Bimetallic Silver–Gold Film Waveguide Surface Plasmon Resonance Sensor

Abstract

It is desirable that a surface plasmon resonance (SPR) sensor is highly sensitive to binding interactions within the sensing region, generate evanescent fields with long penetration depths, and utilize a metal film that is very stable even in extreme environmental conditions. In this study, we present the first example of a wavelength-modulated waveguide SPR sensor with a bimetallic silver–gold film for surface plasmon excitation. The underlying silver yields better evanescent field enhancement of the sensing surface, while the overlying gold ensures that the stability of the metallic film is not compromised. It is shown experimentally that in terms of dλ/dn, the bimetallic film waveguide SPR configuration has a sensitivity of 1232 nm/RIU, greater than two times improvement from the 594 nm/RIU achievable with single gold film waveguide SPR sensor. The higher sensitivity, compact nature, and better evanescent field enhancement of this configuration provides the potential to biosensing applications.     

Influence of Quantum Effects on the Magnetic Field Behavior in Overdense Plasma

We study the conditions for the anomalous transmission of electromagnetic waves through quantum overdense plasma. We show that this anomalous transmission is triggered due to the excitation of surface waves, as was observed in the classical overdense plasma. The conditions for the excitation of surface waves are obtained by studying the dispersion relation within the framework of quantum hydrodynamics. The corresponding consequences at the classical limits are consistent with the previous studies. In comparison with the classical regimes, the quantum dispersion curve exhibits an asymptotic behavior which indicates significant effects, in particular, at large wavelengths. Herein, to create the required evanescent waves, we consider the quantum plasma to be placed between two ordinary prisms and dielectrics. The effects of the main parameters, such as the permittivity of the prisms and dielectrics and the Fermi velocity, on the rate of the transmission and the magnetic field propagation are also evaluated.     

Bimetallic Silver-Gold Film Waveguide Surface Plasmon Resonance Sensor

It is desirable that a surface plasmon resonance (SPR) sensor is highly sensitive to binding interactions within the sensing region, generate evanescent fields with long penetration depths, and utilize a metal film that is very stable even in extreme environmental conditions. In this study, we present the first example of a wavelength-modulated waveguide SPR sensor with a bimetallic silver-gold film for surface plasmon excitation. The underlying silver yields better evanescent field enhancement of the sensing surface, while the overlying gold ensures that the stability of the metallic film is not compromised. It is shown experimentally that in terms of dλ/dn, the bimetallic film waveguide SPR configuration has a sensitivity of 1232 nm/RIU, greater than two times improvement from the 594 nm/RIU achievable with single gold film waveguide SPR sensor. The higher sensitivity, compact nature, and better evanescent field enhancement of this configuration provides the potential to biosensing applications.     

Subwavelength imaging enhancement through a three-dimensional plasmon superlens with rough surface

In this Letter we investigate the subwavelength imaging of a three-dimensional plasmon superlens based on the full vector wave simulations of optical wave propagation and transmission. The optical transfer functions are computed. Comparisons are made between the results of lenses with flat and periodic/random rough surfaces. We also study the problem of practical imaging system geometry using laser as an illumination source. Results show that the lens with periodic or random roughness can reduce the field interference effects, and provide improved focus on the transmission field and the Poynting flux. We illustrate that the subwavelength roughness in a plasmon lens can enhance the image resolution over a flat lens for both matched and unmatched permittivity conditions. The enhancement of resolution occurs because the introduced subwavelength roughness can amplify the evanescent wave components and suppress the surface plasmon resonance peaks.     

Nanosphere Templated Metallic Grating Assisted Enhanced Fluorescence

In this paper, enhanced fluorescence from a silver film coated nanosphere templated grating is presented. Initially, numerical simulation was performed to determine the plasmon resonance wavelength by varying the thickness of the silver film on top of a monolayer of 400 nm nanospheres. The simulation results are verified experimentally and tested for enhancing fluorescence from fluorescein isothiocyanate whose excitation wavelength closely matches with the plasmon resonance wavelength of the substrate with 100 nm silver film over nanosphere. The 12 times enhancement in the intensity is attributed to the local field enhancement in addition to the excitation of surface plasmon polaritons along the surface.     

