Simulation and micro-fabrication of optically switchable split ring resonators

The effect of conductivity variation as a proposed method for the investigation of photoconductive switching properties of split ring resonators (SRRs) is simulated. Three different systems that are applicable under certain fabrication and/or optical excitation conditions are described. The simulated transmission spectrum indicates that for a large range of dark conductivity values, complete switching of the SRR resonance is possible. One of the simulated systems, involving split ring resonators on Si substrate, was fabricated and characterized. The transmission spectrum of that system was measured, with the Si in its high-resistivity state, and a −60 dB dip between 108 and 115 GHz, due to SRRs magnetic resonance, was observed.     

Identifying magnetic response of split-ring resonators at microwave frequencies

In this study we provide experimental methods to identify the magnetic resonance of split ring resonantors (SRR) at the microwave frequency regime. Transmission measurements were performed on both single SRR unit cell and periodic arrays of SRRs. The magnetic response of the SRR structure was demonstrated by comparing the transmission spectra of SRRs with closed ring resonators (CRR). Effects of the changes in the effective dielectric constant of the SRR medium on the band-gaps of SRR are investigated experimentally. SRRs not only exhibit a magnetic resonance band gap but also a band gap due to the electric resonance. Finally, we present the effect of electric coupling to the magnetic resonance of bianisotropic SRRs by utilizing SRRs with different orientations, and incident electromagnetic wave polarizations.     

新型八边形谐振环金属线复合周期结构左手材料奇异性质研究

通过实验及仿真研究了基于C环的新型八边形开口谐振环金属线复合周期结构左手材料.以金属铜八边形谐振环(SRRs)为基本单元的周期结构负磁导率材料,与闭口环(CSRRs)结果对比发现八边形谐振环能产生很好的谐振效果即能产生负磁导率;复合结构仿真结果显示,八边形谐振环金属线复合结构实现负折射具有可行性.设计、制作并实验和仿真研究了两种尺寸的八边形谐振环金属线复合周期结构左手材料,实验结果显示,分别在9.8—15GHz和9.5—15GHz出现良好负折射效应,表明小尺寸材料负折射频段较宽但整体能量透过率较小.通过与尺寸相近的传统C环样品实验对比发现八边形样品损耗较大,但其负折射区域能量分布比例较大,具有一定的优越性.该研究对新型周期结构左手材料的研究、设计和研制具有重要的科学意义,在国防、通信等领域也具有广阔的应用前景.     

Infrared response of a metamaterial made of gold wires and split ring resonators deposited on silicon

Periodic arrays of gold wires and split ring resonators (SRR) with a minimum feature size of 50 nm are fabricated on low-doped silicon. To our knowledge, the periodic arrangement of SRRs and wires considered in this work has not been studied in the near-infrared domain yet. For normal-incidence conditions, this metamaterial structure exhibits resonances at 70 and 170 THz (i.e., at λ ≈ 4.3 and 1.75 μm), which are identified as LC- and Mie resonances, respectively. These resonances are also observed for the SRRs alone, but the amplitude of the Mie resonance is reinforced due to the coupling between the SRRs and wires. The structure is simulated using finite-element software, while transmission and reflection measurements are performed with a Fourier transform infrared spectrometer. Numerical simulations are found to be in very good agreement with experimental characterizations, thereby showing that the Drude model used in calculations is well suited to simulate gold structures at near-infrared frequencies. Theoretical calculations predict that the metamaterial has a negative permittivity and a negative permeability near each resonance.     

一维负磁导率材料中的缺陷效应

理论和实验已经表明，金属开口谐振环（split ring resonators,简称SRR）可以实现材料的有效磁导率μeff在某一频率范围内为负.采用波导法实验研究了不同几何尺寸的六边形SRR在X波段微波作用下的近邻相互作用.实验发现，相同尺寸的六边形SRR周期性排列而成的一维负磁导率材料存在一个谐振频率；当缺陷谐振环引入一维负磁导率材料时，主谐振频率和缺陷环谐振频率同时发生移动；主谐振频率和缺陷环谐振频率的移动量随缺陷与双环谐振频率之差的增加而减小，当缺陷环谐振频率与双环谐振频率接近时，环间的相互作用增强，频率的移动量增大. 关键词： 负磁导率 缺陷效应 开口谐振环     

A wide band left handed material with high transmission

In this study, we propose guidelines to optimize the transmission and bandwidth of left handed materials (LHM) composed of two-dimensional lattice of metallic wires interleaved with double split ring resonators (SRRs). Several LHM structures are fabricated to test the proposed rules of design and operate between 7 and 14 GHz. In a first step, we show that adding extra SRR layers within each period of wire layers (i.e. interleaving the wire lattice with several SRR lattices) increases the transmittance of the material. In a second step, we demonstrate that adding extra SRR layers in a non-equidistant manner (i.e. inserting several SRR lattices shifted one another by different distances) widens the LHM frequency bandwidth. The results of experimental characterization are found to be in excellent agreement with theoretical predictions. A LHM behavior is obtained in a wide frequency band of 1 GHz around 8.2 GHz with a transmission of −1.2 dB measured at resonance. The negative refraction of the material is verified from measurements at oblique incidence.     

