Tunable Fano resonances and plasmonic hybridization of gold triangle–rod dimer nanostructure

A gold dimer structure consisting of a notched triangle nanoslice and a rectangle nanorod is proposed to produce distinct Fano resonance. Owing to the coupling between the dipole plasmon mode of the nanorod and the dipole or quadrupole plasmon mode of the nanoslice, the extinction spectrum with a deep Fano dip is formed and can be well fitted by the Fano interference model for different geometry parameters. In addition, Fano resonance of the gold dimer nanostructure also intensely depends on the polarization direction of incident light. Moreover, Fano resonance of the triangle–rod trimer is also analyzed by adding another nanorod into the former dimer and exhibits the splitting of plasmonic resonant peak in high order coupling modes. The plasmonic hybridizations in these nanostructures have been analyzed for revealing the physical origin of the Fano resonance.     

Fano resonances in dipole-quadrupole plasmon coupling nanorod dimers

We theoretically investigate the plasmon coupling in metallic nanorod dimers. A pronounced dip is found in the extinction spectrum due to plasmonic Fano resonance, which is induced by destructive interference between the bright dipole plasmon of a short nanorod and the dark quadrupole plasmon of a long nanorod. This Fano interference can also be explained as the coupling between the bright and dark modes both supported by the whole dimer. The Fano resonance can be tuned by adjusting the spatial or spectral separation between two nanorods in the dimer.     

Tunable Fano resonances in heterogenous Al–Ag nanorod dimers

We theoretically investigate the plasmonic coupling in heterogenous Al–Ag nanorod dimers. A pronounced Fano dip is found in the extinction spectrum produced by the destructive interference between the bright dipole mode from a short Al nanorod and the dark quadrupole mode from a long Ag nanorod nearby. This Fano resonance can be widely tuned in both wavelength and amplitude by varying the rod dimensions and end geometry, the separation distance and the local dielectric environment. The Al–Ag heterogeneous nanorod dimer shows a high sensitivity to the surrounding environment with a local surface plasmon resonance figure of merit of 7.0, which enables its promising applications in plasmonic sensing and detection.     

同心椭圆柱-纳米管结构的双重Fano共振研究

提出了一种同心椭圆柱-纳米管复合结构,该结构由金纳米管中内嵌椭圆形金柱构成,利用时域有限差分法分析了尺寸参数、周围环境及纳米管内核材料对该结构光学性质的影响.结果表明,调节椭圆柱芯的旋转角度可产生双重偶极-偶极Fano共振,其主要是由椭圆柱芯的纵向或横向偶极共振模式与纳米管的偶极成键和反成键模式杂化形成的超辐射成键模式和亚辐射成键模式之间的相互作用产生的,且共振特性可通过调节复合结构的尺寸参数控制,随椭圆柱长轴或短轴的增大而红移,随纳米管外径的增大或整体尺寸的减小而蓝移,当纳米管内径增大时高频Fano共振随着红移,而低频Fano共振先蓝移再红移,同时其对外界环境的变化不敏感,但对纳米管内核材料变化有着较好的响应.利用等离激元杂化理论对该现象进行了解释.这些结果可为构造其他类型的多波段Fano共振二维或三维纳米结构提供一种新的方式.     

Fano-like resonance characteristics of asymmetric Fe_2O_3@Au core/shell nanorice dimer

A geometrical configuration of Fe2O3/Au core-shell nanorice dimer is proposed and its multipolar plasmon Fano- like resonance characteristics are theoretically investigated by generalizing the plasmon hybridization model of individual nanorice to the bright and dark modes of the nanorice dimer. Under the irradiation of polarization light, the extinction spectra of the nanorice dimer are numerically simulated by using the finite element method （FEM）. Our studies show that the Fano-like resonance of the nanorice dimer results in an asymmetric line shape of the Fano dip in the extinction spectrum which can be controlled by varying the structure parameters of the nanorice dimer. Meanwhile, there is a giant field enhancement at the gap between the two nanorices on account of the plasmonic coupling in the nanorice dimer. The aforementioned two characteristics of the nanorice dimer are useful for plasmon-induced transparency and localized surface plasmon resonance sensors.     

