Tunable coupled plasmon modes via nanoshell particle chains

Coupled plasmon modes have been studied theoretically in periodic chains of nanoshell particles embedded in a graded dielectric host. These chains not only sustain a variety of localized modes as unshelled nanoparticle chains, but also offer precise control of the localization-delocalization transition among these modes by varying the permittivity contrast and/or core-shell radius ratio. By optimizing these parameters, the upper band can be tuned into higher frequencies while the lower band can be tuned into the optical communication frequencies for practical application. We also discuss the Ohmic loss effects in the metallic component of the nanoshells.     

Energy transport in plasmon waveguides on chains of metal nanoplates

An interest in energy transport in 3D chains of metal nanoparticles is oriented towards future applications in nanoscale optical devices. We consider plasmonic waveguides composed of silver nanoplates arranged in several geometries to find the one with the lowest attenuation. We investigate light propagation of 500-nm wavelength along different chains of silver nanoplates of subwavelength length and width and wavelength-size height. Energy transmission of the waveguides is analysed in the range of 400–2000 nm. We find that chain of short parallel nanoplates guides energy better than two electromagnetically coupled continuous stripes and all other considered nonparallel structures. In a wavelength range of 500–600 nm, this 2-μm long 3D waveguide transmits 39% of incident energy in a channel of λ × λ/2 cross section area.     

Optical field enhancement effects in laser-assisted particle removal

We report on the role of local optical field enhancement in the neighborhood of particles during dry laser cleaning (DLC) of silicon wafer surfaces. Samples covered with spherical colloidal particles (PS, SiO₂) and arbitrarily shaped Al₂O₃ particles with diameters from 320–1700 nm were cleaned using laser pulses with durations from 150 fs to 6.5 ns and wavelengths ranging from 400–800 nm. Cleaned areas were investigated with scanning electron and atomic force microscopy. Holes in the substrate with diameters of 200–400 nm and depths of 10–80 nm, depending on the irradiation conditions, were found at the former positions of the particles. For all pulse durations analyzed (fs, ps, ns), holes are created at laser fluences as small as the threshold fluence. Calculations of the optical field intensities in the particles’ neighbourhood by applying Mie theory suggest that enhancement of the incident laser intensity in the near field of the particles is responsible for these effects. DLC for sub-ns pulses seems to be governed by the local ablation of the substrate rather than by surface acceleration. Received: 31 May 2000 / Accepted: 7 September 2000 / Published online: 22 November 2000     

Surface plasmon enhancement of an optical anisotropy in porous silicon/metal composite

We report on the theoretical results obtained by applying the modified effective medium theory to a composite material consisting of mesoporous Si on (110)-oriented substrate with pores filled with silver through, e.g., electroplating process. The theory developed permits a self-consistent determination of the effective dielectric permittivity tensor of such materials. It is shown that the optical anisotropy of such a composite can be greatly enhanced at some wavelength ranges. While this anisotropy is generally uniaxial as in non-metal-filled mesoporous Si etched on (110) substrate, the sign of the anisotropy (i.e., positive or negative) changes in some portions of the spectrum. The optimum material parameters for an experimental observation of the theoretically predicted effects are determined. PACS 78.20.-e; 78.20.Bh; 78.20.Ci; 78.20.Fm; 78.30.Fs; 78.55.Mb     

Field enhancement effect of metal probe in evanescent field

Field enhancement effect of metal probe in evanescent field, induced by using a multi-layers structure for exciting surface plasmon resonance （SPR）, is analyzed numerically by utilizing two-dimensional （2D） TM- wave finite difference time-domain （FDTD） method. In this letter, we used a flmdamental mode Gaussian beam to induce evanescent field, and calculated the electric intensity. The results show that compared with the nonmetal probe, the metal probe has a larger field enhancement effect, and its scattering wave induced by field enhancement has a bigger decay coefficient. The field enhancement effect should conclude that the metal probe has an important application in nanolithography.     

Slow light with symmetric gap plasmon guides with finite width metal claddings

We study the dispersion relation and the modes of a symmetric gap plasmon guide, where a dielectric planar slab is coated with finite metallic layers on both top and bottom. The finite conductivity of the metal is taken into account. The modes of the structure exhibit significant differences from those of dielectric waveguides with air or metal as the bounding media. Avoided level crossing phenomenon between the plasmon and the guided modes is shown to exist, leading to leaky modes. The structure sandwiched between two high index media is shown to lead to slow light in transmission. The group delay is shown to be larger for higher order modes.      