谐振频率可调的环状开口谐振器结构及其效应

理论和实验研究表明，开口谐振环(SRRs)中可以激励磁谐振从而实现负磁导率.通过在SRRs结构中引入与其开口边平行的金属短杆设计并制备了新的磁谐振单元，采用波导法系统研究了短杆对SRRs和左手材料的微波透射特性以及左手效应的影响.实验和数值模拟表明：金属短杆和SRRs开口边形成附加电容，导致SRRs开口电容增大从而引起谐振频率降低.随短杆长度l和短杆与SRRs间距d的增大，SRRs谐振频率也随之减小和增加.短杆的加入不影响SRRs的负磁导率特性，改变短杆与SRRs间距d< 关键词： 开口谐振环 金属短杆 调控 左手材料     

非对称开口六边形谐振单环的微波透射特性

受自然界树型结构的启发，设计了分叉树型微结构单元，用电路板刻蚀技术制作了非对称开口六边形谐振单环及其组合结构.采用计算机模拟和实验研究了单个和多个谐振环在微波段(7—12 GHz)的电磁响应行为.研究结果表明：该结构具有负磁导率特性，开口谐振环几何尺寸影响环的磁谐振频率；两环环间距较小时出现二次谐振；带高级分支的辐射状环列相对于不带分支环列透射峰向低频移动.实验和模拟结果相符. 关键词： 负磁导率 开口谐振环 透射率     

新型小型化的平面左手介质微带线及其后向波特性验证

基于裂缝谐振环结构的降频技术,首先设计了一种电尺寸较小的左手介质微带线单元,并根据电磁波在微带线上的传输和反射数据,分别计算了左手介质的有效介电常数和有效磁导率.之后针对左手介质八元阵列进行三维电磁仿真实验,结果表明该八元阵列在左手介质频段上具有独特的后向波效应,从而证实了该左手介质频段的存在.与传统的左手介质微带单元相比,阐述的左手介质单元的电尺寸减小了60%,而且结构简单,便于加工,适用于平面电路器件的小型化等应用研究工作. 关键词： 左手介质 小型化 双负特性 后向波特性     

射电天文用太赫兹三通带频率选择表面设计

本文设计一种基于单屏改进型开口谐振环(SRR)的太赫兹频率选择表面(FSS). 改进型 SRR 谐振单元由具有开口缝的金属贴片组成, 开口缝的物理尺寸会影响其阶跃特征阻抗特性. 本文通过对改进型 SRR 单元结构建立LC等效电路模型, 提取等效电路模型参数, 并结合传输线理论, 得到 FSS 的基频计算公式和谐波关系式. 相比于传统均匀 SRR, 本文所提出的改进型 SRR 多频带传输的控制更为灵活. 基于此特点, 设计了一款中心频率依次为 0.46 THz, 0.86 THz和 1.03 THz, 可应用于射电天文的三通带太赫兹 FSS. 采用电磁仿真软件对影响该 FSS 传输特性的关键参数、周期间隔、小型化程度以及入射角敏感性等重要指标进行分析研究. 结果表明, 改进型 SRR 三通带 FSS 在三个通带内的反射系数分别为 -37.6 dB, -13 dB和 -19.6 dB, 在0°–60° 范围内均具有稳定的频率响应特性, 且具有小型化程度高、损耗低等特点. 这种三通带 FSS 在太赫兹频段射电天文方面有潜在的应用价值.     

垂直入射条件下厚金属环结构的负磁导率与左手材料行为

用矩形波导法实验研究了微波垂直入射于具有一定厚度的金属铜圆环结构的微波电磁响应行为.结果表明,当金属铜环的厚度增加到一定值时,在中心频率为145GHz处出现透射禁带,并且相位在透射禁带处发生跃变.采用双各向异性媒质理论计算了铜圆环结构的磁导率随频率变化关系,在透射禁带处磁导率为负.对厚金属圆环与金属线一一对应组合的样品,微波沿环面垂直入射时测量到左手透射通带和相位超前,通带带宽达到15GHz,强度为-21dB. 关键词： 负磁导率 垂直入射     

Terahertz wave polarization rotation in bianisotropic metamaterials

Utilizing a polarization sensitive terahertz detection method where the detector is rotated by either 0° or 90° to measure the electric field E_{p, s} (t) of each polarization component, we have characterized the properties of split ring resonators. The strong polarization dependence of the bianisotropic-circular-current-driven and linear-polarization-induced resonances is in excellent agreement with the simulation when the p-polarized terahertz transmission is measured. However, these electromagnetic responses vanish when the s-polarized terahertz transmission is measured. There is only a transmission minimum at 1.64 THz and the terahertz polarization rotation angle of about 90° is observed. The polarized terahertz transmission amplitudes and spectra detected at orthogonal orientations show that these behaviours are probably attributed to the birefringent effect of the sample.     

