Controlling the interaction between light and gold nanoparticles: selective suppression of extinction

The interaction of visible light with the particle-plasmon resonance of metallic nanoparticles can be controlled by geometrical arrangement of nanoparticle arrays. These arrays are placed on a substrate that supports guided modes in the wavelength range of the particle plasmon. Coupling of this particle-plasmon resonance to the directly incident light and to the waveguide modes results in almost complete suppression of light extinction within narrow spectral bands due to destructive interference. Variation of the structure parameters allows continuous tuning of these high-transmission bands across the particle-plasmon resonance.     

Evaluation of gold nanowire pairs as a potential negative index material

A metamaterial consisting of pairs of metallic nanowires, separated by a dielectric spacer, has been fabricated and spectrally characterized in the visible and near-infrared spectral domain. It is shown, that the structure exhibits both a plasmonic and a magnetic resonance depending on its geometry and orientation with respect to the illuminating wave field. In particular, we investigate the influence of the thickness of the spacer layer on the spectral position of the resonances and show that, for an appropriate adjustment, both resonances coincide. Measurements of the amplitude and the phase of the transmitted wave are presented. It is also shown that the material is highly anisotropic with respect to the angle of incidence, as the plasmonic resonance wavelength depends strongly on it whereas the magnetic resonance does not show this sensitivity. All experimental results are supported by numerical simulations. PACS 81.07.-b; 78.67.-n; 78.20.-Ci     

碳纳米管光学天线的有效波长和谐振特性

将经典金属自由电子气模型应用于金属型碳纳米管, 基于光学天线有效波长理论, 得出了金属型碳纳米管光学天线响应的有效波长与碳纳米管介电特性之间的普适关系. 在对碳纳米管介电特性进行第一性原理计算的基础上, 以金属型4 Å碳纳米管为例, 进一步研究了金属型碳纳米管光学天线响应的有效波长与入射波长之间的关系, 以及金属型碳纳米管光学偶极子天线的谐振特性. 通过将已有传统金属光学天线和碳纳米管天线有效波长的研究结果进行对比, 验证了本文理论的正确性. 结果表明, 碳纳米管光学天线响应的有效波长与入射波长呈近似线性关系, 与传统金属材料构成的同直径光学天线相比, 碳纳米管天线显示出了更强的波长压缩能力, 并且在可见光到红外波段内易于发生谐振. 该研究方法可为碳纳米管光学天线研究提供新的思路.     

Linear and nonlinear properties for a dilute suspension of coated ellipsoids

The effects of geometric structures of coated particles on the dielectric properties of composites are derived. For a dilute suspension of coated ellipsoids with dielectric core and metallic shell embedded in a linear dielectric host, we find that the optical absorption peak of maximum wavelength shifts due to the core-shell structure, and has a significant red shift from that of pure metallic particles, especially for thin metallic coating. Meanwhile, the shape of coated ellipsoid can tune the absorption peak in a wide frequency range by properly choosing the depolarization factor. When the composite is made of linear materials, we obtain the condition of partial resonance for coated ellipsoids, which is relative to the particle shape. Under such a condition, the property of the inner core can be extended to the outer shell. When the inner core is a weakly nonlinear material, the nonlinearity of the composite can be greatly enhanced at the linear partial resonance. To achieve the condition of partial resonance, a metallic shell may be suitable to reach the case if the frequency is far away from the plasmon frequency. It indicates that the nonlinearity can also be enhanced in terms of the geometric structure of materials as well as the properties of themselves.     

非局域颗粒复合介质的相干完美吸收效应

研究了两束相干光以相同的入射角从左、右两侧分别入射到Au-SiO_2复合介质板时,在不同的体系参数下该复合材料体系发生相干完美吸收的情形.运用有效媒质理论推导出了复合介质的有效介电常数以及有效磁导率;在得到有效电磁参数的基础上进一步推导得到平面波入射复合介质板时的反/透射系数.通过比较分析非局域和局域情况下颗粒复合介质的相干完美吸收现象,发现当颗粒尺寸很小时非局域效应的影响会导致复合介质产生相干完美吸收的入射光的频率范围显著变宽.在进一步的解析计算中,通过调节复合介质板的厚度、入射光波长、金属颗粒体积分数等参数得到了不同情况下产生的相干完美吸收现象,并由此分析非局域情形下对于相干完美吸收现象的调控.     