Oscillatory behavior and enhancement of the surface plasmon linewidth in embedded noble metal nanoparticles

We study the Landau damping of the surface plasmon resonance of metallic nanoparticles embedded in different environments of experimental relevance. Important oscillations of the plasmon linewidth as a function of the radius of the nanoparticles are obtained from numerical calculations based on the time dependent local density approximation. These size-oscillations are understood, within a semiclassical approximation, as a consequence of correlations in the spectral density of the nanoparticles. We treat inert matrices, as well as the case with an unoccupied conduction band. In the latter case, the plasmon lifetime is greatly reduced with respect to the inert case, but the non-monotonous size-dependence persists.     

Tunable multiple plasmon resonances and local field enhancement of nanocrescent/nanoring structure

According to the plasmon hybridization theory, the plasmon resonance characteristics of the gold nanocrescent/nanoring(NCNR) structure are systematically investigated by the finite element method. It is found that the extinction spectra of NCNR structure exhibit multiple plasmon resonance peaks, which could be attributed to the result of the plasmon couplings between the multipolar plasmon modes of nanocrescent and the dipolar, quadrupolar, hexapolar, octupolar,decapolar plasmon modes of nanoring. By changing the geometric parameters, the intense and separate multiple plasmon resonance peaks are obtained and can be tuned in a wide wavelength range. It is further found that the plasmon coupling induces giant multipole electric field enhancements around the tips of the nanocrescent. The tunable and intense multiple plasmon resonances of NCNR structure may provide effective applications in multiplex biological sensing.     

Microscopic model of the THz field enhancement in a metal nanoslit

We discuss the strong THz-field enhancement effect in a metal slit of dozens of nanometers sizes reported recently. Proposed simple microscopic model considers electric charges induced at the edges of the slit by a polarized incident wave. These charges contribute then to the field in the slit. The model is capable of explaining peculiarities of the field enhancement phenomenon such as an inverse frequency dependence of the enhancement factor. It provides closed-form expressions for the enhancement factor and field mapping inside the slit having only one fitting parameter. The model predicts influence of the slit shape on the field enhancement.     

分子激发中的表面等离激元增强效应

金属纳米粒子的表面等离激元增强效应是纳米科学领域的一个研究热点.针对染料分子与金属纳米粒子的耦合系统,应用偶极-偶极近似计算分子与金属纳米粒子的库仑相互作用,并应用密度矩阵理论描述在不同极化方向的电场作用下的电荷输运过程,分析了分子与金属纳米粒子在不同相对位置下分子激发态的动力学过程,发现表面等离激元的增强效应与分子和金属钠米粒子的相对位置以及等离激元的耗散系数有密切关系,详细讨论了分子与金属纳米粒子间的耦合强度、外场的极化方向、等离激元的寿命及共振激发条件对分子激发态及表面等离激元增强的影响,分析了分子-金属纳米粒子耦合系统中表面等离激元增强效应的物理本质.     

Digital resolution enhancement in surface plasmon microscopy

The use of photonic crystal and negative refractive index materials is known to improve the resolution of optical microscopy and lithography devices down to the 80 nm level. Here we demonstrate that utilization of well-known digital image recovery techniques allows us to further improve the resolution of optical microscopy down to the 30 nm level. Our microscope is based on a flat dielectric mirror deposited onto an array of nanoholes in thin gold film. This two-dimensional photonic crystal mirror may have either a positive or negative effective refractive index as perceived by surface plasmon polartions in the visible frequency range. The optical images formed by the mirror are enhanced using simple digital filters. PACS 73.20.Mf; 42.70.Qs; 07.60.Pb     

Numerical estimate of the phase diagram of finite ANNNI chains in transverse field

We have estimated numerically the phase diagram of a one dimensional spin 1/2 quantum Ising model with competing nearest and next nearest neighbour interactions in presence of a transverse field. The method essentially is to diagonalise exactly the Hamiltonian for finite (10 spins) open chains and calculate the spin-spin correlations from the ground state eigenvector. The results obtained confirm the transition between ferromagnetic and paramagnetic phases for <0.5 and between antiphase and paramagnetic phase for >0.5. ( is the ratio of next nearest and nearest neighbour interactions.) The results perhaps indicate furthermore that (i) there is a disorder line passing through =0.5; (ii) the zero point quantum fluctuations destroy the order near =0.5 as the transverse field is switched on; and (iii) there is probably also a floating phase with slowly decayling correlation near the order-disorder phase boundary for >0.5.     

