm通光孔径的探针,可分辨最小尺寸为10nm(0.016倍波长)的金属间隙.通过对比金属和介质偶极天线的a-SNOM探针远场辐射速率测量的计算结果,表明天线金属纳米间隙的超分辨测量的实现是由于金属间隙表面等离激元的激发.     

Optical oscillation modes of plasmon particles observed in direct space by phase-contrast near-field microscopy

Light distributions near resonant metal nanoparticles are recorded by a scattering-type scanning near-field optical microscope (s-SNOM), for the first time with a sub-particle-size resolution (<10 nm) and with simultaneous amplitude and phase contrast. The images depict the optical oscillation patterns of single plasmon particles. Examples are presented of particles excited in dominantly dipolar and quadrupolar modes, and also of closely spaced particles sustaining a gap mode. The gap mode can provide enhanced optical fields in nanometric spots for non-linear and single-molecule spectroscopy applications. Received: 20 June 2001 / Revised version: 3 August 2001 / Published online: 19 September 2001     

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。