Plasmon loss structure in synchrotron radiation photoemission from Mg films

Synchrotron radiation excitation has been used to vary the kinetic energy of electrons photoemitted from the 2p core level of polycrystalline magnesium films and the energy-dependence of the accompanying plasmon loss structure has been observed. Difficulties associated with background subtraction are discussed, and it is shown that the method of background subtraction profoundly affects the conclusions drawn from such an experiment on the relative importance of plasmon and single particle excitation mechanisms for electron scattering. Very thin films of Mg have also been studied in an attempt to separate the intrinsic and extrinsic modes of plasmon creation. Although the plasmon loss structure is dependent on film thickness implying a considerable contribution from extrinsic processes, we are not able to observe plasmons excited in the thin film by electrons originating in the substrate core level. The implications of these results are discussed and further experiments suggested.     

Experimental measurement of acoustic plasmons in polycrystalline palladium

An experimental study of collective oscillations in Pd covering the region of very low energy and momentum transfers is reported. Through Dynamic Electron Scattering spectroscopy, structure factor spectra were measured from 80?K to 298?K on a bulk polycrystalline Pd sample. Here we report the first experimental evidence of damped acoustic plasmons and their evolution to the single-particle excitation continuum. The acoustic plasmons follow a linear dispersion and are experimentally shown to be a separate and distinct resonance mode from acoustic surface plasmons. Calculations of the dielectric function employed a model that incorporates complete mixing of two conduction bands with contributions from both interband and intraband transitions. The model was used in computational studies that focused on specific experimental results to aid the characterization and understanding of the plasmon behavior. We found that the Pd acoustic plasmon energy matched the longitudinal phonon anomaly that has sparked numerous theoretical reports on the possible energetic coupling of these modes. Further experimental evidence of plasmon and phonon dynamical processes are found in the linewidth analysis of the data. The primary decay mechanism of the plasmons is interpreted to be strong phonon-assisted interband transitions. Further spectral features and the plasmon velocity are also reported.     

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.     

Comparison of the spectra of a blackbody and thermally stimulated surface plasmon polaritons in the infrared range

The emission spectra of thermally stimulated surface plasmon polaritons (thermally stimulated surface plasmon polaritons) and a blackbody have been analyzed and compared, and the temperature dependence of these spectra has been studied. It has been found that the total energy of the entire ensemble of surface plasmons is proportional to the cube of temperature and their spectrum is red-shifted from the blackbody spectrum. It has been shown that the spectrum of thermally stimulated surface plasmon polaritons obeys the Wien’s displacement law, yet with another constant. The fraction of the photon energy of the conducting layer transferred to the surface plasmons has been estimated. It has been demonstrated numerically by the example of a gold layer that this fraction can exceed 10% for a layer thickness of less than 1 mm.     

Radiative plasmon polaritons in multilayer structures with a two-dimensional electron gas

Two-dimensional plasmon polaritons are analyzed for a typical experimental configuration in which a layer of two-dimensional electrons with a finite mobility lies on the top of a dielectric waveguide formed by the substrate (a wafer of finite thickness). Two-dimensional plasmons couple strongly to the radiative modes of this dielectric waveguide. It is shown that, as a result of the competition between collisional and radiative processes, a family of eight quasi-stationary normal modes arises. Six of them decay carrying energy to infinity. The two remaining plasmon-polariton modes are nonradiative. One of these modes, the TM-type plasmon polariton, in the limiting case where retardation is disregarded corresponds to the conventional longitudinal two-dimensional plasmon. The other mode, the TE-type plasmon polariton, exists only for a finite thickness of the substrate. All of them are characterized by different dispersion relations of the complex frequency ω(q) = Reω + iImω and differ in both polarization (longitudinal and transverse) and symmetry with respect to the direction of decay (symmetric and asymmetric). The latter modes decay slowly, propagating into free space to plus or minus infinity. The conditions under which the Q factors of certain modes are arbitrarily high are found. In this case, Imω(q ₀) = 0, and dissipative losses in the two-dimensional electron gas are compensated by external sources. As a result, the reflection coefficient for a plane wave whose angle of incidence is determined by the vector q ₀ vanishes.     

