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.     

Fabry-Perot腔谐振对横电波激励下亚波长一维金属光栅的异常透射性的作用

利用时域有限差分法,对横电波(TE波)激励带电介质的亚波长一维金属光栅的光场分布进行了模拟分析,发现TE波在所研究的模型下具有异常透射现象.探究其物理本质,确定类导模共振理论是第一个峰和第二个峰产生的主要原因.在此基础上,从麦克斯韦方程出发,通过有效折射率法,确定了类Fabry-Perot(F-P)腔谐振是产生第三个峰的主要原因.从而完善了TE波在所研究的模型下产生异常透射现象的物理本质.为进一步研究TE波异常透射性的物理本质提供了一种完整的理论依据. 关键词： 金属光栅 横电波 异常透射 类Fabry-Perot腔谐振     

横电波激励下亚波长一维金属光栅的异常透射性

针对一维亚波长金属光栅异常透射现象实现的问题,利用时域有限差分法,对横电波（TE波）激励带电介质的亚波长一维金属光栅的光场分布进行了模拟分析,得到了TE波的透射率与电介质折射率的变化关系,从而发现TE波在所研究的模型下具有异常透射现象.基于导模共振理论建立了类导模共振理论,并应用该理论较好地解释了TE波在所研究模型下的异常透射现象,确定类导模共振是TE波产生异常透射性的主要原因.应用所建立的理论解决了传统透射理论无法解决的问题.类导模共振理论揭示了异常透射现象的物理本质,为进一步研究异常透射性的物理本质提 关键词： 金属光栅 横电波 异常透射 类导模共振     

级联亚波长平面介质栅偏振分束器研究

提出了一种纳米厚度金属层连接的级联亚波长平面介质光栅三明治结构,利用该结构中纳米金属连接层上下界面处的表面等离子共振的互作用形成多波长共振的特点设计了适用于光通信波段（1 300～1 600nm）高性能的宽带偏振器和宽带偏振分束器。利用严格耦合波理论分析发现偏振分束器带宽主要由金属连接层厚度控制,光栅厚度基本不影响偏振分束器带宽,只影响共振深度和共振波长。该结论为后续的制备提供了理论指导和借鉴作用。     

Surface plasmon resonance transmission filters at 1053nm based on metallic grating with narrow slit

Metallic gratings with narrow slits can lead to special optical properties such as strongly enhancing the transmission and considerably strengthening the polarized effect. A narrow-band filter suitable for application in optical communication is designed by sandwiching a metallic grating between two identical dielectric films. The maximum transmission can reach 96% after optimizing the parameters of films and grating at a central wavelength of 1053 nm. It is the first time, to our knowledge, that such high transmission has been reported since the discovery of the extraordinarily high transmission through periodic holes or slits; moreover, the extremely polarized effect is also found in P mode of this symmetric grating.     

Polarization-insensitive surface plasmon resonance sensor by cross-slit metallic periodic arrays

We propose a plasmonic structure to obtain polarization-insensitive localized surface plasmon resonance (LSPR) sensor, which consists of cross-slit metallic periodic arrays embedded in the background material. Numerical simulation illustrates that the mechanism of the LSPR sensor is based on the shift of the Fabry–Perot cavity mode resonance peak in the spectrum as the change of the dielectric material properties for the near fields. And one of the transmission dips of the structure is very sensitive to the background materials; the structure could gain the sensitivity (nm/RIU) more than 500 nm/RIU. Meanwhile, the structure holds great potential to achieve high-performance sensors in practical application due to polarization-insensitive virtue.     

Design and analysis of nano-deep corrugated waveguide grating-based dual-resonant filters in visible and infra-red regions

A theoretical analysis of nano-deep corrugated long-period waveguide gratings on a SU-8 polymer-based channel waveguide with NOA61 optical epoxy coated upper- and lower cladding is presented. The transmission spectra of the gratings show strong rejection bands both at visible (at wavelength region of 450?460 nm) and infra-red (at wavelength region of 1530?1540 nm) regions when a grating period of ?68 μm with optimized grating tooth height is considered. Phase-matching graphs are studied to find the relationship between resonance wavelength and grating period. These results show that the grating parameters significantly affect the characteristics of transmission spectra as well as the resonance wavelength of the grating. Long-period waveguide grating-based band pass filter made by use of same polymer materials are also designed and analyzed. These types of waveguide grating-based filters can widely be used for visible and infra-red wavelength sensing applications.     

