Surface plasmon resonance spectroscopy on rotated sub-micrometer polymer gratings generated by UV-laser based two-beam interference

Two-beam interference method was applied to generate gratings having periods of 416 nm and 833 nm by the forth harmonic of a Nd:Yag laser on thin poly-carbonate films spin-coated onto silver layer-covered substrates. The dependence of the modulation depth on the fluence and number of laser pulses was investigated by atomic force microscopy. A secondary pattern appeared on very thin polymer layers thanks to the “p” polarized laser beam illumination induced self-organized processes. The conditions of the emergence of grating-coupling caused additional plasmon resonance peak were determined for the sub-micrometer periodic polymer gratings. Surface plasmon resonance measurements were performed in attenuated total reflection arrangement to determine the effect of the angle between the plasmon propagation direction and the polymer groves on the grating-coupling. The effect of the modulation depth on the grating-coupling caused additional resonance minimum was also analyzed. We found coupling phenomena according to our calculations, the differences between the measured and theoretically predicted resonance curves were explained by the scattering on the complex surface structure.     

Experimental observation of a practical limit on enhancement of the spontaneous emission rate in blue GaN-LEDs by mediating surface plasmons

High modulation speed of light-emitting diodes (LEDs) is of primary importance for applications in optical communication. To this end, we experimentally investigated enhancement behaviors of the spontaneous emission rate (SER) of electron–hole pairs in blue InGaN/GaN LEDs by mediating surface plasmons (SPs). The coupling strength of the electron–hole recombination into SPs is controlled by etching the p-GaN layer between the active and metal layers to form thicknesses between 40 nm and 10 nm. While a tendency of increasing SER is theoretically expected for a smaller separation, the maximum value SER enhancement has a practical limit of about 2.5 at λ = 441 nm, and separation of 20 nm due to damage on the p-GaN layer caused by the etching process.     

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.     

基于表面等离子体效应的光开关研究现状和进展

目前表面等离子体(surface plasmons, SPs)效应在光传感、光存储及生物光子学等领域的应用前景受到了广泛关注,通过计算模拟或实验基于SPs效应的光开关也层出不穷.文章较为系统地介绍了各种基于SPs效应的光开关原理和优缺点,对SPs全光开关做了重点介绍.     

Dispersion relation of surface plasmons on gold- and silver gratings

The propagation velocity and damping of surface plasmons on sinusoidally modulated gold- and silver surfaces with different grating constants (ca. 4000 Å, 8000 Å and 15000 Å) are measured at wavelengths of 6471 Å and 5682 Å as function of the amplitude of the grating (up to ca. 300 Å). Earlier measurements of the changes of the dispersion relation produced by the sinusoidal profile could be improved by reducing the contribution of a second harmonic to the grating profile to less than about 0.06. Observed and calculated data, obtained with the Rayleigh method, show good agreement.     

On the non-existence of hydrodynamic solution of surface plasmons at the bimetallic interface

It is shown that for an abrupt bimetallic interface a hydrodynamic solution for interface plasmons does not exist. It appears that this result is valid irrespective of the choice of of the additional boundary condition, thereby suggesting a careful look at the use of usual hydrodynamic equations for a bimetallic interface.     

Coupling of spontaneous emission from GaN-AlN quantum dots into silver surface plasmons

We have demonstrated the decay of spontaneous emission (SE) from AlN-GaN quantum dots (QDs) into silver surface plasmon (SP) modes in the ultraviolet at approximately 375-380 nm. Using time-resolved photoluminescence (PL), we show that the electron-hole recombination rate in AlN-GaN QDs is enhanced when SE is resonantly coupled to a metal SP mode, corresponding to the dip in the continuous-wave PL spectrum. Exciton recombination by means of silver SP modes is as much as 3-7 times faster than in normal QD SE and depends strongly on emission wavelength and thickness of the silver.     

Tarnishing of silver subwavelength slit gratings and its effect on extraordinary optical transmission

We study the impact of tarnishing of silver subwavelength gratings on their optical performance. We report that in the course of months under regular laboratory conditions, the gratings undergo tarnishing very differently from plain silver films: instead of thin layer of evenly distributed silver sulfide, a random pattern of rare larger crystals is formed across the grating area. As typical of plasmonic metamaterials, the gratings appear to be very sensitive to the arising structural disorder and show a very specific modification of the optical transmittance spectra – total degradation of the Wood diffraction anomaly and attenuation of the near-infrared transmission peak. The identified ‘optical fingerprints’ of the microscopic grating contamination pave a way for prospective plasmonic sensor applications.     

