左手材料的反射特性与负折射率行为

利用平板波导法研究了不同入射角度下周期排列开口谐振环负磁导率材料、周期排列金属杆负介电常数材料以及左手材料微波反射特性，并利用劈尖法研究了左手材料的负折射特性.实验结果表明：负磁导率材料反射率曲线形成反射峰，其对应的反射峰频率与材料的谐振频率一致；负介电常数材料反射率接近0dB；左手材料出现单个反射较小的反射峰，其峰值反射率随入射角度的增大而变大，即反射能力增强，且反射峰与透射峰有相对频移.劈尖法测量还表明，左手材料在9800MHz频率附近出现负折射现象，其折射率n为-0.796. 关键词： 左手材料 反射 负折射率     

Magnetic metamaterials at telecommunication and visible frequencies

Arrays of gold split rings with a 50-nm minimum feature size and with an LC resonance at 200 THz frequency (1.5 microm wavelength) are fabricated. For normal-incidence conditions, they exhibit a pronounced fundamental magnetic mode, arising from a coupling via the electric component of the incident light. For oblique incidence, a coupling via the magnetic component is demonstrated as well. Moreover, we identify a novel higher-order magnetic resonance at around 370 THz (800 nm wavelength) that evolves out of the Mie resonance for oblique incidence. Comparison with theory delivers good agreement and also shows that the structures allow for a negative magnetic permeability.     

Theory of light amplification in active fishnet metamaterials

We establish a theory that traces light amplification in an active double-fishnet metamaterial back to its microscopic origins. Based on ab initio calculations of the light and plasmon fields we extract energy rates and conversion efficiencies associated with gain and loss channels directly from Poynting's theorem. We find that for the negative refractive index mode both radiative loss and gain outweigh resistive loss by more than a factor of 2, opening a broad window of steady-state amplification (free of instabilities) accessible even when a gain reduction close to the metal is taken into account.     

光波段多频负折射率超材料

从光波段圆孔形双鱼网结构的负折射材料模型出发,采用基于有限积分技术的CST软件系统研究了原胞结构的改变对负折射行为的影响.数值仿真结果表明,对原胞结构做微小调节也可获得负折射率频带的增大效应.将双鱼网结构改为阶梯形孔洞和半球形孔洞结构,可以在更多的频段里出现负折射率,并且谐振频率发生了一定的红移.半球形孔洞的双鱼网结构可以方便地用化学模板法制备,这为从实验上实现红外及可见光波段的多频负折射材料提供了一种简单可行的方法. 关键词： 负折射率 多频段 双鱼网结构     

Influence of the substrate on negative index fishnet metamaterials

We analyze numerically the influence of the dielectric substrate on the optical response and effective macroscopic parameters of fishnet metamaterials. We show that the substrate can suppress the region of the negative refractive index due to the weakening of the gap plasmon associated with the magnetic resonance of the structure. We also demonstrate that the effect of substrate vanishes when the number of the functional layers increases.     

Characterization,slab-pair modeling and phase analysis of circular fishnet metamaterials

Planar metamaterials, which have incident to normal plane excitation unlike SRR-type structures and that are easily fabricated in multilayer form, have received great interest in recent years. In this paper, one-dimensional and polarization independent circular fishnet metamaterials and their equivalent discontinuous slab-pair modeling for tuning resonance frequencies are introduced. After the numerical and experimental demonstration of the inclusions, the standard retrieval characterization methods and the correspondent/related backward-wave propagation observation are realized in order to check the physical explanation mentioned in the paper. In addition, a detailed phase analysis is performed in order to demonstrate the application of the suggested structure as a phase compensator.     