亚波长金属光栅的表面等离子体激元共振特性

亚波长金属光栅在共振波长处有光场局域增强、异常透射等现象,为深入认识其共振机制,本文研究了亚波长金属光栅的表面等离子体激元(SPP)共振特性。通过研究不同金属光栅的几何结构以及金属介电常数对SPP共振波长的影响,获得了3种共振波长的基本物理机制。采用周期边界元法进行数值模拟,在边界积分方程的基础上结合平面波展开方法来处理任意形状的周期性结构。模拟结果表明,3种共振波长可以分别由金属的材料、金属光栅周期和金属光栅厚度所调谐。该研究为微纳米光学器件的设计提供了依据。     

Nonpolarizing guide-mode resonance filter with ultra-narrow linewidth

Ultra-narrow linewidth,nonpolarizing guide-mode resonance (GMR) filters with single and double common resonance wavelengths are designed.The guide-mode resonance filters consist of a single grating layer with asymmetric profiles.By choosing appropriate parameters,same resonance wavelengths for both transverse electric (TE) and transverse magnetic (TM) polarizations can be achieved.Results show that high reflection (more than 99.9%) is obtained at every resonance wavelength,and the full-width at halfmaximums (FWHMs) of TEand TM-polarized light are only 0.008 and 0.215 nm,respectively.     

Enhanced transmittance of a dual pass-band metamaterial filter

A broad pass-band metamaterial-based optical filter is experimentally and numerically studied. The designed structure consists of periodically arranged composite metallic arrays and dielectric layer that exhibits transmission responses composed of two flat pass-bands. The coupling of localized surface plasmon (LSP) modes results in the low-frequency pass-band, while the internal surface plasmon polaritons (ISPPs) between the upper and lower metal layers leads to the high-frequency pass-band. Structural parameters (L and R) are experimentally considered from the viewpoint of exploiting their effects on the pass-bands and resonance frequencies. The bandwidths of these pass-bands both can reach to maximums by optimization of these structural parameters. In addition, the two pass-bands can be modulated to be a single pass-band with a bandwidth of 10.7 THz by optimizing L and R simultaneously.     

Ultra-broad band and dual-band highly efficient polarization conversion based on the three-layered chiral structure

In the paper, a novel three-layered chiral structure is proposed and investigated, which consists of a split-ring resonator sandwiched between two layers of sub-wavelength gratings. This designed structure can achieve simultaneously asymmetric transmission with an extremely broad bandwidth and high amplitude as well as multi-band 90° polarization rotator with very low dispersion. Numerical simulations adopted two kinds of softwares with different algorithms demonstrate that asymmetric parameter can reach a maximum of 0.99 and over than 0.8 from 4.6 to 16.8 GHz, which exhibit magnitude and bandwidth improvement over previous chiral metamaterials in microwave bands (S, C, X and Ku bands). Specifically, the reason of high amplitude is analyzed in detail based on the Fabry-perot like resonance. Subsequently, the highly efficient polarization conversion with very low dispersion between two orthogonal linearly polarized waves is also analyzed by the optical activity and ellipticity. Finally, the electric fields are also investigated and further demonstrate the correctness of the simulated and calculated results.     

Polarizing beam splitter of two-layer dielectric rectangular transmission gratings in Littrow mounting

We theoretically investigated the design of polarizing beam splitters (PBS) of two-layer dielectric rectangular transmission gratings in Littrow mounting. The PBS grating is aimed to separate TE and TM polarized waves into two different transmitted orders, i.e. the 0th and −1st orders. The polarization-dependent diffraction process is analyzed by using a simplified modal method. The mode reflection and transmission at the interface of the two layers are illustrated. Design equations are given based on the average mode indices of the even and odd modes inside the grating region, which are verified by using rigorous coupled wave analysis. Moreover, with simulated annealing algorithm, two kinds of two-layer PBS gratings are optimized for operation over the C-band (1520-1570 nm). Compared with the single-layer transmission PBS gratings, such two-layer scheme exhibits a great flexibility of design. The PBSs of two-layer dielectric transmission gratings should be useful for practical applications.     

Broad band polarizing beam splitter with a dielectric layer embedded in the groove of metal grating

We described the use of a metal grating embedded with a dielectric layer in the grating groove as a highly efficient polarizing beam splitter (PBS), to reflect s-polarized light and transmit p-polarized light in a wide spectrum band. This novel structure is designed at a wavelength of 1550 nm to be used in the optical communication wavelength band. High polarization efficiency and extinction ratio for both reflected and transmitted light are obtained by optimization of the designed structure. Moreover, it is shown that this structure can exhibit extremely high extinction ratio in the optical communication band with less dependence on the grating parameters and on the incidence angle, which greatly ease the fabrication of such kind of gratings.     

