Negative role of surface plasmons in the transmission of metallic gratings with very narrow slits

It is generally admitted that the extraordinary transmission of metallic grating with very narrow slits is mainly due to the excitation of surface plasmons on the upper and lower interfaces of the grating. We show that the surface plasmon contribution is not the prime effect and that waveguide mode resonance and diffraction are responsible for the extraordinary transmission. Additionally and surprisingly, we reveal that the transmittance of subwavelength metallic gratings is always nearly zero for frequencies corresponding to surface plasmon excitation. This finding implies that surface plasmons play a negative role in the transmission.     

Grating-coupled surface plasmon resonance in conical mounting with polarization modulation

A grating-coupled surface plasmon resonance (GCSPR) technique based on polarization modulation in conical mounting is presented. A metallic grating is azimuthally rotated to support double-surface plasmon polariton excitation and exploit the consequent sensitivity enhancement. Corresponding to the resonance polar angle, a polarization scan of incident light is performed, and reflectivity data are collected before and after functionalization with a dodecanethiol self-assembled monolayer. The output signal exhibits a harmonic dependence on polarization, and the phase term is used as a parameter for sensing. This technique offers the possibility of designing extremely compact, fast, and cheap high-resolution plasmonic sensors based on GCSPR.     

Classifying the transmission resonances of a subwavelength aperture within a thin metallic film by breaking the symmetry

Here by introducing an I-shape structure to break the symmetry of a thin metallic film, the contributions of Fabry–Perot (FP) resonance-like modes and surface plasmon polariton (SPP) resonance-like are determined. The finite difference-time domain (FDTD) simulation is utilized to provide a clear picture of the electromagnetic field distribution and transmission profile. The FP matrix method is employed to specify the contribution of FP resonance. Some of the predicted resonances of the FP matrix method coincide with the simulation result which is labeled as FP resonance-like modes. The remaining resonance modes in transmission profile are considered as SPP resonance-like modes. Our results provide explicit evidence that both FP resonator modes and SPP resonance are responsible for extraordinary optical transmission (EOT) through a subwavelength aperture.     

一维金属光栅的光学反射吸收

利用RCWA(rigid coupled-wave analysis)方法研究了一维金属光栅的反射特性,考察了 瑞利反常、表面等离激元驻波共振和几何共振三种共振吸收机理,分析了这三种机理的相互作用,如表面等离激元驻波共振和几何共振可以形成混合模式. 在反射式复合金属光栅中,确认了第四种共振形式,即相位共振. 数值计算表明相位共振对光学吸收的影响有两种形式: 当光栅周期大于一个波长时,相位共振导致尖锐的吸收峰,峰位在几何共振吸收峰一侧;当光栅周期小于一个波长时,相位共振导致混合模式的共振吸收峰发生劈裂. 对一维金属光栅反射特性的研究增加了对金属光栅共振吸收模式及其相互作用的认识. 关键词： 一维金属光栅 瑞利反常 表面等离激元 相位共振     

Analytical Investigation of Transmission Properties of Metallic Gratings

We re-examine the classical one-dimensional transmission grating to explain the enhanced transmission in the surface impedance approximation. The nearly zero transmission and extraordinary transmission phenomena related to the surface plasmon are presented by analysing the scattering amplitude of waveguide mode at the output surface of grating. It is revealed that the transmission peaks are related to the Fabry-Perot factor and the interaction of surface plasmon and other diffractive orders.     

基于表面等离子体共振增强的硅基锗金属-半导体-金属光电探测器的设计研究

金属-半导体-金属光电探测器的光栅结构可激发表面等离子体, 有效增强探测器的吸收. 为深入研究器件结构对于表面等离子体的激发及共振增强的影响, 本文提出了一种具有超薄有源层的硅基锗金属-半导体-金属光电探测器的设计方法. 采用时域有限差分的方法详细分析了光栅周期、光栅厚度、 光栅间距及有源层厚度对于表面等离子体共振增强器件性能的影响, 通过仿真模拟获得了器件的最佳结构, 详细地分析了各个界面激发的表面等离子体及其共振模式对于光谱吸收增强的机理. 仿真结果表明, 有源层锗的厚度为400nm的超薄器件在通信波段具有较高的吸收, 尤其在1550nm波长处器件的归一化的光谱吸收率可以高达53.77%, 增强因子达7.22倍. 利用共振效应能够极大地提高高速器件的光电响应, 为解决光电探测器响应度与响应速度之间的相互制约关系提供了有效途径. 关键词： 表面等离子体 锗探测器 时域有限差分仿真     