级联亚波长平面介质栅偏振分束器研究

提出了一种纳米厚度金属层连接的级联亚波长平面介质光栅三明治结构,利用该结构中纳米金属连接层上下界面处的表面等离子共振的互作用形成多波长共振的特点设计了适用于光通信波段（1 300～1 600nm）高性能的宽带偏振器和宽带偏振分束器。利用严格耦合波理论分析发现偏振分束器带宽主要由金属连接层厚度控制,光栅厚度基本不影响偏振分束器带宽,只影响共振深度和共振波长。该结论为后续的制备提供了理论指导和借鉴作用。     

Guided mode resonance in subwavelength metallodielectric free-standing grating for bandpass filtering

We present the experimental study of a free-standing metallic guided-mode resonant structure, for bandpass filtering applications in the mid-IR wavelength range. Structure consists of a subwavelength gold grating with narrow slits deposited on a silicon nitride membrane. High optical transmission is measured with up to 78% transmission at resonance. Angularly resolved spectra are presented revealing Fano-type resonance.     

Dielectric layer-dependent surface plasmon effect of metallic nanoparticles on silicon substrate

The electromagnetic interaction between Ag nanoparticles on the top of the Si substrate and the incident light has been studied by numerical simulations. It is found that the presence of dielectric layers with different thicknesses leads to the varied resonance wavelength and scattering cross section and consequently the shifted photocurrent response for all wavelengths. These different behaviours are determined by whether the dielectric layer is beyond the domain where the elcetric field of metallic plasmons takes effect, combined with the effect of geometrical optics. It is revealed that for particles of a certain size, an appropriate dielectric thickness is desirable to achieve the best absorption. For a certain thickness of spacer, an appropriate granular size is also desirable. These observations have substantial applications for the optimization of surface plasmon enhanced silicon solar cells.     

金微纳阵列表面等离激元中红外波段光谱特性

表面等离激元是金属表面自由电子在入射光激发下产生的电子集体振荡行为及其相应的电磁场分布。目前，金微纳颗粒表面等离激元除了在可见光波段内被大量研究和应用外，在中红外波段内也显示出独特的光谱特性，具备设计生产优良传感器的潜力，因而同样备受瞩目。研究表明，在中红外波段内设计表面等离激元传感器的关键问题在于如何有效地调节共振谱的共振波长、峰值吸收率以及半峰宽等主要特征参数。相比于单个微纳颗粒而言，阵列结构由于拥有良好的周期性，从而能够在上述参数的宽光谱调节方面具有独特的优势。基于此，提出一种基于金微纳颗粒组成的阵列结构，利用时域有限差分方法，在4～18 μm波段范围内，通过分别改变该阵列的结构参数，包括颗粒半径、高度、间距及颗粒形状等，系统地研究了该微纳阵列结构在中红外波段对入射光的反射光谱、透射光谱和吸收光谱等特性的影响。研究发现，在8～10 μm光谱内，入射光能够与其所激发的金微纳阵列表面等离激元产生共振效应，表现出明显的共振峰特性。可以通过分别改变上述结构参数来有效调节吸收率谱线共振峰的共振波长、峰值吸收率和半峰宽等主要特征参数。研究结果对中红外光谱内基于金微纳阵列结构传感器的科学研究和实际设计具有独特的理论应用价值。     

Tunable multi-band terahertz absorber based on graphene nano-ribbon metamaterial

The tunable multi-band selective absorption effect with graphene nano-ribbon metamaterial is investigated. It is achieved by depositing a set of graphene nano-ribbons on a dielectric spacer backed with a metallic mirror. A dual-band and tri-band absorbers are designed to illustrate such multi-band selective absorption behavior. The designed graphene absorbers exhibit near-unity absorption at multiple resonance frequencies, which is attributed to the plasmonic resonance of graphene nano-ribbons with different widths. Moreover, the multiple resonance wavelengths can be tuned in a wide frequency range by changing the Fermi energy and the absorbers possess large angle insensitivity. Last, a Fabry-Perot model is employed to give a physical understanding of such multi-band selective absorption effect. It is believed that the conclusions may be useful for designing of next-generation graphene-based optoelectronic devices.     