Interplay of structural and temperature effects on plasmonic excitations at noble-metal interfaces

The interplay of structural and electronic properties on plasmon modes was investigated for a thin Ag film grown at room temperature on Cu(111). Surface plasmons are confined within Ag grains, as indicated by the analysis of their dispersion relationship, which is dispersionless up to a critical wave-vector. Surface plasmon confinement is removed upon annealing at 400?K. The thermal treatment induces a flattening of the Ag adlayer with a merging of Ag islands, and, moreover, a strong enhancement of surface conductivity.     

Plasmonic structure and electromagnetic field enhancements in the metallic nanoparticle-film system

We investigate the plasmonic structure of a metallic nanoparticle near a metallic thin film. We show that in the thin film limit, a virtual plasmon resonance composed of delocalized thin film plasmons is induced. We investigate how the physical properties of the virtual state depend on polarization, film thickness and nanoparticle-film separation. We show that the electromagnetic field enhancements associated with the virtual plasmon resonance are large, suggesting applications of metallic nanoparticle/thin film systems as substrates for surface enhanced spectroscopies and surface enhanced scanning probe microscopies. PACS 78.67.Bf; 73.20.Mf; 78.30.-j     

A theoretical study of a plasmonic sensor comprising a gold nano-disk array on gold film with a SiO_2 spacer

A plasmonic refractive index(RI) sensor with high RI sensitivity based on a gold composite structure is proposed.This composite structure is constructed from a perfect gold nano-disk square array on a gold film, with a SiO_2 spacer. The reflection spectra of the composite structure, with analyte RI in the range of 1.30 to 1.40, are theoretically studied using the finite-difference time-domain method. The incident light beam is partly coupled to the localized surface plasmons(LSP) of the single nano-disks and partly transferred to the propagating surface plasmons(PSP) by grating coupling. The reflectivity is nearly zero at the valley of the reflection spectrum because of the strong coupling between LSP and PSP. Also, the full width at half maximum(FWHM) of one of the surface plasmon polaritons(SPPs) modes is very narrow, which is helpful for RI sensing. An RI sensitivity as high as 853 nm/RIU is obtained. The influence of the structure parameters on the RI sensitivity and the sensor figure of merit(FOM) are investigated in detail. We find that the sensor maintains high RI sensitivity over a large range of periods and nano-disk diameters. Results of the theoretical simulation of the composite structure as a plasmonic sensor are promising. Thus, this composite structure could be extensively applied in the fields of biology and chemistry.     

扫描隧道显微镜诱导单分子发光热荧光现象的全量子理论

使用量子主方程方法从理论上研究了STM诱导分子电致发光现象,并特别考虑了分子和量子化等离激元相互作用对于单分子光学性质的影响. 分子-等离激元相互作用大大增加了分子的自发辐射速率和分子发光的的强度,而且当分子自发辐射速率和振动弛豫速率可比拟的时候,将会出现热荧光现象.     

Infrared absorption enhancement of monolayer species on thin evaporated Ag films by use of a Kretschmann configuration: Evidence for two types of enhanced surface electric fields

Infrared absorption enhancement of m- and p-nitrobenzoic acid deposited on thin-evaporated silver films has been investigated using the Kretschmann's ATR coupling method. The absorption spectra provide direct evidence that enhancement is prominent only for vibrations of the first monolayer adsorbed on the Ag surface. It is shown that all of the vibrational modes observed obey the normal dipole selection rule. Moreover, it was found that there exist two types of absorption enhancement; the first is enhanced (300) by both p- and s-polarized radiation, and the second is enhanced (60) by p-polarized radiation alone. The Ag film thickness optimum is about 50 Å in the former case and about 200 Å in the latter. The enhancement insensitive to the polarization state of radiation can well be explained by the excitation of the transverse collective electron resonance of the Ag islands, whereas that obtained by p-polarized radiation may be due to the excitation of delocalized surface plasmons modified by surface roughness. The image-dipole effect may also be significant in the mechanisms.     

