Ultrathin plasmonic metamaterial for spoof localized surface plasmons

The multipolar spoof localized surface plasmons (LSPs) on a planar textured metallic disk are proposed and experimentally demonstrated at microwave frequencies. Based on ultrathin metal film printed on a thin dielectric substrate, the designed plasmonic metamaterial clearly shows multipolar plasmonic resonances, including the dipole, quadrupole, hexapole, octopole, decapole, dodecapole, and quattuordecpole modes. Both numerical simulations and experiments are in good agreement. It is shown that the spoof LSP resonances are sensitive to the disk's geometry and local dielectric environments. Hence, the ultrathin textured metallic disk may be used as plasmonic sensors and find potential applications in the microwave and terahertz frequencies.     

High Sensitivity Transmission-Type SPR Sensor by Using Metallic--Dielectric Mixed Gratings

We theoretically investigate transmission-type SPR sensors with novel metallic-dielectric mixed gratings by rig- orous coupled-wave analysis （RCWA）, compared to the conventional dielectric gratings based structure. It is found that the transmittance efficiency and the full width at half-maximum （FWHM） of the transmission curve can be modulated by increasing or decreasing the metallic part. Therefore, appropriate proportion of metal part will induce enhancement factor of sensor merit. Furthermore, this novel structure will also bring enhancement of resonant angle shift, which can be explained by plasmonie interpretation based on a surface limited increase of interaction area and excitation of localized surface plasmons （LSPs）. The proposed configuration has a wide range of potential applications not only as sensor but also other optical devices.     

Tunable coupling of a hybrid plasmonic waveguide consisting of two identical dielectric cylinders and a silver film

The propagation length of surface plasmon polaritons(SPPs) is intrinsically limited by the metallic ohmic loss that is enhanced by the strongly confined electromagnetic field. In this paper, we propose a new class of hybrid plasmonic waveguides(HPWs) that can support long-range SPP propagation while keeping subwavelength optical field confinement. It is shown that the coupling between the waveguides can be well tuned by simply varying the structural parameters. Compared with conventional HPWs, a larger propagation length as well as a better optical field confinement can be simultaneously realized. The proposed structure with better optical performance can be useful for future photonic device design and optical integration research.     

Optical and magneto–optical properties of periodic Co double layer film

Co double layer film （CoDLF） consisting of a disk-array layer and an antidot-array layer, both with square order, was investigated. Both the reflectivity and Kerr spectra of CoDLF show anisotropic effects when the azimuthal angle of incident light changes. From the simulation result of surface plasmon polaritons （SPPs）, we attribute the reflectivity minima and Kerr angle maxima in the spectra mainly to the excitation of different diffractional orders＇ SPPs. More interestingly, the Kerr angle changes sign at specific wavelengths. We attribute these phenomena to the excitation of SPPs and localized surface plasmons （LSPs）, and the interaction between them.     

Local Surface-Plasmons in Nonspherical Metal Nanoparticles

When a small metallic nanoparticle.is irradiated by incident light, the oscillating electric field can cause the conduction electrons to oscillate coherently, which excites the local surface plasmons （LSPs）. As is well known, excited LSPs can gather the energy of incident light to the surface of metallic nanoparticle. Recently, some nonspherical particles, e.g. tetrahedron, are suggested to obtain stronger localized electric field. We employ the discrete dipole approximation method to calculate the optical response of the tetrahedron nanoparticle, including the extinction and distribution of the electric field around the particle. The influences of some parameters, including the nanoparticle size, incident direction and polarization, are investigated to analyse the response modes and to obtain stronger localized electric field.     

Lasing action assisted by long‐range surface plasmons

It is shown that lasing action at subwavelength scales can be achieved in realistic plasmonic systems supporting long‐range surface plasmons (LRSPPs). To this end, a general numerical framework has been developed that is able to accurately account for the full spatio‐temporal lasing dynamics and the vastly different length‐ and time‐scales featured by this class of systems. Starting from a loss compensation regime for propagating LRSPPs, it is shown how the introduction of an optical feedback mechanism induces the formation of a self‐sustained laser oscillation at moderate pump intensities. The simplicity of the proposed subwavelength scale laser offers significant potential as a novel class of planar light sources in complex plasmonic circuits.     

