非对称银纳米双环表面等离激元光谱特性

贵金属纳米材料在入射光激发下能够产生表面等离激元,即金属表面自由电子产生集体振荡。当其振荡频率与入射光频率相同时,发生表面等离激元共振,形成一种特殊的电磁场模式和光谱特性。利用该电磁场模式和光谱特性, 能够调节金属纳米材料的光谱学行为,例如通过改变金属纳米结构的大小、形状以及周围介质介电常数等参数, 在微纳尺度上实现光谱学信号的有效调控。目前,除了具有一定对称性的贵金属纳米材料被大量研究和应用外,非对称纳米结构的表面等离激元光谱特性也受到广泛关注。研究表明,在可见-近红外波段光谱范围内设计表面等离激元光电传感器件的关键问题在于,如何有效地调节其消光谱的共振波长、半峰宽以及峰值强度等主要特征参数。提出一种基于银纳米双环组成的非对称结构,利用时域有限差分方法,在可见-近红外波段内,通过分别改变银纳米双环的尺寸、间距及入射光偏振方向等参数,计算了该纳米结构在不同条件下的消光谱。结果表明,在0.4～3 μm的消光谱内,入射光能够激发产生两个独立的表面等离激元共振峰。通过研究峰值波长处的电场分布图发现,上述共振峰分别对应两种不同的电磁场模式。结果还表明,消光谱内两个独立的共振峰可以通过改变该双环结构的不同参数,被分别地进行调节。其中,可以通过改变该双环结构的半径来有效调节短波长峰的共振波长和半峰宽,同时保持长波长峰的共振波长和半峰宽基本不变。此外,通过改变两环间距或入射光偏振方向,可以分别以不同趋势来调节两个共振峰的峰值强度。在提出的非对称银纳米双环的消光谱中,获得了能够被分别调节的两个表面等离激元共振峰,研究结果能够为可见-近红外波段内基于银纳米材料光电传感器件的开发设计提供理论基础。     

基于超疏水高黏附结构的痕量农药分子现场检测SERS基底研究

多种农药,包括孔雀石绿（MG）作为禁用兽药,存在食用致癌的风险。由于MG低廉的价格和极好的药效,在渔业养殖中一直被不法商贩非法使用,使得鱼类生鲜中时有MG残留检出。针对MG分子痕量残留的检测,目前一般是抽取少量养殖水样,再利用高效液相色谱柱、液相色谱-光谱等方法来评估其是否超标。这类传统的检测方法一般需要依赖价格昂贵的大型设备,且检测过程操作繁琐复杂,单次检测耗时长、价格高,因而与农贸市场中商品流通量大、速度快、价格需亲民低廉等特点和要求不相符合。近年来,表面增强拉曼散射（SERS）检测技术以及便携式拉曼光谱仪的出现,有望实现对痕量农药分子的现场快速检测,进而很好地解决这一问题。SERS检测技术利用金属纳米结构的表面等离激元效应感应位于其结构表面附近的分子,得到分子种类和浓度信息。为了降低可检测的浓度极限,一般会在SERS基底上利用咖啡环效应或其他手段将待测分子蒸发富集,以获得足够高的信号强度。针对亲水基底,液滴与基底相接触后,会在基底表面摊开,使其分布面积扩大,导致其咖啡环周长变长,分子分布浓度随之降低。而当采用疏水基底富集时,由于常规的疏水基底表面黏附性小,液滴在其表面处于随处滚动无法抓取的状态,极大增加了操作的难度。以MG分子痕量残留的检测为例,由于农贸市场人员众多、无专业实验平台,磕碰撞击时有发生,在此环境下采用疏水SERS基底对农药分子进行检测显然是不可取的。该研究提出一种基于超疏水高黏附纳米森林结构的SERS基底用于痕量MG分子的快速现场检测。相比于超疏水SERS基底,所提出的超疏水高黏附基底利用其高黏附性可牢固抓取待测液滴,解决了以往超疏水基底在实际现场检测中存在液滴滚动无法操作的问题。此外,与亲水基底相比,超疏水高黏附基底由于接触角大,可将咖啡环面积缩小5.73倍,继而使分子的富集浓度提高5.73倍,最终使检测极限浓度降低了至少两个数量级。研究所提出的超疏水高黏附SERS基底有望在痕量农药分子快速现场检测中得到应用。     

Dependence of thermodynamic properties of model systems on some dissipative particle dynamics parameters

The influence of systematic perturbation of input interaction parameters on thermodynamic equilibrium properties is studied employing dissipative particle dynamics (DPD) simulations. The values of both the excess pressure and the surface tension are found to be very sensitive to the values of the soft repulsion parameter between unlike DPD particles for high values of the coarse-graining level (number of water molecules per DPD particle). For the case in which a molecular surfactant is present at the interface we have determined the dependence of these properties on the values of the parameters that characterize the bonding force between polymer beads. No significant differences were found between linear and branched surfactants.     