Collective excitations and quantum size effects on the surfaces of Pb(111) films: An experimental study

Pb(111) film is a special system that exhibits strong quantum size effects in many electronic properties. The collective excitations, i.e., plasmons, in Pb(111) films are also expected to show signatures of the quantum size effect. Here, using high-resolution electron energy loss spectroscopy, we measured the plasmons on the surface of Pb(111) films with different film thicknesses and analyzed the plasmon dispersions. One surface plasmon branch exhibits prominent damping in the small momentum range, which can be attributed to the interaction between the top and bottom interfaces of the Pb(111)films. With the film thickness increasing, the critical momentum characterizing the damping in Pb(111) films decays not only much slower in Pb(111) films than in other metal films, and even in films with the thickness up to 40 monolayers the damping still exists. The slow decay of the surface plasmon damping, manifesting the strong quantum size effect in Pb(111) films, might be related to the strong nesting of the Fermi surface along the(111) direction.     

Delocalization of phonon-plasmon modes in GaAs/AlAs superlattices with tunnel-thin AlAs barriers

The technique of Raman spectroscopy has been used to investigate doped (n-type) and undoped GaAs/AlAs superlattices with AlAs barrier thicknesses from 17 to 1 monolayers. The peak corresponding to the scattering by a two-dimensional plasmon was found in the Raman spectrum of a doped superlattice with relatively thick barriers. The position of the experimental peak corresponded to the value calculated in the model of plasma oscillations in periodic planes of a two-dimensional electron gas. The electron tunneling effects played an increasingly prominent role as the AlAs barrier thickness decreased. The peaks corresponding to the scattering by coupled phonons with three-dimensional plasmons were found in the Raman spectra for a superlattice with an AlAs thickness of 2 monolayers; i.e., the delocalization of coupled modes was observed. In this case, the folding of acoustic phonons was observed in the superlattice under consideration, indicative of its good periodicity, while the localization of optical phonons in GaAs layers was observed in undoped superlattices with an AlAs thickness of 2 monolayers.     

Resonant magneto-optical Kerr effect induced by hybrid plasma modes in ferromagnetic nanovoids

With nanovoids buried in Co films, resonant structures were observed in spectra of polar magneto-optical Kerr effect(MOKE), where both a narrow bandwidth and high intensity were acquired. Through changing the thickness of Co films and the lattice of voids, different optical modes were introduced. For a very shallow array of voids, the resonant MOKE was induced by Ag plasma edge resonance, for deeper ones, hybrid plasma modes, such as void plasmons in the voids, surface lattice plasmons between the voids, and the co-action of them, etc. resulted in resonant MOKE. We found that resonant MOKE resulted from the void plasmons resonance which possesses the narrowest bandwidth for the lowest absorption of voids. The simulated electromagnetic field(EF) distribution consolidated different effects of these three optical modes on resonant MOKE modulation. Such resonant polar MOKE possesses high sensitivity, which might pave the way to on-chip MO devices.     

环形狭缝腔阵列光学特性的研究

设计了一种六角密排的二维环形纳米腔阵列结构, 利用时域有限差分算法对该结构的光学特性进行了探究. 仿真结果表明, 在线性偏振光入射时, 环形腔内可以形成多重圆柱形表面等离激元谐振, 谐振波长的个数和大小与环形腔的结构参数相关. 根据透、反射光谱, 电场矢量的模式分布及截面电荷密度的分布, 谐振波长处形成圆柱形表面等离激元, 谐振波长处入射光能量大部分在环形腔内损耗, 此时反射率为极小值, 环形腔内的电场增强效应为极大值(光强增强可达1065倍). 谐振波长与环形腔的结构参数(狭缝内径、狭缝外径、膜厚、环境介质折射率、金属的材质)相关, 通过调节结构参数, 谐振波长在350–2000 nm范围内可调. 通过对比相同结构参数的单个环形腔和环形腔阵列的仿真结果, 周期排布对环形腔内的圆柱形表面等离激元吸收峰位置影响不明显. 该结构反射光谱对入射光电矢量偏振方向不敏感. 谐振波长的可调控性对于表面拉曼增强和表面等离激元共振传感器的设计与优化具有指导性意义, 且应用于折射率传感器时灵敏度可达1850 nm/RIU.     

Long-range surface plasmon polaritons propagating on a dielectric waveguide support

In the traditional long-range surface plasmon geometry, an ultrathin metal film is sandwiched between two layers having identical dielectric constants. Here we demonstrate the long-range surface plasmon polariton (LRSPP) properties for a new structure where a thin layer with a dielectric constant exceeding that of the surroundings is inserted within the sandwich, provided the layer thickness d satisfies the condition k(⊥)d=mπ where k(⊥) is the component of the guide wavevector perpendicular to the layer and m is an integer. The resulting plasmon modes have smaller losses and nearly the same phase velocity as the original LRSPP. This provides a strategy to support silver films having thicknesses of 10's of nanometers to create plasmonic devices for sensor applications.     

