Achieving a multi-band metamaterial perfect absorber via a hexagonal ring dielectric resonator

A multi-band absorber composed of high-permittivity hexagonal ring dielectric resonators and a metallic ground plate is designed in the microwave band.Near-unity absorptions around 9.785 GHz,11.525 GHz,and 12.37 GHz are observed for this metamaterial absorber.The dielectric hexagonal ring resonator is made of microwave ceramics with high permittivity and low loss.The mechanism for the near-unity absorption is investigated via the dielectric resonator theory.It is found that the absorption results from electric and magnetic resonances where enhanced electromagnetic fields are excited inside the dielectric resonator.In addition,the resonance modes of the hexagonal resonator are similar to those of standard rectangle resonators and can be used for analyzing hexagonal absorbers.Our work provides a new research method as well as a solid foundation for designing and analyzing dielectric metamaterial absorbers with complex shapes.     

Magnetic medium broadband metamaterial absorber based on the coupling resonance mechanism

In this paper, we present a design, simulation and experimental measurement of a metamaterial absorber (MMA) in the microwave regime. The proposed MMA structure consists of periodic cross electric resonators separated from the ground metal plane using a magnetic composite layer. The broadband absorption can be ascribed to the periodic cross electric resonators. The anti-parallel currents are observed at the peak frequency on the surface of the MMA and the ground metal plane, respectively, and thus the coupled resonance magnetic field occurs in the magnetic medium resulting in the magnetic loss. The new absorption peak located at 2.8 GHz broadens the whole absorption spectrum. The frequency of this peck is lower than that of the cross resonator of 3.7 GHz, suggesting the distinguish resonance mechanism: the absorbing properties are ascribed to the phase cancellation, Ohmic loss, dielectric loss at the end of the cross pattern, and the magnetic loss caused by the above mentioned coupled magnetic field. The obvious absorption peak at 2.8 GHz is also observed experimentally verifying the simulation result. All these results indicate the proposed MMA structure is promising for microwave absorbing application.     

The Evaluation of Dielectric Resonators Containing H2O or D2O as RF Coils for High-Field MR Imaging and Spectroscopy

Electromagnetic resonators consisting of low-loss dielectric material and/or metallic boundaries are widely used in microwave technologies. These dielectric resonators usually have high Q factors and well-defined field distributions. Magnetic resonance imaging was shown as a way of visualizing the magnetic field distribution of the resonant modes of these resonators, if the dielectric body contains NMR sensitive nuclei. Dielectric resonators have also been proposed as RF coils for magnetic resonance experiments. The feasibility of this idea in high-field MR is discussed here. Specifically, the dielectric resonances of cylindrical water columns were characterized at 170.7 MHz (4 T¹H Larmor frequency), and evaluated as NMR transmit and receive coils. The dielectric resonance of a cylindrical volume of D₂O was used to image a hand at 170.7 MHz. This study demonstrated that MRI is an effective way of visualizing the magnetic field in dielectric structures such as a water cylinder, and can potentially be generalized to solid-state dielectric devices. The possible applications of dielectric resonators other than simple cylindrical volumes in MRI and MR solution spectroscopy at high field strengths are also discussed.     

Low-mode submillimeterwave resonators for piezoelectric surface excitation of sound

The efficient excitation of coherent THz phonons requires an electric field as high as possible at the surface of the piezoelectric transducer material. This can be done below 10¹¹Hz by means of a coaxial resonator or a cavity resonator, respectively. At still higher frequencies new kinds of suitable resonators must be developed. In this paper two types of low-mode submillimeterwave resonators, a dielectric waveguide resonator and a two-layer Fabry-Perot resonator, are investigated with respect to their electromagnetic properties in the THz range.     