All-Optical Modulation with a Surface Plasmon Mach-Zehnder Interferometer

We use two parallel nano-slits in a silver film to form a surface plasmon Mach Zehnder interferometer （MZI）, based on the interference of two surface plasmon waves propagating along the two surfaces of the silver film. Coating the silver film with a photoinduced birefringence polymer film, we achieve optical modulation of the MZI output by changing the refractive index of the polymer film with a pump beam. An on/off ratio 0f 2. 7 is obtained for a probe wavelength of 865 nm.     

Fresnel coefficients for a layer of NIM

Reflection and transmission of traveling and evanescent waves by a layer of material with a negative index of refraction (NIM) is studied by means of the Fresnel coefficients. We derive their values in the “NIM limit”, and we show that this limit is consistent with the exact solution. It is also indicated that simply substituting the negative values of the relative permittivity and permeability of the NIM material into the exact solution leads to incorrect results for evanescent waves.     

Transmission mode of one-dimensional phononic crystal based on coupling of total evanescent waves

In order to obtain the transmission properties of one-dimensional phononic crystal under total evanescent waves, we design structure model. Basing on the basic acoustic wave equations and boundary condition as well as the Bloch theory, we study the band structure of one-dimensional phononic crystal. We summarize the properties of the mode based on the coupling of total evanescent waves and explain its physical mechanism. There are three transmission modes in phononic crystal. Based on the coupling of total evanescent waves, the number of perfect transmission peaks is just equal to the number of structure period, and the thickness of period can be much less than the wave length.     

Electromagnetic energy transport in a chain waveguide of silver nanoshell cylinder array

Abstract

Plasmonic wave propagation in a chain waveguide created by a triangular array of silver nanoshell cylinder was investigated using finite element method and Finite Difference Time Domain Method. It is observed that the trimer nanoshell cylinder array provides better field propagation compared to single-chain and pair-chain nanoshell cylinder arrays. Results show that the resonant wavelength of the structure is highly sensitive to the permittivity of either the core or the surrounding medium. It is also observed that as the core thickness of the waveguide is increased surface plasmon resonance shows a red shift.     

Control of the distribution of surface plasmon local field by altering the surrounding material

The field distribution of the surface plasmon (SP) around a silver nanowire greatly influences its applications for both spectrum enhancement and nano-waveguide. This paper demonstrates the change of the SP local field distribution of a silver nanowire on glass substrate with the refractive index of the covering medium by numerical simulation. The hot energy site is observed focusing on the interface between the nanowire and the surround material of larger refractive index. Moreover, we also find that the surface plasmon field becomes more confined as the refractive index between the substrate and medium has a larger gap, while a homogeneous distribution of the surrounding dielectric material around the nanowire shows a uniform local field distribution with a larger surface plasmon propagation length. The change of related parameters, such as effective index, mode area, and propagation length are also comprehensively investigated against the refractive index for nanowires with 100, 200, 300 nm diameter. Considering the wide use of polymethyl methacrylate (PMMA) material, the field distribution of silver nanowire partially imbedded in PMMA against the PMMA thickness is also studied.     