周期排列的电介质小球所诱发的金属-电介质表面上的表面等离子激元的光学性质

研究了金属板的上下表面附近各放置一层按周期排列的电介质小球的体系的光学性质.用多重散射法计算的结果显示金属上侧的周期性排列的电介质小球可诱发金属-电介质表面上的表面等离子激元.这些表面等离子激元的存在可通过非常尖锐的吸收峰反映出来.对于无限厚的金属板，这些吸收峰的峰值位置主要与电介质小球的周期有关，且与解析理论符合得相当好.在有限厚度的金属板中，金属板的两侧表面会产生对称和反对称的两种表面等离子激元，从而使原来在无限厚的金属表面上所出现的单一频率的表面等离子激元劈裂为双频率.由于对称和反对称的表面等离子激元模式在金属板的两侧表面均有相当强的电磁场，因而它们可导致强的电磁波穿透.通过在金属板的下侧加入玻璃球层可将表面等离子激元的电磁场引导出金属，并产生透射波.用多重散射法计算的结果证实，在此体系中由表面等离子激元所引起的透射可达到相当的强度. 对该体系中的物理机理进行了详细分析，从而能够通过调节该体系中的一些参数来控制表面等离子激元出现的频率，使强吸收峰或强透射峰出现在所希望的频率上. 关键词： 表面等离子激元 吸收谱 透射     

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.     

Resonant surface plasmons of a metal nanosphere treated as propagating surface plasmons

On the assumption that the resonant surface plasmons on a spherical nanoparticle are formed by standing waves of two counter-propagating surface plasmon waves along the surface, by using Mie theory simulation, we find that the dispersions of surface plasmon resonant modes supported by silver nanospheres match with those of the surface plasmons on a semiinfinite medium-silver interface very well. This suggests that the resonant surface plasmons of a metal nanosphere can be treated as a propagating surface plasmon wave.     

A large-area photolithography technique based on surface plasmons leakage modes

An atomic force microscope (AFM) assisted surface plasmons leakage radiation photolithography technique has been numerically demonstrated by using two-dimensional finite-difference time-domain (2D-FDTD) method. With the aid of a metallic AFM tip, particular characteristic of the Kretstchmann configuration to excite surface plasmons (SPs) is utilized to achieve large-area patterns with high spatial resolution and contrast, the photoresist could be exposed with low power laser due to the remarkable local field enhancement at the metal/dielectric interface and the resonant localized SPs modes near the tip. Good tolerance on the film thickness and incident angle has been obtained, which provides a good practicability for experiments. This photolithography technique proposed here can realize large-area, high-resolution, high-contrast, nondestructive, arbitrary-structure fabrication of nanoscale devices.     

Improving the emission efficiency of periodic plasmonic structures for lasing applications

We report on a two dimensional plasmonic structure that utilizes an Ag film for the generation of surface plasmons and a layer of the organic semiconductor tri(8-hydroxyquinoline) aluminum (Alq₃) doped with the laser dye 4-dicyanmethylene-2-methyl-6-(p-dimethylaminostyryl)-4 H-pyran (DCM) as an active medium. The dispersion diagram of this structure exhibits a plasmonic bandgap in the dye emission wavelength range. At the flat band-edge, the group velocity tends to zero, so that the density of surface plasmon modes is high. This may yield a lasing action. However, the device suffers from the energy dissipation due to metal absorption and unwanted radiation. We examine how some of them may be overcome. Firstly, we propose the use of long range surface plasmons (LRSPs) characterized by a low loss coefficient. To this end, we investigate theoretically and experimentally the best conditions for the excitation of these modes. A strong emission is observed compared to that from a planar structure. These modes provide a high performance-an enhancement factor of 3-when the dye thickness is about 100 nm, a value consistent with the numerical findings. We further demonstrate that the use of a spacer layer significantly increases the emission efficiency. Finally, we suggest a specific design for the laser structure for minimal radiation loss.