Broadband Plasmon-Induced Transparency to a Electromagnetically Induced Absorption Conversion Metastructure Based on Germanium

A Germanium (Ge) metastructure with switching features from broadband plasmon-induced transparency (PIT) and electromagnetically induced absorption (EIA) is presented, which forms a broadband PIT through the near-field coupling between the localized plasmon resonance and the Mie resonance employing four gold cut wires as bright plasmon resonators and four C-shaped dielectric rings as dark plasmon resonators. A wide transparent window above 0.9 is achieved covering from 0.622 to 0.823 THz with a relative bandwidth of 27.8%. In addition, through phase modulation, a magnetic dipole is generated by the constructive interference of the near-field coupling of the three resonators to realize the PIT to EIA conversion. In addition, a grid-like metastructure is introduced to realize the PIT to EIA conversion by phase modulation. To enhance the EIA bandwidth, another layer of C-shaped reflection plate resonator is introduced to increase the dark mode loss, and a layer of frequency selective surface composed of cross-shaped resonators is also used. Thus under the dual effect, a wide absorption window above 0.75 is achieved covering from 0.647 to 0.756 THz with a relative bandwidth of 15.5%.     

氢离子在固体表面掠角散射与能量损失的数值模拟

利用介电响应理论和镜像反射模型对氢离子在固体表面掠角散射和能量损失进行了数值模拟．离子在表面散射时同时受到表面上原子的库仑排斥作用和表面电子气的动力学相互作用,后者是表面电子受运动的正离子扰动所产生,用线性介电响应理论来确定．在高速和低速情况下,分别采用仅与频率有关的局域介电函数和局域场修正介电函数来确定表面电子气产生的动力学相互作用力．计算结果与实验结果作了比较．发现入射速度很低时能量损失随入射角度变化不太明显,而当速度很高时能量损失随入射角度的变大而有所增加. 关键词：     

Numerical analysis of complex impedance and microwave absorption of metamaterials composed of split cut wires on grounded dielectric substrate

The microwave absorption of metamaterials composed of split cut wire (SCW) on grounded dielectric substrate has been investigated on the basis of equivalent transmission line circuit. S-parameters (S ₁₁ and S ₂₁) and input impedance are numerically simulated with variations of the thickness and dielectric loss of the substrate and the geometry of the SCW. Magnetic resonance resulting from antiparallel currents between SCW and ground plane was observed at the frequency of minimum reflection loss. The simulated resonance frequency and reflection loss can be explained well on the basis of the circuit theory of an LC resonator. Analysis of the input impedance of the high impedance surface has shown that perfect absorption can be obtained at the optimized impedance-matching condition, which is dependent on SCW width, thickness and the dielectric loss of the substrate. Better insight into the absorption mechanism of metamaterial absorbers can be attained through the parametric analysis on complex impedance of SCW and its relationship with reflection loss.     

Combined dielectric and plasmon resonance for giant enhancement of Raman scattering

Combined dielectric/metal resonators for colossal enhancement of inelastic light scattering are developed and their properties are investigated. It is shown that a record enhancement factor of 2 × 10⁸ can be obtained using these structures. The dielectric resonators are fabricated on Si/SiO₂ substrates where periodic arrays of square 10- to 200-nm-high dielectric pillars are produced via electron-beam lithography and plasma etching. The lateral size a of the pillars varies between 50 and 1500 nm, and their period in the array is 2a. To make a combined dielectric/metal resonator, a nanostructured layer of silver is deposited onto the fabricated periodic dielectric structure by thermal evaporation. It is established that, for a fixed height of the dielectric pillars, the Raman scattering enhancement factor experiences pronounced oscillations as a function of the period (and size) of the pillars. It is shown that these oscillations are determined by the modes of the dielectric resonator and governed by the relation between the excitation laser wavelength and the planar size of the dielectric pillars.     

Absorption of surface plasmons in a metal-cladding layer-air structure in the terahertz frequency range

The effect of a thin dielectric cladding layer of a metal on the absorption of surface plasmons (SPs) in the terahertz frequency range is studied experimentally and numerically. It is found that, as the radiation wavelength increases, the attenuation of SPs caused by the cladding layer can increase by a factor of ～10⁴ as compared to the absorption of SPs propagating along the unperturbed metal-air interface. Data obtained in experiments with germanium-cladded aluminum specimens using radiation from a terahertz free-electron laser (v = 90 cm^?1) confirm that application of a dielectric cladding on the metal surface causes the SP absorption to increase.     

