Ability and Limitation of Effective Medium Theory for Subwavelength Gratings

In the recent researches of subwavelength relief gratings, the effective refractive indices are used for calculating the coefficients of reflection and transmission of light waves. The effective refractive indices are given by the effective medium theory for light waves propagating in a bulky stratified medium. This paper describes the ability and limitation of the theory applied to estimate the performance of such subwavelength gratings. The limitation is revealed by numerical calculations using a rigorous electromagnetic grating theory and the effective medium theory. As a result, an understanding of the wave behavior in the subwavelength grating has been acquired, which explains the limitation of the effective medium theory.     

Temperature or strain induced adjustable-chirp characteristics of uniform fibre grating with tapered metal coating

Temperature and strain characteristics of uniform fibre grating with tapered metal coatings have been analysed theoretically, by which adjustable chirp characteristics of such gratings are shown. Electroplating is adopted to fabricate such gratings, and the tapered metal coating is obtained by gradually drawing the fibre grating out of the solution during the process of electroplating. The gradually changing cross-sectional area of the metal coating is calculated by a newly suggested numerical method. By combining the theoretical and numerical simulation analyses, the gratings' characteristics are given at various temperatures and strains. The results obtained using such a method are also testified by experiments.     

亚波长衍射微透镜的设计

设计了连续浮雕衍射微透镜和与其等效的二元亚波长衍射微透镜,采用旋转体时域有限差分法对通过计算机辅助设计出的元件进行了严格的矢量分析,给出了各元件在焦平面的场强分布.分析结果表明,本文设计的二元亚波长微透镜对连续浮雕微透镜有很好的等效效果;填充因子的脉冲宽度调制法要优于填充因子的线性近似法;制作相对误差不大时深度误差和宽度误差对微透镜聚光能力的影响不大.     

Broadband Terahertz Polarization Converter and Asymmetric Transmission Based on Coupled Dielectric‐Metal Grating

Coupled dielectric‐metal gratings are investigated for broadband terahertz (THz) wave polarization conversion and asymmetric transmission by the experiments and numerical simulations, which are composed of the subwavelength Si grating and metallic wire grating layers. The dielectric grating layer with a large artificial birefringence and low dispersion is employed as a phase engineered waveplate, and the metal wire grating arranged with a 45° angle to the dielectric grating is utilized as a high‐efficiency polarizer. Due to the subwavelength integration, this coupled grating presents a local resonance coupling mechanism between dielectric and metal gratings, which greatly enhances the polarization rotation and expands the bandwidth, not a simple combination with dielectric and metallic gratings. The results demonstrate that a broadband asymmetric transmission with an extinction ratio of 30dB from 0.2 to 1.2 THz is achieved and the highest transmission of 90% can be obtained. It provides a simple way towards practical applications for THz artificial dispersion materials, polarization control and asymmetric transmission.     

Coupling of surface waves by two crossed dielectric waveguides

The coupling of surface waves by two crossed dielectric slab sections with curved transition waveguides is investigated by the method of staircase approximation. the coupling problem of the nonuniform structure is analyzed in terms of the scattering of incident surface wave by the crossed structure as a whole. The scattering characteristics of the crossed waveguides are calculated with a particular attention directed to the effect of radiation due to bending of the curved transition waveguides on coupling properties of the structure. Some useful guidelines for the design of the coupling structures are thereby suggested.     

亚波长介质光栅的制作误差分析

利用严格耦合波理论（RCWA）分析了方向误差和面形误差对亚波长光栅衍射效率的影响. 通过分析发现,方向误差和图案边缘钝化对光栅的衍射效率影响不大,而刻蚀过程中由于侧壁倾斜而产生的面形误差对光栅的衍射效率影响非常大.在制作亚波长光栅时,可以通过选取合理的刻蚀系统或增大占空比的方法来避免基底型误差的出现.该结论对于制作亚波长光栅具有重要的指导作用.同时根据得出的结论,选用专门用于硅深刻蚀的等离子体辅助刻蚀系统制作出了红外30 μm亚波长抗反射光栅,检测结果显示,光栅沟槽侧壁陡峭且透过率和设计值吻合得比较好.     