从光子晶体效应到表面等离子波的实时演变

文章报道了激光诱导太赫兹表面等离子谐振效应。采用激光抽运-太赫兹波探测技术，实时改变单晶硅中的载流子浓度，使其介电特性从类绝缘体演变为类金属导体，以支持表面等离子谐振效应，进而实现太赫兹波在周期性亚波长单晶硅孔阵列中的实时可控制谐振增强传输。同时还通过实验观测到太赫兹波从光子晶体效应到表面等离子波的实时演变。文章作者采用Fano模型对实验结果进行模拟分析，获得了与实验数据一致的理论拟合。     

基于导模共振效应提高石墨烯表面等离子体的局域特性

本文构建了一种包含石墨烯和亚波长光栅的复合结构, 借助衍射光栅的导模共振效应, 在石墨烯表面激发高局域性表面等离子体激元, 研究了石墨烯与光栅结构对表面等离子体激元局域特性的影响规律, 并借助基于有限元法的COMSOL软件, 分析了缓冲层厚度、光栅周期、载流子迁移率和费米能级对石墨烯的表面电场、品质因子Q和有效模式面积S_eff的影响. 结果表明, 石墨烯表面等离子体激元的局域性在特定的参数点获得显著提高: 当μ = 0.7 m²/(V·s)时, 品质因子达到最大值Q_max = 1793; 当p = 235 nm或E_F = 0.72 eV时, 表面电场达到了入射光的3000倍以上. 强烈的局域性导致强烈 的光-物质相互作用, 因而本文提出的复合结构可实现高灵敏度传感器和高效率的非线性光学设备, 极大地扩展了石墨烯在纳米光学领域中的应用.     

A Compact Optical Switch via Plasmonics of Subwavelength Circular‐Sharp Hole Arrays in Metal Films

We studied numerically the enhanced optical transmission (EOT) through periodic subwavelength circular‐sharp hole arrays in metallic films with different edge sharp distribution features of unit structures. Detailed studies indicate that the unit structure edge sharp distribution features strongly influence the surface plasmons (SPs). These results demonstrate that the number of edge sharp activated the localized surface plamons (LSP) resonance on the unit structure is changed by rotating the polarization of the incident light, leading to change the infrared transmittance of the array. Moreover, a compact plasmonic switch via periodic circular‐sharp hole arrays based on the dependence of SPs on unit structural edge sharp distributions is proposed. The finding provides a new idea for designing plasmonics devices, and expands the application range of metal micro‐nano structure in the field of optical communications and information processing.     

衍射极限尺度下的亚波长电磁学

作为波的本性之一,衍射是现代物理学的重要研究内容.衍射导致自由空间中波的能量不能被无限小地聚集,从而为成像、光刻、光存储、光波导等技术设定了一个原理性的障碍——衍射极限.对于电磁波和光波而言,尽管通过提高介质的折射率可以压缩衍射效应,但由于自然界中材料的折射率有限,该方法存在很大限制.近年来,随着表面等离子体光学的兴起,表面等离子体在超越传统衍射极限方面的能力和应用前景受到了学术界的关注.本文从亚波长电磁学的角度出发,介绍衍射极限研究的历史,综述了突破衍射极限的理论方法.首先,利用金属介质表面等离子体激元的短波长特性,可将等效波长压缩一个数量级以上,在纳米尺度实现光波的聚焦或定向传输;更进一步,通过人为设计超构材料和超构表面,利用结构化金属和介质中的局域谐振、耦合等特殊电磁响应,可实现亚波长局域相位调制、超宽带色散调控、近完美吸收、光子自旋轨道耦合等,从而突破传统理论的诸多局限,为下一代电磁学和光学功能器件奠定重要基础.     