Possible implementation of epsilon-near-zero metamaterials working at optical frequencies

Metamaterials (MTMs) exhibiting a near-zero real part of the permittivity function in a given frequency range have been demonstrated to be useful in several application fields, including field localization and focusing. So far, however, the realistic implementations of such materials working at optical frequencies and exhibiting a reasonable level of losses are rare. In this work, we propose a possible implementation of optical epsilon-near-zero (ENZ) MTMs based on the employment of an array of core-shell nano-spheres embedded in a dielectric medium. The core of the nano-spheres and the host medium are both made of silica, while the shell is formed by a plasmonic material (i.e. silver). Using classical homogenization formulas, we show that it is possible to design the array in such a way to exhibit near-zero values of the effective real permittivity with relatively low losses at optical frequencies. These results are supported and confirmed by proper full-wave simulations and design examples.     

Theoretical and experimental investigation of negative index fishnet metamaterial multilayers in the Q-band

We investigate the electromagnetic properties of stacked metamaterial layers composed of modified fishnet structures at around 40?GHz. The designed planar multilayer exhibits theoretically high transmission of ?0.5?dB, low reflection, and a large negative index bandwidth with a record figure of merit higher than 36 at 38.5?GHz. Good agreement between simulation and measurement results is observed. We also verify experimentally the resolution of the negative index metamaterial stack and the focusing ability of the plano-concave lens configuration.     

An improved fishnet three-dimensional metamaterial with multiband left-handed characteristics at terahertz frequencies

We present the design of a multiband left-handed three-dimensional (3D) metamaterial based on improved fishnet structure at terahertz frequencies. The design realizes a three-dimensional material by mechanical stacking of multiple layers. The electromagnetic properties of the metamaterial have been investigated by numerical simulation. The results show that simultaneously negative values of permittivity, permeability and refractive index are found around the frequencies of 0.73, 0.85 and 1.12 THz for the electromagnetic wave normal incidence. The proposed metamaterial with independent polarization and compact effect offers a way to develop THz 3D materials and devices suitable for multifrequencies.     

光频三维各向同性左手超材料结构单元模型的仿真设计

基于树枝结构单元思想,提出了光频三维各向同性左手超材料球刺结构单元模型.采用金属的Drude原理,运用等效媒质理论,仿真研究了结构的电磁响应特性.在晶格间距与工作波长的比值满足等效媒质理论要求的前提下,通过调节结构单元的几何参数,发现它能在光波段出现介电常数和磁导率同时为负值的区域,实现折射率小于零的左手超材料响应特性.该模型结构简单,具有各向同性特点.研究结果为采用"由下向上"方法制备光波段的三维左手超材料指出了新的途径. 关键词： 球刺模型 光波段 三维 各向同性     

微波超材料隐形结构及其新型快速实验方案

基于Maxwell方程组在坐标变换下的协变性,利用柱坐标变换以及开口谐振环周期排列而形成的超材料设计了一个工作于10.14GHz频率下的隐形结构.采用一种新型快速实验方案测量微波信号强度.实验结果表明在外围增加了隐形结构之后,圆柱金属的阴影和散射效应都被削弱,得到了预期的隐形效果.该实验系统降低了系统的结构复杂性并缩短了实验周期,为该领域的研究提供了一种新的简单有效的实验方案.     

Effective medium representation and complex modes in 3D periodic metamaterials made of cubic resonators with large permittivity at mid-infrared frequencies

We review some of the techniques that lead to the effective medium representation of a three-dimensional (3D) periodic metamaterial. We consider a 3D lattice of lead telluride cubic resonators at mid-infrared (MIR) frequencies. Each cubic resonator is modeled with both an electric and a magnetic dipole, through a method called the dual dipole approximation. The electric and magnetic polarizabilities of a cubic resonator are computed via full-wave simulations by mapping the resonator's scattered field under electric/magnetic excitation only to the field radiated by an equivalent electric/magnetic dipole. We then analyze the allowed modes in the lattice, with transverse polarization and complex wavenumber, highlighting the attenuation that each mode experiences after one free space wavelength. We observe the presence of two modes with low attenuation constant, dominant in different frequency ranges, able to propagate inside the lattice: this allows the treatment of the metamaterial as a homogeneous material with effective parameters, evaluated by using various techniques. We then show that the metamaterial under analysis allows for the generation of artificial magnetism (i.e., relative effective permeability different than unity, including negative permeability with low losses) at MIR frequencies.     