Area-coded effective medium structures, a new type of grating design

We propose a new way to design gratings with desired diffraction properties by using subwavelength feature sizes perpendicular to the ordinary superwavelength grating period. This is different from well-known one-dimensional binary-blazed gratings that use a structuring along the grating period and thus opens new flexibility in generating arbitrary effective-index distributions in the direction of the grating period. Since the subwavelength features form contiguous areas, they are called area-coded effective medium structures (ACES). Compared with well-known binary subwavelength structures in two-dimensional arrangements consisting of pillars, ACES are more stable and have comparable efficiency properties. As an example we show how to design in principle a four-level area-coded effective medium grating, compare the efficiency of ACES with binary-blazed and échelette gratings, and optimize the subwavelength period of ACES.     

Nanofiber Fabry-Perot microresonator for nonlinear optics and cavity quantum electrodynamics

We experimentally realize a Fabry-Perot-type optical microresonator near the cesium D2 line wavelength based on a tapered optical fiber, equipped with two fiber Bragg gratings that enclose a subwavelength diameter waist. Owing to the very low taper losses, the finesse of the resonator reaches F=86 while the on-resonance transmission is T=11%. The characteristics of our resonator fulfill the requirements of nonlinear optics and cavity quantum electrodynamics in the strong coupling regime. These characteristics, combined with the demonstrated ease of use and advantageous mode geometry, open a realm of applications.     

Broadband Antireflection Subwavelength Gratings for Polymethyl Methacrylate Fabricated with Molding Technique

We fabricated a two-dimensional subwavelength grating (SWG) on a polymethyl methacrylate (PMMA) film using a molding technique. A method of fabricating SWGs with high accuracy using a replica technique in an atmospheric environment was proposed. The SWG consisted of tapered gratings with a 200 nm period and a 200 nm deep groove. The reflectivity at wavelengths from 400 nm to 800 nm was measured and compared with the results calculated on the basis of rigorous coupled-wave analysis. At these wavelengths, the reflectivity decreased to approximately half that of the PMMA film.     

Beam focusing through a tapered subwavelength aperture surrounded by dielectric surface gratings

A novel plasmonic nanolens formed by a tapered subwavelength metal slit surrounded by surface dielectric gratings is proposed and demonstrated numerically. By patterning surface corrugations on the output surface, the beam can be focused, by regulating the aperture, the focal length can be controlled effectively. Numerical simulations using Finite-Difference Time-Domain (FDTD) method coupled with anisotropic perfectly matched layer (APML) boundary conditions verify that the proposed metallic lens can focus the radiation on the scale of a wavelength below the substrate and the method is effective for the design of nano-optical devices such as optical microprobes.     

Calculation of Diffraction Characteristics of Sub wavelength Conducting Gratings Using a High Accuracy Nonstandard Finite-Difference Time-Domain Method

Gratings with subwavelength groove depth and period are frequently used in optics for various purposes. The polarization dependent diffraction characteristics of subwavelength (high frequency) gratings can only be calculated by solving Maxwell’s equations of electromagnetism. In this paper, we calculate the classical diffraction characteristics of subwavelength conducting gratings numerically, using a new high accuracy version of nonstandard finite-difference time-domain (NS-FDTD) algorithm. For the purpose of analysis, we employ a gold sinusoidal grating with light incident at a large angle. We have compared high accuracy NS-FDTD simulation results with those obtained from standard finite-difference time-domain (S-FDTD), and the finite element method (FEM) simulations.     

Hydrophobic and antireflective characteristics of thermally oxidized periodic Si surface nanostructures

We reported the antireflective properties of periodic silicon (Si) subwavelength gratings and flat substrates in combination with thermal oxidation for an effective surface passivation. Moreover, theoretical calculations and analysis based on rigorous coupled-wave analysis simulation were performed. For thermally oxidized Si subwavelength grating structures, the reflectance was significantly reduced compared to that of the thermally oxidized flat Si substrates in wide ranges of wavelengths and angles of incidence. As the oxidation time increased, the reflectance became lower with increasing SiO _x thickness on Si subwavelength gratings due to the more graded effective refractive index profile from air to the Si via the thermally oxidized Si subwavelength gratings with increased height. By incorporating a thermal oxidation process into the Si subwavelength gratings, a more hydrophobic surface with good surface passivation was obtained.     