Energieverlustmessungen an Silber mit hoher Energieauflösung

In energy loss experiments on Ag-films with monochromatic fast electrons the separation of surface and volume loss is demonstrated. The surface loss is found at 3.63 0.02 eV with a half-width of ≈250 mV. The volume loss at 3.78±0.02 eV has a halfwidth of 75±15 mV. The energy loss spectrum is compared with that computed from optical data by means of the dielectric theory and shows good agreement. Also the dependance of the surface loss from the scattering angle and the angle of incidence is in agreement with calculated values. The shift of the volume loss with temperature of ?2.5·10^?4 eV/degree agrees with values obtained from experiments on plasma radiation.     

High-frequency investigation on nonradiative single-mode dielectric resonators

High-frequency investigations of the recently proposed nonradiative single-mode dielectric resonators are presented. In particular, the TE₀₁₁ mode of high-density polyethylene and single-crystal quartz discs has been experimentally characterized, by means of a simple setup, in a frequency interval around 190 GHz. The obtained unloaded merit factors of 1400 for polyethylene and 2480 for single-crystal quartz lead to magnetic field conversion factors of 15 G/W^1/2 and 49 G/W^1/2, respectively. In the latter case, the obtained value represents the state of the art among the room temperature conversion efficiencies. The close agreement with the theoretical predictions demonstrates that the performances of the proposed resonators are only limited by intrinsic factors, as finite conductivity of metallic mirrors and dielectric losses of the employed materials. Ideal performances are then expected in suitably realized nonradiative resonators, also in the high-frequency regime.     

Whispering gallery mode dielectric resonators in EMR spectroscopy above 150 GHz: Problems and perspectives

At frequencies above 150 GHz conventional microwave techniques become unsuitable for electron magnetic resonance (EMR) spectroscopy applications. Quasi-optical techniques are the general frame into which the different approaches to high-field high-frequency EMR (HF²-EMR) have been developed. Beyond the single-pass approach the existing HF²-EMR spectrometers make use of Fabry-Perot resonators. In this paper the use of an alternative resonating structure on the basis of the use of whispering gallery mode dielectric resonators is shown. The physics of these resonators and their implementation in a HF²-EMR probe head are described. Their use in HF²-EMR experiments performed on standard samples is described and the results of these experiments critically discussed in order to have insight into the open problems of the employed spectrometer; the more promising solutions to these problems, actually under development, are briefly outlined.     

Study on Planar Whispering Gallery Dielectric Resonators. II. A Multiple-Band Device

The basic theory underlying the realization of simple multiple-band non-homogeneous dielectric resonators, whose spectral response is the overlap of single-resonator frequency bands, is developed exploiting a general approach discussed in the previous companion paper. The limit frequencies of the proposed devices, given only by the dielectric properties of the involved materials, can differ in principle by several decades. Experimental confirmations have been obtained on a composite structure built up with teflon and polyethylene; as predicted by the theory, the overall band includes frequencies which range about from 20 GHz to more than 400 GHz, when high frequency resonances are selectively excited. The localization of the higher frequency radiation between the positive steps of the dielectric constant, which is the basic properties of these non-homogeneous resonators, has been experimentally verified by mapping the electromagnetic field intensity. Possible applications of multiple-band Whispering Gallery dielectric resonators are finally outlined.     

石英中Al3+-空穴取向变化弛豫过程中的红外发散响应

基于α石英的晶体结构，将红外发散响应模型和双势阱模型应用到含Ａｌ杂质的α石英中Ａｌ³⁺－空穴的取向变化弛豫过程，研究其低温介电损耗特性，结果表明Ｔ＜６．５Ｋ时，介电损耗的主要贡献来自于单声子助隧道弛豫过程；Ｔ＞１０Ｋ时，主要贡献来自于热跃迁弛豫过程；而在中间温区，介电损耗是两种过程的迭加，同一弛豫体不同的弛豫过程对应于不同的红外发散响应，还讨论了同一弛豫体引起的超声弛豫损耗。 关键词：     

Effect of K+ ion on the dielectric properties of metallo organic L-alanine cadmium chloride single crystals

The single crystals of pure and Potassium doped L-alanine cadmium chloride (LACC), a metallo organic nonlinear optical material is grown by a slow evaporation technique. The grown crystals were confirmed by single crystal, powder XRD analyses and atomic absorption studies. Dielectric measurements were carried out for different frequencies at different temperatures. The dielectric constant decreases due to the introduction of large ionic radius K⁺ ion in the pure LACC crystal. The low dielectric constant and dielectric loss suggest that it can be used as inter-metal dielectric material.     