Studies of chemical diffusion in La1-X SrXCoO3-δ

Recent investigations of superconductivity in carbon nanotubes have shown that a single-wall zig-zag nanotube can become superconducting at around 15?K. Theoretical studies of superconductivity in nanotubes using the traditional phonon exchange model, however, give a superconducting transition temperature T _c less than 1?K. To explain the observed higher critical temperature we explore the possibility of the plasmon exchange mechanism for superconductivity in nanotubes. We first calculate the effective interaction between electrons in a nanotube mediated by plasmon exchange and show that this interaction can become attractive. Using this attractive interaction in the modified Eliashberg theory for strong coupling superconductors, we then calculate the critical temperature T _c in a single-wall nanotube. Our theoretical results can explain the observed T _c in a single-wall nanotube. In particular, we find that T _c is sensitively dependent on the dielectric constant of the medium, the effective mass of the electrons and the radius of the nanotube. We then consider superconductivity in a bundle of single-wall nanotubes and find that bundling of nanotubes does not change the critical temperature significantly. Going beyond carbon nanotubes we show that in a metallic hollow nanowire T _c has some sort of oscillatory behaviour as a function of the surface number density of electrons.     

Wideband slow light achievement in MIM plasmonic waveguide by controlling Fano resonance

In this paper, we have investigated the characteristics of an asymmetric shaped Fano line in a metal–insulator–metal (MIM) plasmonic waveguide side coupled to two resonating stub structures. The spectral properties of Fano resonance are quite distinct due to the destructive interference between a two propagating plasmon modes. Two structural parameters are carefully adjusted: physical separation between both the resonating stubs and length of resonating stubs. By tailoring the separation between both the resonating structures, coupling between both the plasmon modes is controlled, and hence asymmetric nature of Fano line can be shaped accordingly. Resonance condition of Fano line can be tuned by scaling the length of stubs. A strong red shift in resonating wavelength with varying degree of asymmetry is observed, when length of resonating structures is increased. The sharp resonant peak, due to an asymmetric shaped Fano resonance is generally accompanied by large dispersion that results in reduction of group velocity of light near Fano resonance. By controlling the coupling between resonating stub, or by scaling the length of lower resonating stub, large value of group index (n_g = 75) and delay bandwidth product (DBP = 0.2533) is obtained. The structure can be modified to suit different applications in optical buffers, optical switches and nonlinear optics devices.     

D3h和D4h等离激元超分子的Fano共振光谱的 子集合分解解释

针对D_3h和D_4h对称构型金属纳米多颗粒集合即等离激元超分子表面等离激元共振光谱的子集合分解及其相对应的Fano共振光谱低谷的产生机理, 本文运用群论的方法做出了详细的分析研究. 运用与群论中求解分子简正振动模式类似的方法, 推导证实了在线偏振光入射时, D_nh环形多颗粒只有2个电偶极表面等离激元共振模式, 增加中心颗粒会使模式增加1个. 对D_3h和D_4h等离激元超分子的表面等离激元共振模式进行不可约表示基向量正交分解分析表明, Fano共振光谱低谷是由于两个起主要作用的相邻模式包含有共同的正交基向量, 并形成相消干涉而产生. 这进一步验证了Fano共振光谱低谷的起源除传统观点(即源自于宽频超辐射亮模式和窄频低辐射暗模式之间的耦合)之外的另一种解释视角.     

Shape of Fano resonance line spectra calculated for silver nanorods

Using theoretical tools, we numerically demonstrated Fano line shapes in the scattering spectra of silver rods resulting from different mechanisms. One of the Fano line shapes is due to the coupling of an in-plane quadrupole and a dipole mode in a single rod. Two nodes were observed at the resonance wavelength, each of which is located at a quarter of the rod length from the two ends. The Fano resonance is strengthened when the silver rod is cut at the two nodal positions. The second mechanism that gives rise to a new Fano resonance peak occurs when the symmetry of the rod is broken and is a result of the asymmetric coupling between the two excited dipoles.     

Characteristics and mechanism analysis of Fano resonances in Π-shaped gold nano-trimer

Fano interference of metallic nanostructure is an effective way to reduce the irradiation loss and improve the spectral resolution. A Π-shaped gold nano-trimer, which is composed of a gold nanorod and two gold nanorices, is presented to investigate the properties of Fano resonances in the visible spectrum by using the finite element method(FEM). The theoretical analysis demonstrates that the Fano resonance of the Π-shaped gold nano-trimer is attributed to the near-field interaction between the bright mode of the nanorice pair and the dark quadrupole mode of the nanorod. Furthermore, by breaking the geometric symmetry of the nanostructure the line-shape spectrum with double Fano resonances of Π-shaped gold nano-trimer is obtained and exhibits structure-dependent and medium-dependent characteristics. It is a helpful strategy to design a plasmonic nanostructure for implementing multiple Fano resonances in practical applications.     