Figure-of-merit enhancement of surface plasmon resonance sensors in the spectral interrogation

We show that adding a thin dielectric layer with high refractive index on top of the metallic layer in surface plasmon resonance sensors in the Kretschmann-Raether configuration in the spectral mode causes a redshift of the resonance wavelength, narrowing of the resonance dip, and an enhancement to the spectral sensitivity. Surprisingly, together with the sensitivity enhancement, the dip becomes much narrower and the figure of merit is considerably improved, particularly in the IR range.     

共享孔径人工电磁媒质设计及其在高增益低雷达散射截面天线中的应用

提出了一种具有部分反射特性和吸波特性的共享孔径人工电磁媒质(shared aperture metamaterial, SA-MTM).该媒质由上层斜十字金属图案加载集总电阻的吸波表面、下层开条带缝隙金属面的部分反射表面以及中间介质层构成, 吸波表面和部分反射表面在垂直维度上共享了一个物理孔径使该媒质同时实现了吸波特性和部分反射特性.将SA-MTM与天线一体化设计, 利用SA-MTM的部分反射表面和天线表面构成的法布里-珀罗(Fabry-Perot, F-P)谐振腔提升天线的增益, 利用SA-MTM的吸波表面吸收入射电磁波实现低雷达散射截面(radar section cross, RCS)天线的设计.仿真和实验结果表明, SA-MTM 的加载使天线的前向增益在5.57–5.94 GHz 的工作带宽范围内都提升了3 dB以上, 且天线的后向RCS在2–9 GHz范围内都有明显的减缩.该研究成果克服了天线辐射性能和散射性能无法兼顾的矛盾, 对高增益低RCS天线的设计具有重要的指导意义.     

Transmission of light through metallic Z-shaped slit array with inner metallic bar

The transmission of a normally incident wave through a Z-shaped channel metallic slit array with metallic bar inside has been investigated by using finite-difference time-domain method. It is obtained that transmission spectra are nearly the same of the slit array with straight channel as that with Z-shaped channel in the condition the material of the slit array the same as that of the inner bar. If the Au bar is replaced by Al of the Au slit array, both resonance modes red shift obviously, especially for the structure with bent channel. Along with the width of the inner bar increasing, the localized waveguide resonance mode red shifts regularly with a tiny decrease of the peak value of all the kinds of composed structure introduced here, and the surface plasmon resonance mode red shifts regularly accompanied by peak value changing uniformly for the structures with only one type of metal. However, the surface plasmon resonance behaves different obviously, its center peak first moves to a larger wavelength fast, then red shifts slowly, for the Au-film Al-bar structure, but that moves in a very small wavelength range for the Al-film Au-bar one. The results obtained here are helpful to design subwavelength optical devices.     

Large negative Goos-Hänchen shift from a weakly absorbing dielectric slab

It is theoretically shown that the negative Goos-H?nchen shifts near resonance, Re[k(z)d] = m pi, can be an order of magnitude larger than the wavelength for both TE- and TM-polarized beams reflected from a weakly absorbing dielectric slab if the absorption of the slab is sufficiently weak, which is different from the case for a lossless dielectric slab [Phys. Rev. Lett. 91, 133903 (2003)].     

Evaluation of single virus detection through optical biosensor based on microsphere resonator

Shift of resonance frequency in microsphere optical resonator due to attachment of a desirable particle is obtained. Our 3-D finite element numerical method (FEM) simulations’ results show the path of light through microsphere and its variation due to attachment of particle. It is apparent that after attachment of particle to microsphere's surface, light is inclined to pass through the particle. Subsequently, the path of light becomes longer than previous. Because of this phenomenon, the resonance wavelength shifts to longer wavelengths. It is shown that microsphere optical resonator is a prominent biosensor for single virus detection since we applied characteristics of virus for particle in our simulations. Response of this biosensor depends on the characteristics of particle like its radius as we show in this article. Transmission spectrum of fiber which reveals a selected resonance frequency, have been studied in the frequency range of 106.3 to 107 THz under three different sizes of particles. The results show that the amount of frequency shift rises by enhancement of particle's size.     