Remote excitation and remote detection of a single quantum dot using propagating surface plasmons on silver nanowire

Using propagating surface plasmons(SPs) on a silver nanowire(NW), we demonstrate that a focused laser light at the end of the silver NW can excite a single quantum dot(QD) microns away from the excitation spot. The QD–NW interaction allows the excited QD convert part of its energy into propagating SPs, which then can be detected at remote sites.Simultaneous multi-QD remote excitation and remote detection can also be realized. Furthermore, the tight confinement of the propagating SPs around the NW surface enables the selective excitation of QDs very close in space, which cannot be realized under the conventional excitation condition. This remote excitation and remote detection approach may find applications in optical imaging and the sensing of chemical and biological systems.     

Electroless deposition of silver on synthesized calcite via surface modification

Metallic silver was deposited on the surface of synthesized calcite via a simple electroless deposition method. Calcite with cubic morphology was prepared first by homogeneous precipitation and it was subsequently surface modified using ammonium oxalate. The electroless deposition was carried out using formaldehyde as the reducing agent and silver nitrate as the silver source. Both calcite and the silver deposited calcite were characterized by different techniques. Surface modification of calcite with ammonium oxalate is necessary for the deposition of silver and the size of the deposited silver particles could be controlled by changing the deposition parameters such as concentration of the reagents and the deposition time. Lower concentration of silver ions (e.g. 0.01 M AgNO₃) and shorter deposition times (e.g. 30 min) lead to the formation of silver nanoparticles on calcite.     

Numerical synthesis of metallic nanostructures for enhancing the emission of a dipole through surface plasmon coupling

In this study, we numerically synthesize a two-dimensional metallic nanostructure consisting of a Au half-space and two separate Ag elliptical cylinders by the simulated annealing (SA) method. The simulated nanostructure is so designed that the surface plasmon polariton (SPP) and the localized surface plasmon (LSP) are simultaneously excited at their common resonant wavelength (535 nm), leading to the enhancement of emission of a nearby dipole source. This enhancement effect is more significant than that of the case where only one of the SPP and LSP is excited. In numerically synthesizing a metallic nanostructure, we try to maximize both the downward emission (in the direction away from the metallic structure) and the emission efficiency. A cost function is defined as some combination of the downward emission and the emission efficiency. We adjust the simulated structure by SA to minimize the cost function at a designated resonant wavelength, and calculate and analyze the spectra of downward emission and emission efficiency for the optimal structure. Other structures are also investigated for comparison. From numerical simulations, it is demonstrated that the enhancement of dipole emission is better for optimization at wavelength 535 nm than at other wavelengths. Note that the downward emission and the emission efficiency can reach maxima almost simultaneously when the SPP and the LSP couple effectively at a common resonant wavelength. This implies that the lighting efficiency of green light-emitting diodes (LEDs) can be increased by the coupling effect at a common resonant wavelength of SPP and LSP.     

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...     

Simultaneous excitation of long and short range surface plasmons in an asymmetric structure

An asymmetric multilayer structure allowing nearly 100% simultaneous excitation of both symmetric and antisymmetric surface plasmons is proposed. It is shown that the two surface plasmons can be excited at a fixed angle of incidence with polychromatic light or with monochromatic light at different angles of incidence, depending on the design. The structure can be of interest in high-performance surface plasmon resonance sensing and in all-optical signal processing.     

基于银胶纳米颗粒的痕量三硝基甲苯的表面增强拉曼光谱的实验研究

近年来对作为典型的硝基苯环类炸药的三硝基甲苯(TNT)的痕量检测受到越来越多的关注。将银胶纳米颗粒作为表面增强拉曼散射(SERS)基底,研究了10-6 mol.L-1的TNT的表面增强拉曼光谱的优化实验条件,重点研究了氯化钠溶液(NaCl)的含量以及碱性水解对TNT的表面增强拉曼光谱的影响。实验发现SERS样品中如果没有加入NaCl溶液,将观察不到TNT的SERS谱。加入的NaCl的含量必须在某一个范围之内才能观察到TNT的SERS谱,而且在这个范围之内,随着NaCl含量的增加,TNT的1 392cm-1处的拉曼峰出现先增大后减小的现象。对NaCl在TNT的SERS中的作用给予了理论解释。研究还发现TNT分子经过碱性水解后,与银纳米颗粒之间的吸附作用增强,其SERS谱的强度明显优于未经碱性水解的SERS结果。     

Investigation of transmission resonances on the one-dimensional metallic cylindrical gratings in THz frequency range

In the experiments of THz wave transmitting through the metallic cylindrical gratings fabricated by sub-wavelength brass wires, this paper reports that the discrepancy in the sharp resonances occurred as the grating perpendicular or parallel to the electric vector are observed. A simulation based on the finite difference time domain (FDTD) indicated that the enhanced transmission through the grating is attributed to the combined effects of surface plasmons and cavity modes in the perpendicular condition, while the cavity modes dominate the resonant transmission under the other conditions. Additional experimental data and calculated results show that ～1 enhanced coupling efficiency can be realized in some THz frequency, which could be applied to the design and improvement of various optoelectronic devices, or detection of biological molecule and powder samples, etc.     