金纳米环双体尺寸和耦合效应对表面等离子体共振特性的影响

采用离散偶极子近似方法系统地研究了金纳米环双体的消光光谱及其电场分布. 计算结果表明, 金纳米环双体在耦合作用下的共振消光峰对应着不同振动模式, 改变金纳米环双体的排列方式、 间距和尺寸大小, 其表面等离子体共振消光峰发生红移或蓝移. 因此可以通过对金纳米环双体结构参数和排列方式的设定, 调节其表面等离子体共振消光峰的位置. 电场分布表明, 水平排列的金纳米环双体较单个金纳米环产生更强的局部表面增强电场. 适当的小间距, 较大的内外半径的金纳米环水平阵列更适合做表面增强拉曼散射的衬底, 在生物分子检测等领域具有潜在的应用.     

Surface‐enhanced Raman scattering studies of carbon nanotubes using Ag‐core Au‐shell nanoparticles

Surface‐enhanced Raman scattering from carbon nanotube bundles adsorbed with plasmon‐tunable Ag‐core Au‐shell nanoparticles (Ag@Au nps) was carried out for the first time. By utilizing nanoparticles whose plasmon resonance peak (541, 642 nm) closely matches the commonly used Raman excitation sources (532, 632.81 nm), we can observe a large enhancement in the Raman signatures of carbon nanotubes. We obtain greater enhancement in the Raman signal for the above case when compared to nanotubes adsorbed with conventional Ag, Au or other ‘off resonant’ Ag@Au nps. The power‐dependent SERS experiment on single‐walled nanotubes (SWNTs) with resonant Ag@Au nps reveals a linear behavior between the G‐band intensity and the photon flux density, which is in agreement with the vibrational pumping model of SERS. The observed enhancement by resonance matching is pronounced for carbon nanotubes and may lead to insights into understanding nanotube–nanoparticle interaction. Copyright © 2009 John Wiley & Sons, Ltd.     

Optical Tunability of Silver-Dielectric-Silver Multi-Layered Cylindrical Nanotubes Using Quasi-Static Approximation

Local surface plasmon resonances(LSPRs) of silver-dielectric-silver multi-layered(SDS-ML) nanotubes are studied by theoretical calculations. Based on quasi-static approximation, the absorption cross section of SDS-ML nanotubes is plotted as a function of wavelength. The results show that SDS-ML nanotubes exhibit strong coupling between the cylindrical silver and nanotubes. The absorption spectra of LSPRs are strongly influenced by changing the radius of the inner core and outer nanotube shell. The longer wavelength is red-shifted by increasing the radius of the inner core and outer shell, while the short wavelength shows the opposite properties.These phenomena are explained by the plasmon hybridization theory. In addition, for clarity, the distributions of electric field intensity at their plasmon resonance wavelengths are also studied.     

Fano resonances in heterogeneous dimers of silicon and gold nanospheres

We theoretically investigate the optical properties of dimers consisting of a gold nanosphere and a silicon nanosphere. The absorption spectrum of the gold sphere in the dimer can be significantly altered and exhibits a pronounced Fano profile. Analytical Mie theory and numerical simulations show that the Fano profile is induced by constructive and destructive interference between the incident electric field and the electric field of the magnetic dipole mode of the silicon sphere in a narrow wavelength range. The effects of the silicon sphere size, distance between the two spheres, and excitation configuration on the optical responses of the dimers are studied. Our study reveals the coherent feature of the electric fields of magnetic dipole modes in dielectric nanostructures and the strong interactions of the coherent fields with other nanophotonic structures.     

Surface exciton-plasmons and optical response of small-diameter carbon nanotubes

We study theoretically the interactions of excitonic states with surface electromagnetic modes of small-diameter (≲1 nm) semiconducting single-walled carbon nanotubes. We show that these interactions can result in strong exciton-surface-plasmon coupling. The exciton absorption lineshape exhibits the line (Rabi) splitting ∼0.1–0.3 eV as the exciton energy is tuned to the nearest interband surface plasmon resonance of the nanotube so that the mixed strongly coupled surface plasmon-exciton excitations are formed. We discuss possible ways to bring the exciton in resonance with the surface plasmon. The exciton-plasmon Rabi splitting effect we predict here for an individual carbon nanotube is close in its magnitude to that previously reported for hybrid plasmonic nanostructures artificially fabricated of organic semiconductors deposited on metallic films. We expect this effect to open up paths to new tunable optoelectronic device applications of semiconducting carbon nanotubes.     