光学共振增强的表面铯激活银纳米结构光阴极

金属光阴极因其超短脉冲发射和运行寿命长的特性从而具有重要应用价值,但是较高的功函数和较强的电子散射使其需要采用高能量紫外光子激发且光电发射量子效率极低.本文利用Mie散射共振效应增强银纳米颗粒中的局域光学态密度,提升光吸收率和电子的输运效率,并利用激活层降低银的功函数,从而增强光阴极在可见光区的量子效率.采用时域有限差分方法分析银纳米球阵列的光学共振特性,采用磁控溅射和退火工艺在银/氧化锡铟复合衬底上制备银纳米球,紧接着在其表面沉积制备铯激活层,最后在高真空腔体中测试光电发射量子效率.实验结果表明平均粒径150 nm的银纳米球光阴极在425 nm波长的量子效率超过0.35%,为相同激活条件下银薄膜光阴极的12倍,峰值波长与理论计算的Mie共振波长相符合.     

Optical characteristics of silver-doped polarizing glass

According to the theory of scattering and absorption of light, an anisotropic Ag colloid particle, which is small compared with the wavelength of the incident light, absorbs light differently in different directions. The underlying mechanism is regarded as electron plasma resonance. The optical anisotropy of a single Ag particle or of a group of the same particles has been analyzed. An equation for estimating the distribution of the particle shapes in space is proposed. Based on this equation and the work of other researchers, a program was designed to calculate the major principal transmittance and the minor principal transmittance of Ag-doped polarizing glass. The results show that the polarizing property of glass with particles with different aspect ratios is better than that of glass with identical particles. Also, the effective wavelength range is different if the particles' aspect ratios change, and this range tends to narrow as the parameter R (the rate of change of the aspect ratio) is increased.     

Transmission through array of subwavelength metallic slits curved with a single step or mutli-step

The transmission of normally incident plane wave through an array of subwavelength metallic slits curved with a sin- gle step or mutli-step has been explored theoretically. The transmission spectrum is simulated by using the finite-difference time-domain method. The influences of surface plasmon polaritons make the end of finite long sub-wavelength metallic slit behaves as magnetic-reflecting barrier. The electromagnetic fields in the subwavelength metallic slits are the superpo- sition of standing wave and traveling wave. The standing electromagnetic oscillation behaves like LC oscillating circuit to decide the resonance wavelength. Therefore, the parameters of adding step may change the LC circuit and influence the transmission wavelength. A new explanation model is proposed in which the resonant wavelength is decided by four factors： the changed length for electric field, the changed length for magnetic field, the effective coefficient of capacitance, and the effective coefficient of inductance. The effect of adding step is presented to analyze the interaction of two steps in slit with mutli-step. This explanation model has been proved by the transmission through arrayed subwavelength metallic slits curved with two steps and fractal steps. All calculated results are well explained by our proposed model.     

Unidirectional and wavelength-selective photonic sphere-array nanoantennas

We design a photonic sphere-array nanoantenna (NA) exhibiting both strong directionality and wavelength selectivity. Although the geometric configuration of the photonic NA resembles a plasmonic Yagi-Uda NA, it has different working principles and, most importantly, reduces the inherent metallic loss from plasmonic elements. For any selected optical wavelength, a sharp Fano resonance by the reflector is tunable to overlap spectrally with a wider dipole resonance by the sphere-chain director, leading to high directionality. This Letter provides design principles for directional and selective photonic NAs, which are particularly useful for photon detection and spontaneous emission manipulation.     

Radiation pressure over dielectric and metallic nanocylinders on surfaces: polarization dependence and plasmon resonance conditions

Multiple scattering calculations of the electromagnetic force and the potential energy exerted by an evanescent field on a nanometric cylinder over a dielectric interface, as well as by a propagating Gaussian beam, are carried out. These calculations constitute a model that describes the gradient, scattering, and absorption components of the force in an elongated particle. The attractive or repulsive nature of the force is strongly dependent on the polarization of the incident field for a metallic particle, whereas a dielectric particle is only weakly attracted to high-intensity regions. Excitation of plasmon resonance in a metallic particle enhances both the scattering and the absorption components of the force, whereas it diminishes the gradient-force component.