Absorptive optical bistability at microwave frequencies

Summary Intrinsic absorptive bistability and hysteresis cycles in a steady-state regime are obtained at microwave frequencies. A theory is presented which takes into account the Gaussian shape of the fundamental mode excited into the resonator and is compared with the experimental results.

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Riassunto Sono state osservate la bistabilità assorbitiva intrinseca e l’isteresi nel caso stazionario, operando alle frequenze delle microonde. Si presenta una teoria che tiene conto della forma gaussiana del modo fondamentale eccitato nel risuonatore e che è confrontata con i risultati sperimentali.

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Резюме На микроволновых пастотах наблюдаются внутренняя абсорбционная бистабильность и гистерезисные циклы в стационарном режиме. Предлагается теория, которая учитывает гауссову форму основной моды, возбужденноц в резонаторе, и которая сравнивается с экспериментальными резулстатами.

     

Nuclear magnetic resonance at microwave frequencies

Instrumental requirements for pulsed NMR studies of hyperfine splittings in rate-earth metals and compounds are considered, with particular attention to factors affecting the design of the specimen cavity. A spin-echo spectrometer for hyperfine studies in the 2 to 8 GHz frequency range is described.     

Negative refractive index in chiral spiral metamaterials at terahertz frequencies

A three-dimensional chiral metamaterial consisting of arrays of the multi-layered mutually twisted metallic spirals is proposed. We theoretically demonstrate such a chiral spiral structure exhibiting negative refractive index at terahertz frequencies. The chirality with varied refractive index can be obtained by change of configurations of the structure. The presented design offers flexibility for investigation of electromagnetic properties of chiral metamaterials in the terahertz regime and thus leads to a unique route to terahertz device applications.     

Transmission properties of electromagnetic metamaterials: From split-ring resonator to fishnet structure

We experimentally presented the electromagnetic behavior of transformative magnetic metamaterials: from the early invented split-ring resonator to its improvement, the cut-wire pair, for providing a negative magnetic permeability. By adding the continuous wire to cut-wire pair structure, the left-handed (LH) transmission made by doubly negative permittivity and permeability was demonstrated in combined structure. Interestingly, until the width of cut-wire pair increases to be physically merged with the adjacent continuous wire, in other words, to form the so-called fishnet structure, the LH behavior is still observed. This result indicates that in a broad sense, the essence of the electromagnetic response in the fishnet structure is similar to that in a combined structure. Our experimental results show a good agreement with previous theoretical study.     

Theoretical and experimental investigations on erbium-doped fiber amplifiers

A comprehensive model of erbium-doped fiber amplifiers (EDFA) is proposed in this paper, taking exited-state absorption (ESA), amplified spontaneous emission (ASE), background absorption, mode field, and Erbium ion radial distribution into account. Computer programs capable of handling forward, backward, and different wavelength bidirectional pumping are developed. Among other things, it is found that under typical conditions, Gaussian field approximations may cause considerable errors. The optimum V-value for small signal gain is much smaller than that for minimum Gaussian or Desurvire spot size; ASE propagating opposite to the pimp beam can be several times higher than at propagating along the pump beam, resulting in a faster-than-linear decrease of the pump power at its beginning portion. Experimental results of domestic-made erbium-doped fibers pumped by 980 nm and 1480 nm laser diodes are also given.     

Nonlinear waves in waveguides at microwave frequencies

The process of nonlinear-wave formation in a waveguide at microwave frequencies is studied theoretically. It is shown that a nonlinear wave in the form of a pulsating soliton is formed under conditions of uniform spectrum-line broadening. The possibility of experimental observation of the phenomenon is considered.Tbilisi State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 35, No. 5, pp. 475–480, May, 1992.     

External reflection from dielectrics at microwave frequencies

3 cm microwave are being used to investigate the beam shifts which occur for external reflection from a planar dielectric interface. The adaptation of a planar wave theory to the microwave case has proved successful in predicting the magnitudes and direction of such shifts.