Comparison of subwavelength focusing properties of diffraction and Fresnel zone plate plasmonic planar lenses

The subwavelength focusing properties of diffractive plasmonic planar lenses (DPLs) and Fresnel zone plate plasmonic planar lenses (FZPs) have been compared in this paper. To that end, we use the same lens material, incident wavelength, thickness and focal length for comparison. Both DPLs and FZPs consist of central circular slits surrounded by transparent and opaque zones and can get clear focusing performances. By using the rigorous electromagnetic numerical method, the fields in the focal region are analyzed in detail, and our results demonstrate that FZP can generate higher transmission efficiency, and higher peak field intensity at the focal plane. Focusing polarization properties of the lens illuminated by linearly polarized state, are calculated and analyzed also. The numerical results show that both the DPL and the FZP produce asymmetrical focal spot distributions with a low f-number. To the FZP, the full-width at half-maximum (FWHM) varies from 256 nm (along y-axis) to 516 nm(along x-axis) and to the DPL, the FWHM varies from 256 nm(along y-axis) to 580 nm(along x-axis), respectively. But for a high f-number, the asymmetrical performance of the focal spot will be reduced due to complicated electromagnetical field interferences and micro waveguide effect. Otherwise, the DPL can get a higher depolarization effect than the FZP does.     

Optical transmission through an interface between subwavelength slits

This paper studies optical transmission through an interface between two slits with different widths in a sheet composed of an ideal conductor. Such a structure is of potential use in fabricating optical diode and may be the simplest one compared to other designs. Our calculations show that there is a critical wavelength. When the light wavelength is below the critical wavelength, the transmissivity is unidirectional. The expression of the stable transmissivity as a function of the ratio of the widths of the two slits was obtained analytically. Particularly, at the critical wavelength, the transmissivities are zero. This phenomenon has great potential for application in the manufacture of wavelength blockers.     

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.     

Tunable multi-function broadband splitter with optical bistability based on surface plasmon

A tunable multi-function broadband splitter consisted of a silver film, Kerr medium and a silver grating sandwiched between linear dielectrics waveguides is proposed and numerically investigated. This splitter can realize the functions of the beam splitter, the polarization splitter and wavelength beam splitter. This surface plasmon based device provides a unique approach for polarization sensitive manipulation of light in an integrated circuit and will be essential for future classical and quantum information processes.     

Guided surface modes in a hollow slab waveguide with a left-handed material substrate or cover

The structural dispersion characteristics of guided surface modes in a hollow slab waveguide with a left-handed material substrate or cover are investigated. Dispersion relation is derived by using normalized parameters, and universal dispersion curves have been obtained analytically, by solving transcendental dispersion equations in a reverse way. Existence condition, mode degeneracy and other dispersion properties of guided surface modes have been discussed for different ? or μ of three layers for this substrate or cover layer.     

Net lateral shift of reflected beam due to surface plasmon resonance in Kretschmann configuration

When surface plasmon resonance is excited in Kretschmann configuration, the maximum Goos–Hänchen shift occurs at the resonant angle which is associated with the minimum value of reflectivity, but the maximum angular shift occurs at two incident angles beside the resonant angle. When the reflected beam is detected at Rayleigh range, the maximum net lateral shift of the reflected beam does not encounter the resonant angle and its magnitude is lager than that of Goos–Hänchen shift. Numerical simulation shows that theory agrees well with numerical results when the beam width is large enough.     

Contactless Spectroscopy of Deep Levels in Semiconducting Materials: Gaas

Abstract

The surface photo voltage (SPV) and photocurrent (PC) transients as a result of the excitation by the short high-intensity light pulses from semiconductor's intrinsic absorption spectral region are investigated in semi-insulating GaAs. It is shown that the mathematical convolution of SPV transients and arbitrary form double-pulse integrator (lock-in, double-boxcar) in a wide temperature range allows to receive the deep-level (DL) spectrum without the need to form electrical contacts to the crystal investigated. The use of such a procedure while scanning the crystal surface with a light spot at a temperature, corresponding to some DL maximum in the spectrum, makes possible the con tactless determination of this DL density distribution profile along the scanning direction.