Fluorescent silver nanoparticles via exploding wire technique

Aqueous solution containing spherical silver nanoparticles of 20–80 nm size have been generated using a newly developed novel electro-exploding wire (EEW) technique where thin silver wires have been exploded in double distilled water. Structural properties of the resulted nanoparticles have been studied by means of X-ray diffractometer (XRD) and transmission electron microscopy (TEM). The absorption spectrum of the aqueous solution of silver nanoparticles showed the appearance of a broad surface plasmon resonance (SPR) peak centered at a wavelength of 390 nm. The theoretically generated SPR peak seems to be in good agreement with the experimental one. Strong green fluorescence emission was observed from the water-suspended silver nanoparticles excited with light of wavelengths 340, 360 and 390 nm. The fluorescence of silver nanoparticles could be due to the excitation of the surface plasmon coherent electronic motion with the small size effect and the surface effect considerations     

基于表面等离子激元的超薄金属减色滤波器的研究

基于表面等离子激元的新型超薄金属减色滤波器具有可靠、易制备、高透射率等诸多优点.本文对银、铜、铝、镍等不同金属材料的超薄一维光栅减色滤波器进行了系统研究.仿真获得了透射光谱,理论计算了对应透射光谱的色品坐标,并推导出了银、铜、铝三种材料透射率最低点波长与膜厚、周期关系的经验公式.仿真研究发现:除了镍的选波性较差外,银、铜、铝均表现出较好的、不同的滤波特性,其中银的滤波范围较广,而铜、铝则分别在长、短波段表现出较好的滤波性能.为了验证仿真结果,利用磁控溅射技术和聚焦离子束刻蚀技术进行了三种材料的一维光栅减色滤波器的制备实验,并在显微镜下观察滤波器的颜色效应,结果与理论仿真相符合.研究结果表明:不同金属材料在相同微结构下的颜色效应存在较大差异,可根据实际滤波需要,选择材料和微结构参数来获得最佳性价比的滤波器.     

银纳米粒子阵列中衍射诱导高品质因子的四偶极晶格等离子体模式

金属纳米颗粒阵列中形成的四偶极晶格共振模式具有低辐射损耗、高品质因子的特性,因此广泛应用于纳米激光、传感、固态照明等领域.基于时域有限差分法在均匀环境下研究了银纳米圆柱阵列的光谱与近场特性.研究结果表明,在x偏振光直入射下,通过调节阵列x方向的周期,共振强度先增加后降低,当两个方向上的周期相等时,提出的阵列结构能够产生一个线宽约0.4 nm、品质因子高达1815的四偶极晶格共振模式,这种共振模式呈现出Fano线型的透射谷;调控y方向的周期能够实现从Fano线型的透射峰到透射谷的转变.本文说明了粒子大小、晶格周期对四偶极晶格共振模式的重要性,同时为银纳米颗粒在可见光波段设计高品质因子共振提供了优化策略.     

Near-field Moiré effect mediated by surface plasmon polariton excitation

We have demonstrated a surface plasmon polariton mediated optical Moiré effect by inserting a silver slab between two subwavelength gratings. Enhancement of the evanescent fields by the surface plasmon excitations on the silver slab leads to a remarkable contrast improvement in the Moiré fringes from two subwavelength gratings. Numerical calculations, which agree very well with the experimental observation of evanescent-wave Moiré fringes, elucidate the crucial role of the surface plasmon polaritons. The near-field Moiré effect has potential applications to extend the existing Moiré techniques to subwavelength characterization of nanostructures.     

Metal-polymer composite films based on polyacrylonitrile and silver nanoparticles. Preparation and properties

The structures consisting of silver nanoparticles and polyacrylonitrile (PAN) are synthesized by the photopolymerization method yielding a homogeneous dispersion and a small size spread of nanoinclusions in PAN matrices, which is observed in images of transmission electron microscopy. The optical properties of the nanocomposites are studied to depend upon conditions of their obtaining. The minimum in the transmission spectra in a wavelength region of 430 nm, which is attributed to the surface plasmon resonance of silver nanoparticles, is found. The absorption peaks in the infrared range are observed at ∼820 and ∼1110 cm⁻¹ for the silver nitrate (AgNO₃) and the photoinitiator, respectively.     

