金属光栅对表面等离子体波的辐射抑制研究

对金属光栅进行严格耦合波理论计算, 得到了780和1500 nm波长入射光条件下不同光栅调制深度(20-80 nm)对应的反射谱. 根据Fano理论推导了描述反射谱线的经验公式, 最后应用有限元法计算光栅表面近场电场分布, 验证了公式的正确性. 反射谱线公式反映出光栅耦合表面等离子体的各个物理效应, 其中最重要的是反映出光栅在某一调制深度下对表面等离子体反耦合的抑制作用, 这一发现为设计光栅能量约束器件提供了物理依据.     

Complex Modulation in Plasmon-Assisted Transmission Spectra of a Two-Slit Structure

We study the plasmon-assisted transmission of two kinds of slit structures in a 290-nm-thick silver film on a glass substrate. For the two-slit structure, the total transmission intensity spectra displays a complex modulation, which is attributed to different propagation constants of the surface plasmon polaritons （SPPs） on the silver-air and silver-glass interfaces. Replacing one of the two slits by a shallow corrugation results in a simple sinusoidal intensity modulation because of the only effective SPP excitation and propagation on the silver-air interface. These confirm the role of different SPP propagations and interference in the transmission properties of metal nanoslits.     

周期地嵌入电介质球壳的金属表层的表面等离子激元及其与电介质腔体模式的耦合

通过观察金属底板中周期地嵌入电介质球壳的体系的光学吸收性质，研究了表面等离子激元 以及与其他电磁模式的耦合特性.在这种周期结构的金属表面，发现存在两种响应频率，分 别对应于表面等离子激元模式和金属中的电介质腔体模式.在这些响应频率上，可观察到与 它们相对应的吸收峰.由于金属的表面模式不能与平面入射光直接耦合，而腔体模式与平面 入射光和表面等离子激元模式的耦合一般较弱，因而通常情况下这些吸收峰的峰值有限.然 而，通过调整体系中的某些参数，可以使腔体模式和表面模式的频率非常接近，这时二者之 间的耦合强度将大大提高.此时，在相应的频率附近可观察到极强的吸收峰.详细地研究了介 质球壳的物理和几何参数对此共振吸收的影响. 关键词： 腔体模式 表面等离子体模式 共振吸收     

Long-range propagation of plasmon polaritons in a thin metal film on a one-dimensional photonic crystal surface

We present experimental results on ultralong-range surface plasmon polaritons, propagating in a thin metal film on a one-dimensional (1D) photonic crystal surface over a distance of several millimeters. This propagation length is about 2 orders of magnitude higher than the one in the ordinary Kretschmann configuration at the same optical frequency. We show that a long-range surface plasmon polaritons propagation may take place not only in a (quasi)symmetrical scheme, where a thin metal film is located between two media with (approximately) the same refraction index, but also in a scheme where the thin metal film is located between an appropriate 1D photonic crystal and an arbitrary (air, water, etc.) medium. The ultralong-range surface plasmon polaritons are potentially important for biosensors, plasmonics, and other applications.     

基于类表面等离子体激元的矩形金属光栅色散特性的研究

等离子体激元(surface plasmon polaritons, SPP)因其独特的光学和物理特性, 使其具有诸如透射增强和局域共振等一系列新颖现象, 已成为当前国内外学者研究的热点. 本文对基于类表面等离子体激元(Spoof Surface Plasmons, SSP)的矩形金属光栅色散特性和模式分布进行了研究. 利用本征函数法并结合场匹配条件, 获得了矩形栅表面SSP的场表达式、色散关系和模式分布, 并通过电磁仿真进行了验证. 在此基础上分析了矩形栅各参数对SSP色散及模式分布的影响, 研究结果表明: 由本征函数法获得的SSP色散特性与仿真结果基本符合; 增大金属栅高度或减小排列周期能减小SSP的相速度; 而增大金属栅周期占空比能在一定程度上拓展SSP与电子束互作用的带宽; 改变金属盖板高度对慢波SSP色散模式基本没有影响; 减小金属栅侧面宽度能增大模式之间的间隔, 从而能有效避免模式竞争的发生. 本文对基于SSP的矩形金属光栅色散特性的研究将为进一步研究SSP与电子束的相互作用, 形成高效、宽带的新型太赫兹源奠定良好的理论基础.     