Bragg scattering induces Fano resonance in photonic crystals

We present a study of a Fano resonance between a narrow Bragg band and disorder-induced continuum in photonic crystals where the continuum is either of the broad band Fabry-Pérot scattering in an imperfect one-dimensional photonic crystal or Mie scattering in an imperfect three-dimensional photonic crystal. Our experimental studies of synthetic opals have demonstrated how the Fano resonance may lead to a transmission spectrum exhibiting a Bragg dip with an asymmetric profile or a Bragg rise.     

Plasmon resonance coupling in strongly coupled gold nanotube arrays with structural defects

We theoretically investigate the transmission spectra and the field distributions with different defects in the gold nanotube arrays by using the finite-difference time-domain method.It is found that the optical properties of the nanotube arrays are strongly influenced by different defects.When there are no defects in the central nanotube,the values of peaks located at both sides of the photonic band gap have their maxima.Based on the distributions of electric field component E x and the total energy distribution of the electric and the magnetic field,we show that mainly a dipole field distribution is exhibited for the plasmon mode at the long-wavelength edge of the band gap but higher order modes of the composite are excited at the short-wavelength edge of the band gap.The plasmon resonant modes can also be controlled by introducing defects.     

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].     

Ag-Al纳米球二聚体的光学性质研究

金属纳米材料因其表面等离子体共振特性而备受关注。异质结构的金属纳米材料的光学特性相比于同质结构因其材料的不同破坏了原有结构的对称性,对称性的破坏将引起光学性质的改变,相邻两个颗粒之间的相互作用会产生Fano共振。Fano共振是由异质纳米结构的表面等离子体共振耦合引起的,通过合理地调控表面等离子体共振的耦合,将进一步调控Fano共振的强度同时促使异质结构的电场增强特性和辐射特性得到进一步优化。受金银等贵金属的带间跃迁影响,金属铝纳米材料成为研究紫外-近紫外光区的表面等离子体共振研究最佳选择。采用有限时域差分方法研究了Ag-Al纳米球二聚体的光学特性。研究了Ag和Al纳米球组成的二聚体的吸收光谱与入射光偏振方向、纳米球半径、颗粒间距和介质折射率等几何结构及物理参数的关系,并深入讨论了二聚体的局域场分布规律;讨论了获取更高效的Fano共振光谱的方法。由于材料的对称性被破坏,异质二聚体的光学性质与同质二聚体明显不同,Ag-Al异质纳米球二聚体呈现出在紫外和可见光区的双Fano共振现象。Ag-Al二聚体表面等离子体互相耦合引起Fano共振从而导致表面等离子体的共振抑制和增强。研究结果对在紫外-可见光区的表面等离子体应用、纳米光学器件的设计与开发及基于表面等离子体共振的表面增强光谱、生物传感和检测研究等有一定参考价值。     

Strong near-field couplings of anapole modes and formation of higher-order electromagnetic modes in stacked all-dielectric nanodisks

We theoretically study the near-field couplings of two stacked all-dielectric nanodisks, where each disk has an electric anapole mode consisting of an electric dipole mode and an electric toroidal dipole (ETD) mode. Strong bonding and anti-bonding hybridizations of the ETD modes of the two disks occur. The bonding hybridized ETD can interfere with the dimer's electric dipole mode and induce a new electric anapole mode. The anti-bonding hybridization of the ETD modes can induce a magnetic toroidal dipole (MTD) response in the disk dimer. The MTD and magnetic dipole resonances of the dimer form a magnetic anapole mode. Thus, two dips associated with the hybridized modes appear on the scattering spectrum of the dimer. Furthermore, the MTD mode is also accompanied by an electric toroidal quadrupole mode. The hybridizations of the ETD and the induced higher-order modes can be adjusted by varying the geometries of the disks. The strong anapole mode couplings and the corresponding rich higher-order mode responses in simple all-dielectric nanostructures can provide new opportunities for nanoscale optical manipulations.