一维亚波长周期结构物成像的光子学研究

当用波长为λ的单色平行光垂直照射到光栅常数d<λ的亚波长周期结构衍射物时,会产生隐失波。由于快速衰减,这种亚波长周期物是不能成像的。但是,通过对亚波长周期结构物进行适当的编码后可得到均匀波,使这种携带了隐失波信息的均匀波通过经特别设计的光学系统并被放大到CCD所能识别的像素大小后,再进行必要的解码以滤掉编码波,即可得到原亚波长周期结构衍射物的像,最终达到超分辨的目的。基于这种新颖的成像技术,用常规仪器就实现了对亚波长周期结构物的成像。在运用光子学方法对实验和成像过程进行较为详尽的理论分析的同时,对编码器、解码器的位置以及它们相对物光栅的取向给出了设计性研究,对滤波器的选择给予了必要的说明。实验结果验证了该设计理论的正确性。     

Tunable directional beaming assisted by asymmetrical SPP excitation in a subwavelength metallic double slit

We present a design for tunable directional beaming through a subwavelength metallic double slit surrounded by dielectric surface-relief gratings.On-axis and off-axis beaming can be switched by controlling the incident angle to asymmetrically excite surface plasmon polaritons(SPPs) that are subsequently coupled out to propagating beams by the two gratings on the left and the right sides of the double slit.Furthermore,the division of optical power into two off-axis beaming directions can be tuned smoothly by varying the incident angle while keeping the total power almost unchanged.The mechanism of this effect is analyzed theoretically and verified using rigorous numerical simulations.     

Resonances on metallic compound transmission gratings with subwavelength wires and slits

We investigate the response of gratings formed by subwavelength wires in a dielectric medium. For gratings with a single wire per period, most of the incident power is transmitted through the structure. If the distance between adjacent wires is small enough, the excitation of waveguide modes can lead to reflection resonances. However, only when the period is formed by several wires (compound grating), new degrees of freedom are introduced for the phase distribution of the field inside the slits, allowing a new class of resonances analyzed in this letter. We give numerical examples illustrating the combined effect produced by both types of resonances in the grating response.     

对正交浮雕光栅傅里叶模方法的快速实现

采用“逆规则”傅里叶展开法来提高正交浮雕双周期光栅问题的收敛性,把用在单周期光栅中的反射透射系数阵算法推广到正交双周期光栅中,并充分利用光栅的对称性和对角周期性,在保证计算准确度和稳定性的条件下,把计算速度提高了几十倍,从而把对这类问题计算从大型计算机中解脱出来,而可用微机加以处理。     

一种设计环形汇聚光栅反射镜的新方法

提出了一种利用数学变换来快速设计环形汇聚光栅反射镜的方法.通过分析具体的物理场景,抽象出已有条形汇聚光栅的"线"汇聚特性与所要设计的"点"汇聚特性在数学上对应的变换关系,然后用该数学变换对条形汇聚光栅进行外形上的变换,外形变换后的条形光栅即为可以实现"点"汇聚的环形光栅.用有限元算法对设计的环形汇聚光栅进行仿真,仿真证明采用该方法设计的环形光栅可以很好地实现高反、高汇聚.采用这一方法,设计了直径为29.788μm的环形光栅反射镜,当垂直入射的径向偏振光从设计的环形光栅表面反射回来后将发生汇聚,汇聚焦点位于环形光栅表面10μm处.经计算,反射镜的数值孔径为0.8302,反射率为0.9163,在焦点所在的汇聚面上,汇聚光栅电场分布的半高宽为1.5548μm.     

Guiding, focusing, and sensing on the subwavelength scale using metallic wire arrays

We show that tapered arrays of thin metallic wires can manipulate electromagnetic fields on the subwavelength spatial scale. Two types of nanoscale imaging applications using terahertz and midinfrared waves are enabled: image magnification and radiation focusing. First, the tapered wire array acts as a multipixel TEM endoscope by capturing an electromagnetic field profile created by deeply subwavelength objects at the endoscope's tip and magnifying it for observation. Second, the image of a large mask at the endoscope's base is projected onto a much smaller image at the tip.     

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.     

亚波长金属偏振分束光栅设计分析

结合有效介质理论和薄膜光学的抗反射设计方法,设计了基于0.65μm工作波长的亚波长金属偏振分束光栅,给出了光栅的优化设计参数,采用严格耦合波理论分析了光栅的偏振分束特性.结果表明,亚波长金属光栅对TE偏振表现为金属膜特性,具有高反射,对TM偏振表现为介质膜特性,具有高透射,在-30°<θ<30°的大入射角范围和0.47μm<λ<0.80μm的宽入射波谱内,该光栅的透射光和反射光均具有高偏振消光比和低插入损耗的特点. 关键词： 亚波长金属偏振分束光栅 有效介质理论 薄膜光学 严格耦合波理论