Nonlinear resonance-enhanced excitation of surface plasmon polaritons

We theoretically investigate nonlinear resonance-enhanced excitation of surface plasmon polaritons in a metal coated by a one-dimensional photonic crystal. Tunneling modes above the air-light line can be directly excited in this structure. Then, with suitable parameters, photon energy and momentum conservation between the tunneling mode and the surface plasmon polaritons can be realized by means of nonlinear four-wave mixing. Compared with the nonlinear excitation of surface plasmon polaritons in a bulk metal [Phys. Rev. Lett. 103, 266802 (2009)], the conversion efficiency in our structure is noticeably enhanced.     

嵌入电介质小球的金属薄片的电磁波透射特性

研究了金属板上下两个表面内侧都周期性地嵌入电介质小球体系的透射和吸收性质.结果显示：对于嵌入深度很小的电介质小球,电磁波可通过隧穿效应进入到电介质小球内,并以腔体本征模式的形式存在.当嵌入深度很小时,周期排列的电介质小球会对金属表面做有效的周期性调制,使体系在特定的频率出现金属表面等离子激元.当腔体本征模式与金属表面等离子激元模式的频率相近时,它们之间的耦合将使两种电磁模式大幅度地增强,从而使上层的电介质小球内具有非常强的电磁场.这些高强度的电磁场有相当一部分可通过隧穿效应进入到下层的电介质小球内,并通过 关键词： 腔体模式 表面等离子激元 透射     

Surface plasmon polariton propagation in nanoscale metal gap waveguides

Based on finite-difference time-domain simulation of the propagation characteristics of surface plasmon polaritons (SPPs) in optical circuits made from metal gap waveguides (MGWs) with nanometric gap widths, we theoretically demonstrate that two structures that consist of splitting and recombining MGWs and of coupling MGWs can be used as nanoscale Mach-Zehnder interferometers. MGW arrays show capabilities for array imaging and for controlling the flow of SPPs. Other potential applications of coupling MGWs, as SPP switches, directional couplers, and even as a nanoscale counterpart for observing linear and nonlinear dynamic behavior of electromagnetic fields, are also predicted and discussed. Our results point to an interesting way to manipulate optical signals and provide efficient sensing in nanophotonic architectures.     

Anomalous transmission of terahertz wave through one-dimensional lamellar metallic grating

The anomalous transmission through one-dimensional lamellar metallic gratings was investigated in terahertz (THz) regime. The extraordinary optical transmission (EOT) is identified to originate from two possible ways: coupling of incident light with waveguide resonances and coupling of surface plasmon polaritons (SPPs) at the upper and lower interfaces of metal grating. The dual effects of SPPs have been clarified in this study: (i) the excitation of SPP modes at each individual interface results in the weakness of the THz wave transmission; and (ii) the coupling of SPP modes at two interfaces of metal grating is attributed to enhancement of THz wave transmission. The enhanced transmission is dominated by the coupling of incident light with transverse waveguide resonances. Numerical simulation based on finite-difference time-domain (FDTD) agrees well with experimental results.     

Suppression of radiative losses of surface polaritons on nanostructured thin metal films

The strong electromagnetic coupling between surface plasmon polariton modes on opposite interfaces of a finite thickness periodically nanostructured metal film has been studied. Surface polariton dispersion and associated electromagnetic field distributions have been analyzed. It was shown that at a frequency that corresponds to the crossing of film Bloch modes of different symmetries, the radiative losses of surface polaritons that are related to the polaritons' coupling to light during propagation on the structured surface are suppressed.     

Dispersion relation for surface plasmon polaritons in metal/nonlinear-dielectric/metal slot waveguides

We present the first (to our knowledge) exact dispersion relation for the transverse-magnetic surface plasmon polariton (SPP) modes of a plasmonic slot waveguide, which is formed by a nonlinear Kerr medium sandwiched between two metallic slabs. The obtained relation is then simplified to the case of small field intensities, while retaining nonlinear terms, to derive approximate dispersion equations for the symmetric and antisymmetric SPP modes.     

Experimental evaluation of reflectance of surface plasmon polaritons at metal step barrier

We have proposed a way to evaluate the reflectance of surface plasmon polaritons (SPP) at a Ag step barrier from an experiment using the attenuated total reflection structure consisting of ultra-thin metal films sandwiched by dielectrics with nearly identical refractive index. At a wavelength of 632.8 nm, for a long-range SPP (L-SPP) excited in a Ag film of 5.7 nm in thickness, the reflectance at Ag step barriers of 766 nm in height was evaluated to be 7.9 × 10⁻².     