Beam focusing from double subwavelength metallic slits filled with nonlinear material surrounded by dielectric surface gratings

Based on the radiation properties of surface plasmon polaritons (SPPs) can be controlled by adjusting the refractive indexes of dielectric materials in the metallic slits, a novel plasmonic focusing structure formed by two subwavelength metal apertures filled with Kerr nonlinear material surrounded by surface dielectric gratings is proposed and demonstrated numerically. Directions of radiation fields are determined by the phase difference of the surface waves at the exit interface and resonance property of each surface grating. Numerical simulations using two-dimensional (2D) Finite-Difference Time-Domain (FDTD) method verify that the deflection angle and focal length can be controlled easily by changing the intensity of incident light, dynamically tunable on-axis and off-axis focusing effects can be achieved.     

Surface plasmon polariton waveguides with subwavelength confinement

Surface plasmon polaritons(SPPs) are evanescent waves propagating along metal-dielectric interfaces, which provide an effective way to realize optical wave guiding with subwavelength confinement. Metallic nanostructures supporting SPPs,that is, plasmonic waveguides, are considered as required components to construct nanophotonic devices and circuits with a high degree of miniaturization and integration. In this paper, various types of plasmonic waveguides operating in the visible, infrared, and terahertz regions are reviewed, and the status of the research on their fundamentals, fabrications,and applications is provided as well. First, we discuss the mechanisms of SPPs beyond the diffraction limit, and their launching methods. Then, the characteristics of SPPs on various plasmonic waveguides are reviewed, including top-down and bottom-up fabricated types. Considering applications, certain prototypes of plasmonic devices and circuits constructed by plasmonic waveguides for bio/chemo sensing, router, and light modulation are demonstrated. Finally, a summary and future outlook of plasmonic waveguides are given.     

金属微结构纳米线中等离激元传播和分光特性

本文从理论和实验两方面探讨了具有微结构的金属纳米线系统中表面等离激元传播规律和分光特性. 我们由麦克斯韦方程组出发, 利用严格耦合波近似和有限元差分等方法首先从理论上给出了金属纳米线系统中等离激元的色散关系和能带特征, 然后基于微结构的银纳米线及其等离激元能带结构, 设计并制备出等离激元分光原型器件, 实验展示其将不同频率的光在微小空间分离的特性. 该研究结果是我们前期相关工作的延续和补充, 可应用于构造多功能集成的光子芯片和新型亚波长光电材料和器件.     

Broadly tunable one-way terahertz plasmonic waveguide based on nonreciprocal surface magneto plasmons

One-way-propagating broadly tunable terahertz plasmonic waveguide at a subwavelength scale is proposed based on a metal-dielectric-semiconductor structure. Unlike other one-way plasmonic devices that are based on interference effects of surface plasmons, the proposed one-way device is based on nonreciprocal surface magneto plasmons under an external magnetic field. Theoretical and simulation results demonstrate that the one-way-propagating frequency band can be broadly tuned by the external magnetic fields. The proposed concept can be used to realize various high performance tunable plasmonic devices such as isolators, switches and splitters for ultracompact integrated plasmonic circuits.     

Subwavelength beam manipulation via multiple-metal slits coupled by disk-shaped nanocavity

A novel plasmonic structure consisting of three nano-scaled slits coupled by nano-disk-shaped nanocavities is pro- posed to produce subwavelength focusing and beam bending at optical frequencies. The incident light passes through the metal slits in the form of surface plasmon polaritons （SPPs） ,and then scatters into radiation fields. Numerical simulations using finite-difference time-domain （FDTD） method show that the transmitted fields through the design example can gener- ate light focusing and deflection by altering the refractive index of the coupled nanocavity. The simulation results indicate that the focal spot is beyond the diffraction limit. Light impinges on the surface at an angle to the optical axis will add an extra planar phase front that interferes with the asymmetric phase front of the plasmonic lens, leading to a larger bending angle off the axial direction. The advantages of the proposed plasmonic lens are smaller device size and ease of fabrication. Such geometries offer the potential to be controlled by using nano-positior!i0g systems for applications in dynamic beam shaping and scanning on the nanoscale.     