Extraordinary optical transmission in the ultraviolet region through aluminum hole arrays

We investigated the extraordinary optical transmission phenomenon in the UV range by fabricating large-area, free-standing aluminum hole arrays using extreme UV interference lithography and shadow thermal evaporation. Transmission spectra show strong peaks in the UV region resulting from both surface plasmon polariton and localized surface plasmon excitations. The results indicate that the high plasmon frequency of Al is directly responsible for the presence of strong resonance peaks in the UV region, which supports the role of plasmonic phenomena in the extraordinary transmission. The simple fabrication method enables large-area production of such structures for research and industrial production purposes.     

内嵌矩形腔楔形金属狭缝的增强透射

金属薄膜上亚波长金属狭缝的非共振光增强透射对宽频带光子器件的设计具有重要意义. 为增强金属狭缝的光透射, 设计了内嵌矩形腔楔形金属狭缝阵列, 并用有限元方法研究了该结构的透射特性. 结果表明, 内嵌矩形腔的楔形金属狭缝阵列比无矩形腔的楔形金属狭缝阵列透射率高. 此外, 还研究了矩形腔的结构参数和位置对内嵌矩形腔楔形金属狭缝阵列透射特性的影响. 这些结果将对设计具有更高透射能力的金属狭缝具有一定的指导意义. 关键词： 光异常透射 金属狭缝 表面等离极化激元     

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.     

Dependence of surface enhanced Raman scattering on the plasmonic template periodicity

Surface enhanced Raman scattering has been investigated from rhodamine 6G molecules embedded in polymethyl methacrylate (R6G+PMMA) and coated on one-dimensional and two-dimensional gold-dielectric gratings fabricated by laser interference lithographically. The Raman signals from these plasmonic templates are 200 to 400 times larger than the signal from R6G+PMMA coated on plain gold films. The enhancement of the Raman signal varies almost periodically with the period of the grating. Finite-difference time-domain simulations show that large electromagnetic near fields occur at the metallic edges due to the resonant excitation of localized surface plasmon of the gold patches by the pump laser. These give rise to large enhancements of the Raman signal. The dependence on period is due to the combined effects of the localized surface plasmon and the periodic grating that couples the pump laser to the surface plasmon polariton.     

Design of Highly Sensitive Surface Plasmon Resonance Sensors Using Planar Metallic Films Closely Coupled to Nanogratings

We investigate the sensitivity enhancement of surface plasmon resonance （SPR） sensors using planar metallic films closely coupled to nanogratings. The strong coupling between localized surface plasmon resonances （LSPRs） presenting in metallic nanostructures and surface plasmon polaritons （SPPs） propagating at the metallic film surface leads to changes of resonance reflection properties, resulting in enhanced sensitivity of SPR sensors. The effects of thickness of the metallic films, grating period and metal materials on the refractive index sensitivity of the device are investigated. The refractive index sensitivity of nanograting-based SPR sensors is predicted to be about 543 nm/RIU （refractive index unit） using optimized structure parameters. Our study on SPR sensors using planar metallic films closely coupled to nanogratings demonstrates the potential for significant improvement in refractive index sensitivity.     

Extraordinary optical transmission through metal gratings with single and double grooved surfaces

We investigate the transmission properties of a normally incident TM plane wave through metal films with periodic parabolic-shaped grooves on single and double surfaces using the finite-difference-time-domain method. Nearly zero transmission efficiency is found at wavelengths corresponding to surface plasmon excitation on a flat surface in the case where the single surface is grooved. Meanwhile, resonant excitation of surface plasmon polariton (SPP) Bloch modes leads to a strong transmission peak at slightly larger wavelengths. When the grating is grooved on double surfaces, the transmission enhancement can be dramatically improved due to the resonant tunnelling between SPP Bloch modes.     

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.     

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.     