Efficient excitation and amplification of the surface plasmons

One dimensional (1D) grating has been fabricated (using focused ion beam) on 50 nm gold (Au) film deposited on higher refractive index Gallium phosphate (GaP) substrate. The sub-wavelength periodic metal nano structuring enable to couple photon to couple with the surface plasmons (SPs) excited by them. These grating devices provide the efficient control on the SPs which propagate on the interface of noble metal and dielectric whose frequency is dependent on the bulk electron plasma frequency of the metal. For a fixed periodicity (Λ = 700 nm) and slit width (w = 100 nm) in the grating device, the efficiency of SPP excitation is about 40% compared to the transmission in the near-field. Efficient coupling of SPs with photon in dielectric provide field localisation on sub-wavelength scale which is needed in Heat Assisted Magnetic recording (HAMR) systems. The GaP is also used to emulate Vertical Cavity Surface emitting laser (VCSEL) in order to provide cheaper alternative of light source being used in HAMR technology. In order to understand the underlying physics, far-and near-field results has been compared with the modelling results which are obtained using COMSOL RF module.Apart from this, grating devices of smaller periodicity (Λ = 280 nm) and slit width (w = 22 nm) has been fabricated on GaP substrate which is photoluminescence material to observe amplified spontaneous emission of the SPs at wavelength of 805 nm when the grating device was excited with 532 nm laser light. This observation is unique and can have direct application in light emitting diodes (LEDs).     

Nanoscale guiding for cold atoms based on surface plasmons along the tips of metallic wedges

We propose a novel scheme to guide neutral cold atoms in a nanoscale region based on surface plasmons (SPs) of one pair and two pairs of tips of metallic wedges with locally enhanced light intensity and sub-optical wavelength resolution. We analyze the near-field intensity distribution of the tip of the metallic wedge by the FDTD method, and study the total intensity as well as the total potential of optical potentials and van der Waals potentials for 87 Rb atoms in the light field of one pair and two pairs of tips of metallic wedges. It shows that the total potentials of one pair and two pairs of tips of metallic wedges can generate a gravito-optical trap and a dark closed trap for nanoscale guiding of neutral cold atoms. Guided atoms can be cooled with efficient intensity-gradient Sisyphus cooling by blue-detuned light field. This provides an important step towards the generation of hybrid systems consisting of isolated atoms and solid devices.     

Sensitivity enhancement of surface plasmon resonance sensor using continuous film metallic gratings

A surface plasmon resonance (SPR) sensor based on continuous film metallic gratings is numerically investigated for enhance sensitivity. The results calculated by rigorous coupled-wave analysis (RCWA) present that interplays between localized surface plasmons and surface plasmons polaritons contribute to sensitivity enhancement. The sensitivity enhancement factor (SEF), which represents the influence of metallic grating, increased as the grating period decreased. In addition, several reflection dips can be achieved as the period of metallic grating increased. By double-dips method, the sensitivity SPR sensor based on continuous film grating-based is improved into 153.23°/RIU, which is more sensitive than conventional thin film-based SPR sensor in the same condition. The SPR sensor based on continuous film metallic gratings exhibits good linearity.     

基于人工表面等离激元探针实现太赫兹波的紧聚焦和场增强

为提高太赫兹近场显微成像技术的分辨率,设计了一款在Teflon探针的尖锥形表面镀上厚度渐变、具有相同占空比的超薄金属银制条带的探针,用于实现探针尖端处人工表面等离激元的激发和太赫兹波的亚波长聚焦.研究表明,对于频率为0.1 THz的入射波,厚度渐变镀银条带探针产生的紧聚焦光场的尺寸可稳定在20μm左右(λ/150),探针尖端处最大电场强度为入射电场强度的849倍.研究还发现,周期性金属条带的数目和入射电场的偏振方向可对探针尖端处产生的紧聚焦光斑的尺寸和电场强度等进行灵活有效的调控.     

Optimal design of sub-wavelength metal rectangular gratings for polarizing beam splitter based on effective medium theory

A novel optimal design of sub-wavelength metal rectangular gratings for the polarizing beam splitter (PBS) is proposed. The method is based on effective medium theory and the method of designing single layer antireflection coating. The polarization performance of PBS is discussed by rigorous couple-wave analysis (RCWA) method at a wavelength of 1550~nm. The result shows that sub-wavelength metal rectangular grating is characterized by a high reflectivity, like metal films for TE polarization, and high transmissivity, like dielectric films for TM polarization. The optimal design accords well with the results simulated by RCWA method.