Attenuation,scattering, and depolarization of light by gold nanorods with silver shells

Gold nanoparticles with silver nanoshells are obtained by synthesizing gold nanorods in a growing solution containing cetyltrimethylammonium bromide, subsequent separation in a concentration gradient of glycerol, and reduction of silver nitrate by ascorbic acid under alkaline conditions in the presence of polyvinylpyrrolidone. The formation of silver nanoshells was monitored by the shift of plasmon resonances of extinction and differential light scattering, by the appearance of characteristics peaks of silver in the energy dispersive X-ray (EDX) spectra of samples, by the data of transmission electron microscopy, and by visual changes in the color of colloids. The spectrum of the intensity ratio of the co- and cross-polarized compo- nents of light scattered by gold-silver nanorods is measured for the first time, and it is observed that the maximum is shifted by 80–100 nm compared to previously published spectra of gold nanorods (Khlebtsov et al., J. Phys. Chem. C 112, 12760 (2008)). The extinction and light scattering spectra are calculated by the method of separation of variables using the model of a confocal two-layer spheroid and these calculations are found to agree with spectral measurements. A method for determining the thickness of a silver nanolayer by the spectral shift of an extinction longitudinal resonance is described. The obtained data of optical spectroscopy and transmission electron microscopy and estimations of the mass of the deposited metal show that the aver-age thickness of the silver layer varies from 0.12 to 4 nm as the Ag/Au ratio changes from 2/80 to 90/80 μg/μg.     

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.     

Evanescently coupled resonance in surface plasmon enhanced transmission

The optical transmission through subwavelength holes in metal films can be enhanced by several orders of magnitude by enabling interaction of the incident light with independent surface plasmon (SP) modes on either side of the film. Here, we show that this transmission is boosted by an additional factor of 10 when the energies of the SP modes on both sides are matched. These results, confirmed by a three-dimensional theoretical analysis, give a totally new understanding of the phenomenon of SP enhanced transmission. It is found that the holes behave like subwavelength cavities for the evanescent waves coupling the SPs on either side of the film. In this unusual device, the reflection at either end of the cavity is provided by the SP modes which act as frequency dependent mirrors.     

基于十字连通形环形谐振腔金属-介质-金属波导的折射率传感器和滤波器

提出了由十字连通形环形谐振腔耦合两个金属-介质-金属(metal-insulator-metal, MIM)波导的结构,并用有限元法数值研究了表面等离极化激元在结构中的传输特性.通过对透射谱的研究,系统地分析了MIM结构的传感特性.结果表明,在透射光谱中有三个共振峰,即存在三种共振模式,其中透射峰与材料的折射率呈线性关系.通过对结构参数的优化,得到了折射率灵敏度(S)高达1500 nm/RIU的理论值,相应的传感分辨率为1.33×10~(-4)RIU.更重要的是,灵敏度不受结构参数变化的影响,这意味着传感器的灵敏度不受制造偏差的影响.此外,谐振波长与环形腔中心半径成线性关系,该器件在较大波长范围内实现可调谐带通滤波.透射强度随着波导与环形腔间距的增大而减小,透射带宽同时减小,因此,可以通过控制环形腔与波导的耦合距离来调谐透射强度及透射带宽.研究结果对高灵敏度纳米级折射率传感器和带通滤波器的设计以及在生物传感器方面的应用都具有一定的指导意义.     

Negative index materials and time-harmonic electromagnetic field

We study the evanescent waveʼs behavior on a device made of a plane interface separating vacuum from a perfect negative index material. The system is described by the macroscopic Maxwellʼs equations with frequency-dependent permittivity and permeability. Assuming that electromagnetic sources with sinusoidal time dependence are switched on at an initial time, we show that, as time increases, evanescent waves result in surface modes at the plane interface. The time dependence of these surface modes is oscillating but not harmonic since their amplitude linearly increases with time. As a consequence, we find that the image of a point source is not a point image. The analysis avoids any ambiguity related with causality and finite energy requirements.