Surface plasma resonances in small spherical silver and gold particles

With 51 keV electrons surface plasma losses have been investigated on small spherical Ag and Au particles embedded in a medium with dielectric constant? _a=2.37. The surface loss of particles with radii of about 50 Å is found at 2.99 ±0.03 eV for silver and 2.34±0.03 eV for gold being in good agreement with calculated values. For larger radii the loss shifts to higher energy values which agrees qualitatively with the theory of Fujimoto and Komaki for the free electron gas. The optical extinction bands of the particles have been measured, too. Their maxima are shifted to lower energies, in accordance with Mie's theory, if the particle size increases sufficiently. Comparison of the energy values of the “electric” extinction bands with those of the surface plasmons shows, that they correspond in theory and experiment, if the particles are small. This confirms, that the optical colloid extinction of Ag and Au may be interpreted as plasma resonance absorption and emission of the particles.     

ZnO压敏陶瓷中缺陷的介电谱研究

从理论上证明了介电松弛过程在介电谱上等效于电子松弛过程,认为室温下10⁵Hz处特征损耗峰起源于耗尽层处本征缺陷所形成的电子陷阱.在-130—20℃范围内测量了三种配方ZnO陶瓷的介电频谱,发现ZnO压敏陶瓷室温下10⁵Hz处的特征损耗峰在低温下分裂为两个特征峰,认为它们起源于耗尽层中的本征缺陷(锌填隙或/和氧空位)的电子松弛过程.发现ZnO-Bi₂O₃二元系陶瓷特征峰仅仅由锌填隙引起,而ZnO-Bi_{2关键词： ZnO压敏陶瓷 本征缺陷 介电谱 热处理}     

A wideband metamaterial absorber based on a magnetic resonator loaded with lumped resistors

<正>A wideband metamaterial absorber（MA） based on a magnetic resonator loaded with lumped resistors is presented. It is composed of a one-dimensional periodic array of double U-shaped structured magnetic resonators loaded with lumped resistors,a dielectric substrate,and a metal plate.We simulated,fabricated,measured,and analyzed the MA. The experimental results show that the reflectance(S₁₁) is below -10 dB at normal incidence in the frequency range of 7.7 GHz-18 GHz,and the peak value is about—20 dB.Simulated power loss density distributions indicate that wideband absorption of the MA is mainly attributable to the lumped resistors in the magnetic resonator.Further investigations indicate that the distance between two unit cells along the magnetic field direction significantly influences the performance of the MA.     

Predistortion techniques for synthesizing coupled microring filters with loss

A general method based on the predistortion technique is presented for determining the optimum coupling parameters of multiple-coupled microring filters in the presence of known uniform resonator loss. The technique involves predistorting the filter transfer function by shifting its poles and zeros to compensate for loss in the microring resonators due to material absorption, surface roughness scattering, coupling loss and bending loss. It is shown that by sacrificing some in-band insertion loss, both the amplitude and group delay responses of the filter can be recovered when loss is present. Application of the method to synthesizing lossy microring filters in both the digital z-domain and analog s-domain will be presented. The proposed technique is general in that it can be applied to both amplitude and phase filters constructed of coupled microresonators in the most general two-dimensional coupling topology.     

Metamaterial absorber and extending absorbance bandwidth based on multi-cross resonators

In this paper, we report the design, simulation, and measurements of a broadband metamaterial absorber (MA) based on a periodic array of multi-layer cross-structure resonators. A perfect narrowband MA consists of cross-structure resonator, dielectric substrate, and continuous metal films, and the absorption frequency can be tunable by changing the geometrical parameters based on L-C resonance circuit theory. Furthermore, the absorption band of our design is effectively extended by simply stacking several such structural layers with different geometrical dimensions. Finally, the 4-layer cross-structure MA is only 2 mm, which can achieve a full width at half maximum (FWHM) bandwidth of 2 GHz by numerical simulations, and 90 % bandwidth of 1.9 GHz by experiments.