Parametric oscillator based on a single-layer resonant metamaterial

We perform numerical simulations demonstrating parametric generation and oscillation processes in a single-layer of metamaterial composed of split ring resonators (SRRs). In general, a parametric resonance is achieved by introducing a time-modulation of one of the energy-storing parameters of a resonant system. An individual SRR is a resonant system that can be modeled as an RLC-circuit inductively coupled to an applied time-varying magnetic field. The relatively simple circuit model can be employed to establish the range of parameters providing the growth of parametric oscillations within the SRR medium. We relate the numerically predicted circuit parameters that ensure the buildup of parametric oscillations to those parameters expected to be available in a modified SRR medium, in which lumped element varactors integrated into the SRRs are used as modulable capacitance elements.     

Theoretical study of left-handed behavior of composite metamaterials

We investigate numerically the transmission properties of 1D and 2D composite metamaterials (CMM) consisting of periodically arrangements of circular split-ring resonators (SRR) and wires. The theoretical methods used are the commercially available code Microwave Studio for transmission calculations and a retrieval procedure which gives the effective electric permittivity, , and magnetic permeability, μ, of the system. Our theoretical results are in good agreement with experimental data.     

Parallel laser microfabrication of terahertz metamaterials and its polarization-dependent transmission property

Large-area split ring resonator (SRR) array was fabricated by femtosecond laser micro-lens array lithography at a fast speed. Transmission spectra of the SRR arrays at different incident terahertz wave polarization states were characterized by terahertz time domain spectroscopy. A polarization-dependent transmission property of the SRR array was observed. Polarization-dependent loss (PDL) spectrum was characterized to investigate the polarization properties of the terahertz metamaterials. The PDL characterization can eliminate the substrate effect and provide a flexible platform to study the characteristics of free-standing terahertz metamaterials.     

High-Q series coupled microstrip split-ring resonator device

A quarter wavelength high-Q resonator based on the well-known split-ring resonator (SRR) element in series configuration on a microstrip line is proposed in this letter. The small size is achieved by working at the quasi-static resonant frequency of the SRR. A relatively high value of unloaded-Q of 137 is achieved, which is considerably higher than other microstrip resonators like closed-loop resonators. An equivalent lumped-element circuit model for this new resonator and the ground plane influence in the unloaded-Q particle are studied. A good agreement between simulation, equivalent circuit, and experimental results is shown and discussed, leading to a simple and effective design.     

Extended verification of scaling behavior in split-ring resonators

We present an expanded LC-model for nanoscale split-ring resonators (SRR), including the influence of dielectric host materials. The LC-model is experimentally verified by changing the geometry of the SRR unit cell as well as by optofluidic tuning, where the SRR samples are covered with index oil. The extended model can be used as a general guideline for metal SRR structures with arbitrary dielectric host materials.     

垂直入射条件下金属环的谐振特性

本文研究垂直入射条件下水平极化和垂直极化时金属开口谐振环(split ring resonator, SRR)的电磁响应行为. 通过分析这两种情况下的透射系数、介电常数和磁导率, 发现垂直极化时, SRR可以产生电谐振实现负介电常数, 其频段远高于磁谐振频段; 水平极化时, SRR只能产生磁谐振实现负磁导率, 其频段与水平入射时的SRR的磁谐振频段相对应. 通过仿真对此进行了证明, 并对产生电谐振和磁谐振的原因进行了分析.     

Dual band enhanced transmission through a subwavelength aperture based on two split-ring-resonators

Enhanced transmission through a subwavelength aperture is observed at two frequency bands with employment of two split-ring-resonators (SRR) of different sizes. Each of the SRR is excited to produce resonance and the electric field energy localized in its gap and split region can be coupled into a small hole with a radius of 2.3 mm around the respective resonance frequency. The simulation results show that the energy through the small hole is increased at 5.94 GHz (r/λ ₁=0.045) and 7.03 GHz (r/λ ₂=0.054), where 1042-fold and 88-fold enhancements are obtained, respectively, in comparison with the case of a single isolated hole. Moreover, it is found that placing two identical SRR structures in front of the hole can realize higher enhanced transmission with respect to the case of only one SRR utilized. The electric field coupling-enhancement mechanism is well described by studying the electric field distribution.     

Experimental observation of true left-handed transmission peaks in metamaterials

We report true left-handed (LH) behavior in a composite metamaterial consisting of a periodically arranged split ring resonator (SRR) and wire structures. We demonstrate the magnetic resonance of the SRR structure by comparing the transmission spectra of SRRs with those of closed SRRs. We have confirmed experimentally that the effective plasma frequency of the LH material composed of SRRs and wires is lower than the plasma frequency of the wires. A well-defined LH transmission band with a peak value of -1.2 dB (-0.3 dB/cm) was obtained. The experimental results agree extremely well with the theoretical calculations.