Strong optical transmission through the ellipsoid metal-film nanohole arrays

The transmission characteristics of a metallic film with subwavelength ellipsoid nanohole arrays are investigated by using the three-dimensional finite-difference time-domain (3D-FDTD) method. The extraordinary transmission is attributed to the collaboration of localized waveguide resonance and surface plasmon resonance. The influences of the lattice constant and the hole shape on the transmission are studied. By analyzing the picture of electric field and electromagnetic energy distribution, we show the mechanisms of the two different resonances: Localized waveguide resonance mode can be confined inside the ellipsoid holes region, while electric field and electromagnetic energy are localized separately at the two ends of ellipsoid holes for the surface plasma resonance mode. Supported by the National Natural Science Foundation of China (Grant No. 60708014), the Distinguished Youth Foundation of Hunan Province (Grant No. 03JJY1008), the Science Foundation for Post-doctorate of China (Grant No. 2004035083), and the Natural Science Foundation of Hunan Province (Grant No. 06JJ20034)     

Enhancement of extraction efficiency by metallic photonic crystal embedding in light-emitting diode

The bandgap effect of photonic crystals (PCs) and the effect of grating diffraction can be used to improve the extraction efficiency of light from the light-emitting diode (LED). The transmission of light at certain wavelength through periodic sub-wavelength hole arrays in metal films is extraordinary, surface plasmon (SP) effects effectively. In this letter, silver metallic photonic crystals with square lattice of cylinder unit cells are fabricated in GaN layer of GaN-based blue LED. We use the finite-difference time-domain (FDTD) method to investigate the optical transmission, the results show that the light extraction efficiency is enhanced by more than four times. Then we use the surface plasmon dispersion relation to analyze the mechanism of antireflection.     

High refractive index sensitivity sensing in gold nanoslit arrays

The extraordinary optical transmission(EOT) phenomenon of nano-periodic aperture array in metallic film has been widely investigated and used in biosensors. The surface plasmon resonance and cavity mode in some periodic nanostructures, such as nanohole and nanoslit, cause EOTs at certain wavelengths. This resonance wavelength is sensitive to the refractive index on the surface of periodic nanostructures. Therefore, the metallic nanostructures are expected to be good sensing elements. The sensing performances of gold nanoslit arrays are experimentally and theoretically investigated.Three-dimensional finite difference time domain(FDTD) simulations are utilized to explore their transmission spectra and steady-state field intensity distributions. The electron beam evaporation, electron beam lithography, and ion milling are applied to the gold nanoslit arrays with different widths and periods. The sensing performances of the gold nanoslit array are characterized via transmission spectra in four kinds of refractive index samples. The highest sensitivity reaches726 nm/RIU when the width of the gold nanoslit array is 38.5 nm.     

双层金属纳米光栅的TE偏振光异常透射特性

根据在亚波长金属光栅表面添加电介质会引起TE偏振光的透射异常性, 应用严格耦合波理论和时域有限差分方法, 研究了双层金属纳米光栅在TE偏振光入射时产生的异常透射现象. 利用等效折射率方法建立了双层金属光栅的等效模型, 得到了TE偏振光透射率与聚合物的折射率、厚度以及金属层厚度的变化关系. 确认了结构中聚合物是透射异常出现的必要条件, TE偏振光以波导电磁模式在其中传播, 并认为类Fabry-Perot腔谐振是透射峰值产生的主要原因.     

Effects of a bar on optical transmission through Z-shaped metallic slit arrays

We investigate the effects of a bar on optical transmission through Z-shaped metallic slit arrays by using the finite-difference time domain (FDTD) method. A new hybrid Fabry-Perot (FP) surface plasmon polariton (SPP) mode emerges when changing the geometric parameters of the bar, and this mode can be viewed as a coupling between FP mode and SPP mode. In addition, an obvious dip appears in a featured area when the bar deviates from the central line, and a small displacement of the bar leads to tremendous change of the dip. These behaviors can be attributed to the phase resonance. In short, the structure is very sensitive to the metal bar. Furthermore, it combines photonic device miniaturization with sensitivity, which is useful for making optical switches.     

金孔阵列电介质与金电介质孔阵列的强透射特性

采用时域有限差分（FDTD）法研究了金膜厚度、电介质折射率及其厚度对金孔阵列电介质与金电介质孔阵列两种结构强透射特性的影响。研究发现这两种结构都具有较好的强透射特性,这表明光与金膜表面自由电子的电荷密度波耦合成表面等离子激元（SPP）,对增强透射起到了关键作用。金膜厚度是影响强透射特性的主要因素,其衰减长度为35 nm;而与金膜相邻的电介质膜厚度对强透射特性影响极小。电介质折射率大小对强透射特性影响明显,折射率为1.8时能够获得较好的强透射特性。