Selective excitation of multipolar surface plasmon in a graphene-coated dielectric particle by Laguerre Gaussian beam

Localized surface plasmonic resonance has attracted extensive attention since it allows for great enhancement of local field intensity on the nanoparticle surface. In this paper, we make a systematic study on the excitation of localized surface plasmons of a graphene coated dielectric particle. Theoretical results show that both the intensity and frequency of the plasmonic resonant peak can be tuned effectively through modifying the graphene layer. Furthermore, high order localized surface plasmons could be excited and tuned selectively by the Laguerre Gaussian beam, which is induced by the optical angular orbital momentum transfer through the mutual interaction between the particle and the helical wavefront.Moreover, the profiles of the multipolar localized surface plasmons are illustrated in detail. The study provides rich potential applications in the plasmonic devices and the wavefront engineering nano-optics.     

Broadband amplification of spoof surface plasmon polaritons at microwave frequencies

Efficient amplification of spoof surface plasmon polaritons (SPPs) is proposed at microwave frequencies by using a subwavelength‐scale amplifier. For this purpose, a special plasmonic waveguide composed of two ultrathin corrugated metallic strips on top and bottom surfaces of a dielectric substrate with mirror symmetry is presented, which is easy to integrate with the amplifier. It is shown that spoof SPPs are able to propagate on the plasmonic waveguide in broadband with low loss and strong subwavelength effect. By loading a low‐noise amplifier chip produced by the semiconductor technology, the first experiment is demonstrated to amplify spoof SPPs at microwave frequencies (from 6 to 20GHz) with high gain (around 20dB), which can be directly used as a SPP amplifier device. The features of strong field confinement, high efficiency, broadband operation, and significant amplification of the spoof SPPs may advance a big step towards other active SPP components and integrated circuits.

     

Extraordinary terahertz transmission through subwavelength spindle-like apertures in NbN film

We studied numerically the temperature dependent extraordinary terahertz transmission through niobium nitride(NbN) film perforated with subwavelength spindle-like apertures. Both the resonant frequency and intensity of extraordinary terahertz transmission peaks can be greatly modified by the transition of NbN film from the normal state to the superconducting state. An enhancement of the(±1, 0) NbN/magnesium oxide(MgO) peak intensity as high as 200% is demonstrated due to the combined contribution of both the superconducting transition and the excitation of localized surface plasmons(LSPs) around the apertures. The extraordinary terahertz transmission through spindle-like hole arrays patterned on the NbN film can pave the way for us to explore novel active tuning devices.     

Plasmonic planar lens based on stack of metal–insulator–metal waveguides with height tuning

We introduce a technique capable of focusing electromagnetic (EM) waves through plasmonic nanoslits symmetrically arranged along the indented semi-circular surface in silver background. The EM transports through the tuning slits in the form of surface plasmon polaritons (SPPs), and gets the required phase retardations to focus at the focal plane. Due to the subwavelength nature of planar metallic lens, we present the rigorous electromagnetic analysis by using two dimensional (2D) finite difference time domain (FDTD) method. These height-modulated slits with uniform width are demonstrated to have unique advantages in beam manipulation. In combination with previous studies, it is expected that our structure with small number of slits could lead to realization of optimum designs of plasmonic nano-lens.     

Plasmonic metasurfaces for waveguiding and field enhancement

The explosive progress in nanoscience has led to uncovering and exploring numerous physical phenomena occurring at nanoscale, especially when metal nanostructures are involved so that optical fields and electronic oscillations can be resonantly coupled. The latter is the subject of (nano) plasmonics with implications extending from subwavelength waveguiding to localized field enhancements. In this review paper, we consider making use of various phenomena related to multiple scattering of surface plasmons (SPs) at periodically and randomly (nano) structured metal surfaces. After reviewing the SP waveguiding along channels in nanostructured areas exhibiting band‐gap and localization effects, SP‐driven field enhancement in random structures and plasmonic fractal drums is discussed in detail. SP manipulation and waveguiding using periodic nanostructures on the long‐wavelength side of the band gap is also considered.