Optical properties of spherical gold mesoparticles

Optical properties of spherical gold particles with diameters of 150–650 nm (mesoparticles) are studied by reflectance spectroscopy. Particles are fabricated by laser-induced transfer of metallic droplets onto metal and dielectric substrates. Contributions of higher multipoles (beyond the quadrupole) in the scattering spectra of individual spherical particles are experimentally observed. These observations are performed for particles in a homogeneous environment and for particles located in air on a metal surface. Good agreement between calculations on the basis of Mie theory and experimental results obtained in homogeneous environment is demonstrated. Multipole resonance features in the experimental reflection spectra of particles located on a gold substrate, in the wavelength range of 500–1000 nm, are discussed and theoretically analyzed on the basis of finite-difference time-domain simulations. High-resolution Raman images of mesoparticle pairs at different polarizations of light are also presented.     

一维金属光栅的透射光学特性

利用时域有限差分法模拟了一维金属光栅的透射光场的分布及光栅厚度等因素对透射光谱的影响.结果表明:特定波长的光通过金属光栅中的亚波长狭缝时有异常大的透过率增强.分析其物理起源,认为是类FabryPerot(FP)腔作用的结果.基底介电常量对类FP腔长度的调制导致了对透射光谱的影响.金属光栅表面激发的表面等离子体激元波与多缝干涉一样,对透射光谱有一定的影响,但不是形成增强的原因 关键词： 金属光栅 透射光学特性     

Surface plasmon-assisted optical switching/bistability at telecommunication wavelengths in nonlinear dielectric gratings

In this paper, we numerically investigate nonlinear optical responses in dielectric gratings coated by flat metallic surface, where the structural parameters are chosen for wavelengths of operation at the telecom regime. The presented designs show the output light reflection with respect to the incident intensity of increasing, decreasing, and non-monotonic belt-shape functions. Nonlinear optical responses including switching and bistability appeared in the telecom region owing to assistance of surface plasmon resonance excited at the metal/dielectric interface, which enhances the diffraction efficiency and induces the momentum matching for the leaky modes of the dielectric grating. This results in electric intensity-effective refractive index dependence enhancement in the Kerr nonlinear media. Our design and investigation of the linear and nonlinear optical responses will provide a general guideline for a hybrid metallic-dielectric structure-based optical switching/bistability device design.     

Fibonacci quasi-periodic superstructure fiber Bragg gratings

Superstructure fiber Bragg gratings constructed following the Fibonacci sequence are proposed and simulated by using the transfer matrix method. The reflection spectrum has a multi-fractal structure as a function of the wavelength and is symmetric to the center. The reflectivity has self-similarity and scaling with respect to the Fibonacci sequence order at the central wavelength. For the transmission spectrum, the interaction of the core-cladding mode is investigated and the particular wavelengths for which the resonance condition is satisfied are located by varying the grating parameters. The potential application of the quasi-periodic structure as fiber sensors is also proposed.     

Novel ultracompact wavelength division demultiplexer based on photonic band gap

We propose ultra-compact wavelength division demultiplexer based on photonic band gap in two dimensional photonic crystals. The structure consists of two different types of lattice, which can separate two communication wavelengths, 1310 and 1490 nm. In addition, it also can be used to separate the wavelengths of 1310 and 1550 nm if we change the structural parameters. The total size of the present structure is only 12.5 μm × 12.5 μm. The transmission efficiency is above 97%, and the good performance is verified with plane wave expansion and finite-difference time-domain simulation.     

亚波长金属光栅的表面等离子体激元共振特性

亚波长金属光栅在共振波长处有光场局域增强、异常透射等现象,为深入认识其共振机制,本文研究了亚波长金属光栅的表面等离子体激元（SPP）共振特性。通过研究不同金属光栅的几何结构以及金属介电常数对SPP共振波长的影响,获得了3种共振波长的基本物理机制。采用周期边界元法进行数值模拟,在边界积分方程的基础上结合平面波展开方法来处理任意形状的周期性结构。模拟结果表明,3种共振波长可以分别由金属的材料、金属光栅周期和金属光栅厚度所调谐。该研究为微纳米光学器件的设计提供了依据。     

Matching of quasioptical open resonators with waveguide feeders

We consider a semisymmetric open resonator (OR) included in a waveguide feeder, in which the first higher-order mode is excited. We show that a high efficiency of excitation of the mode can be achieved by appropriate choice of coupler dimensions due to the matching of structures of the exciting and operating fields. We analyze the behavior of the resonance reflection and transmission coefficients as well as the loaded Q-factor of an OR as functions of the parameters of one-dimensional E-polarized diffraction gratings located in a coupler aperture with dimensions of a few of wavelengths. Calculations are made with allowance for the resonance and excitation losses. The numerical results differ from the experimental data by no more than 13%, which allows one to construct open resonance systems with the required properties by changing the parameters of diffraction gratings in the coupler aperture. Institute of Radiophysics and Electronics, National Academy of Sciences of Ukraine, Kharkov, Ukraine Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 43, No. 4, pp. 325–334, September, 2000.