Surface plasmon dispersion relation and local field enhancement distribution for a deep sinusoidal grating

Two features of light scattering from a deep lossless metallic sinusoidal grating are considered in the limit g/d → ∞, where g and d are the height and periodicity of the grating, respectively. It is found that the surface plasmon dispersion relation is comprised of two flat bands with a frequency gap Δω/ω_p = (√2 − 1)/2, where ω_p is the volume plasmon frequency. For the local field enhancement distribution at the grating's surface, the results show the existence of two qualitatively distinct domains, i.e., λ ⪢ d and λ ⪡ d, where λ denotes the radiation wavelength. In both domains, however, the local field enhancement is larger at the bottom of the troughts than at the top of the peaks. The dressed Rayleigh expansion is used throughout for the analysis.     

Experimental observation of plasmon polariton waves supported by a thin metal film of finite width

What are believed to be the first experimental observations of the existence of long-range plasmon-polariton waves, guided by a thin metal film of finite width, are presented. A waveguide composed of an 8-mum-wide, 20-nm-thick, 3.5-mm-long Au metal film embedded in SiO (2) was successfully excited at a free-space wavelength of 1.55 mum in an end-fire experiment. The theoretical nature of the phenomenon is described, and experimental observations of field confinement provided by this metal waveguide are presented in detail.     

End and central plasmon resonances in linear atomic chains

The existence and nature of end and central plasmon resonances in a linear atomic chain, the 1D analog to surface and bulk plasmons in 2D metals, has been predicted by ab initio time-dependent density functional theory. Length dependence of the absorption spectra shows the emergence and development of collectivity of these resonances. It converges to a single resonance in the longitudinal mode, and two transverse resonances, which are localized at the ends and center of the atom chains. These collective modes bridge the gaps, in concept and scale, between the collective excitation of atomic physics and nanoplasmonics. It also outlines a route to atomic-scale engineering of collective excitations.     

长程表面等离子体的增强效应

研究了双层金属薄膜构型中构型参数对长程表面等离子体的影响,并发现了衰减全反射激发方法下长程表面等离子体的增强效应.以特征矩阵算法为基础,通过数值计算构型的反射谱,研究构型参数的变化对反射谱的影响.发现由于衰减全反射激发方法中耦合器的存在导致的非对称特性,会使双层金属薄膜构型中的长程表面等离子体拥有本征模式特性以外的有趣特性,如长程模式得到增强而另一支受到抑制,从而使能量更为集中在希望被激发的一支.研究结果对非对称激发构型中的长程表面等离子体研究具有启发意义.     

Surface plasmon resonance in superperiodic metal nanoslits

A superperiodic metal nanoslits device is a surface plasmon resonance optical diffraction grating in which each line of the grating consists of an array of finite number metal nanoslits. The metal nanoslits, upon optical excitations, support localized surface plasmon resonance. The superperiod of the nanoslits causes the coherent radiation of the surface plasmon resonance into the far field with angular dispersion. Therefore, localized surface plasmon resonance in the metal nanoslits can be measured with a CCD or a linear photodetector array. In this Letter, we describe a surface plasmon resonance spectral sensor using a superperiodic gold nanoslits array without using an external optical spectrometer.     

One-dimensional plasmon in an atomic-scale metal wire

We have measured one-dimensional (1D) plasmons in an atom wire array on the Si(557)-Au surface by inelastic scattering of a highly collimated slow electron beam. The angular dependence of the excitation energy clearly indicates the strong 1D confinement and free propagation of the plasma wave along the wire. The observed plasmon dispersion is explained very well by a quantum-mechanical scheme which takes into account dynamic exchange-correlation effects, interwire interactions, and spin-orbit splitting of the 1D bands. Although the qualitative feature of the plasmon dispersion is reminiscent of that of a high-density free-electron gas, we detected the substantial influence of electron correlation due to strong 1D confinement.