Effects of shape of terminus on excitation of surface plasmon modes on metal nanowires

The effects of the shape of a nanowire terminus on the excited surface plasmon polariton （SPP） modes are investigated. The conical terminus and terminus cut at a certain angle are studied. For the first time, the quantitative mode decompositions are carried out to derive the full information about excited SPP modes. It is demonstrated that tuning the shape of the terminus provides an effective method to control the composition of excited SPP modes on metal nanowires. It is especially found that some important patterns, such as the pure TM0 mode and the superposition of TM0 and HE＋1 or HE-1 modes, can be generated by some specific shapes of the terminus, whereas there is no way to produce these patterns using flat-end nanowires.     

Surface-plasmon-polariton-assisted dipole-dipole interaction near metal surfaces

We investigate the surface plasmon polariton (SPP)-assisted interaction between two dipoles near a metal surface. The radiation energy from a dipole can excite SPPs and transport to another dipole through the channel of the localized SPP modes. This energy transfer can be much more efficient than direct energy transfer via dipole-dipole radiation interaction in free space. A simple analytical model is proposed to describe the underlying physics behind the influence of SPP on the dipole-dipole interaction energy, and it predicts a wide variety of complicated interaction features that agree well with rigorous calculations.     

Study of interference in a double-slit without walls by plasmon tomography techniques

We investigate the free propagation of two parallel surface plasmon polaritons (SPP) beams using plasmon tomography. In the Fourier-plane images, we observed interference features that are not in correspondence with the images of SPPs on the sample's surface. We clearly observed that the interference maxima and minima are distributed over an arc of a circle. We explain the characteristics of the observed interference patterns assuming that each SPP beam can be considered as a “slit without walls”. We discuss important implications of this work for SPP tomography and interferometric plasmonic sensors.     

Anisotropy‐assisted non‐scattering coherent absorption of surface plasmon‐polaritons

The ability to control propagation of electromagnetic guided modes lies at the heart of integrated nanophotonics. Surface plasmon‐polaritons are a class of guided modes which can be employed in integrated optical systems. Here, we present a theoretical design of a coherent surface plasmon absorber which can perfectly harvest energy of coherently incident surface plasmons without parasitic scattering into free space modes. Excitation of free space modes which usually accompanies scattering of a surface plasmon by an interface boundary is avoided due to specially tailored anisotropy of the absorber. The concept of coherent SPP absorber is analyzed numerically for spatially non‐uniform and finite‐size structures. We believe that our results will be important for the development of integrated nanoplasmonic systems.     

Multiple modes of surface plasmonic polaritons in transversely-truncated metal/dielectric superlattices

The surface plasmonic polariton (SPP) of a transversely-truncated metal/dielectric superlattice (SL) structure has been solved with an approximate method. The effect of inter-layer interfaces in the SL is taken into consideration efficiently in comparison with the effective-medium method. The silver/air and silver/SiO₂ SLs with a shorter period are regarded as two specific examples in numerical calculation. A series of separated SPP modes are found and highly localized at the surface, and the highest-frequency mode is the only one also predicated by the effective-medium method. These results obviously show the effect of inter-layer interfaces in the case of short period, whilst the reliability and limitation of the effective-medium method is presented as well. Because the skin depths of the modes are extremely small, the SLs can be used as ideal surface-wave waveguides.     

Observation of propagation of surface plasmon polaritons along line defects in a periodically corrugated metal surface

Propagation of surface plasmon polaritons (SPPs) excited in the wavelength range 720-830 nm at a corrugated gold-film surface with areas of 150-nm-wide and 45-nm-high scatterers arranged in a 380-nm-period triangular lattice containing line defects is investigated by use of near-field optical microscopy. We demonstrate that the SPP at 740-750 nm propagates along 2.2-microm -wide and 16-microm -long line defects with ~50% loss, whereas its propagation along narrower line defects is strongly damped and in periodically corrugated areas is inhibited. We observe significant deterioration of these effects for both longer and shorter wavelengths and conclude that the SPP guiding occurs as a result of the SPP bandgap effect in the structures.     

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.     

Coupling effect of surface plasmon polaritons in single-negative lamellar heterostructure

Propagation characteristics of surface plasmon polaritons (SPPs) in the lamellar heterostructure, which is actually a SPP waveguide array, constructed by two kinds of single negative (SNG) material layers stacked alternatively are investigated. Based on the finite element method (FEM), the negative-refraction (NR) property is demonstrated when the electromagnetic wave penetrates through free space into such SNG lamellar structure. A clear view of the underlying physics of NR is presented qualitatively that is mainly related to the coupled SPPs. The strong coupling effect leads to the novel SPP dispersion curves and then the anomalous propagation characteristics.