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.     

High frequency single-mode resonators for EPR spectroscopy enabling rotations of the sample about two orthogonal axes

A novel single-mode resonant structure which enables the rotation of the sample about two orthogonal axes is investigated in view of electron paramagnetic resonance applications. The proposed solution is based on cylindrical non-radiative resonators laterally loaded by the sample holder. The resulting structure can still operate in non-radiative regime, although no longer rotationally invariant. These theoretical predictions, based on symmetry considerations, are confirmed by means of a finite element numerical modelling. Theoretical and computational results are then substantiated by experimental investigations at millimeter wavelengths. As a result, a single-mode resonator which enables all the relevant rotations of the sample is demonstrated at millimeter wavelengths for the first time. In this resonator the intensity of the microwave field on the sample and its orientation with respect to the static magnetic field can be kept constant during the rotations. Therefore, a complete characterization of anisotropic systems is possible at the highest sensitivity, without the need of split-coil magnets. Possible applications at very high frequencies are discussed.     

High-frequency EPR applications of open nonradiative resonators

A new class of open single-mode cavities, the nonradiative (NR) resonators, has recently been proposed in order to overcome the limitations of standard cylindrical cavities and Fabry-Perot resonators at millimeter wavelengths. This paper presents the first applications of a NR resonator in W-band pulsed electron paramagnetic resonance spectroscopy. It consists of a cylindrical cavity having a lateral aperture that represents about 35% of its total height. Electron-spin-echo measurements performed on different samples show that the signal-to-noise ratio and the optimal pulse length obtained with the proposed device are comparable to those obtained with the closed cavity used in the commercial W-band spectrometer, at both cryogenic and room temperature. Similar results have been obtained for paramagnetic species optically activated by means of an optical fiber inserted in the aperture of the resonator. The insertion losses estimated for the probe employed with the NR resonator are higher than those of the commercial probe, hence, demonstrating that the proposed cavity holds the promise of improved resonator performance.     

EPR Uniform Field Signal Enhancement by Dielectric Tubes in Cavities

The dielectric tube resonator (DTR) for electron paramagnetic resonance spectroscopy is introduced. It is defined as a metallic cylindrical TE₀₁₁ microwave cavity that contains a dielectric tube centered on the axis of the cylinder. Contour plots of dimensions of the metallic cylinder to achieve resonance at 9.5 GHz are shown for quartz, sapphire, and rutile tubes as a function of wall thickness and average radius. These contour plots were developed using analytical equations and confirmed by finite-element modeling. They can be used in two ways: design of the metallic cylinder for use at 9.5 GHz that incorporates a readily available tube such as a sapphire tube intended for NMR and design of a custom procured tube for optimized performance for specific sample-size constraints. The charts extend to the limiting condition where the dielectric fills the tube. However, the structure at this limit is not a dielectric resonator due to the metal wall and does not radiate. In addition, the uniform field (UF) DTR is introduced. Development of the UF resonator starting with a DTR is shown. The diameter of the tube remains constant along the cavity axis, and the diameter of the cylindrical metallic enclosure increases at the ends of the cavity to satisfy the uniform field condition. This structure has advantages over the previously developed UF TE₀₁₁ resonators: higher resonator efficiency parameter Λ, convenient overall size when using sapphire tubes, and higher quality data for small samples. The DTR and UF DTR structures fill the gap between free space and dielectric resonator limits in a continuous manner.     

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.     

Dynamic nuclear polarization of liquid helium-three

At Kazan State University a number of experiments have been proposed using solid paramagnetic compounds to produce nuclear polarization in liquid helium-three, for which the nuclear resonance frequency isf _n=32.4 MHz T^?1. This note suggests the application of magnetic resonance in antiferromagnetic compounds to this problem. In addition to a range of 3d ions, two compounds of lanthanide ions which become antiferromagnetic at liquid helium temperatures are discussed. Also, electron paramagnetic resonance is proposed using trivalent erbium ions at low abundance in metallic silver.     

A High-Conversion-Factor, Double-Resonance Structure for High-Field Dynamic Nuclear Polarization

This contribution presents a novel design of a double-resonance structure for high-field dynamic nuclear polarization operating at 95 GHz and 144 MHz, in which a miniaturized radiofrequency coil is integrated within a single-mode nonradiative dielectric resonator. After a detailed discussion of the design principles, the conversion factors of this system are determined by means of microwave and radiofrequency measurements. The obtained results, 1.68 mT/W^1/2 for the microwave conversion factor and 0.8 mT/W^1/2 for the radiofrequency conversion factor, represent the state-of-the-art among the double-resonance structures. Simultaneous electron paramagnetic resonance and liquid-state ¹H nuclear magnetic resonance experiments are performed on samples of nitroxide radical 2,2,6,6-tetramethylpiperidine-1-oxyl dissolved in a mixture of water and dioxane. A maximum dynamic nuclear polarization enhancement of about ?16 is obtained at a microwave power of 70 mW with a radical concentration of 10 mM in nanoliter-sized sample volumes. These results are discussed in view of further improvements and applications of the proposed double-resonance structure.     

应用于气体传感器的具有铝/金电极的单模式两端对声表面波谐振器

为改善气体传感器性能,通过器件优化设计获得了一种应用于气体传感器的具有低损耗、高品质因子(Q)的单模式两端对声表面波(SAW)谐振器。该谐振器由两个换能器、分置于换能器两边的短路栅反射器以及在换能器之间分布的用于敏感膜镀膜的约2.5mm金属薄层构成。谐振器采用铝/金双层电极以降低测试气体环境的腐蚀影响。利用经典耦合模(COM)理论对器件性能进行了仿真以提取优化的结构设计参数。基于仿真结果,实验研制了基于300MHz频率的新型铝/金电极SAW两端对谐振器,测试结果显示所研制器件具有较低损耗(〈7dB),较高Q值(-3000)以及单一谐振模式的特点,并且,以所研制的新型谐振器为频率控制单元的谐振器型振荡器表现出良好的频率稳定度(t15Hz/h),这对于改善气体传感器的检测下限及稳定性等性能指标具有重要意义。      

Development of two-port surface acoustic wave resonator with Al/Au electrodes for gas sensing

Simple and efficient surface acoustic wave(SAW)two-port resonators with low insertion loss and high Q-values on ST-X quartz substrate using a corrosion-proof A1/Au-stripe electrode structure are developed for gas sensing.It was composed of two shorted grating reflectors and adjacent intedigital transducers(IDT),and an active metal film in the cavity between the IDTs for the sensitive film coating.The devices are expected to provide good protection towards metal electrode for gas sensors application in chemically reactive environments.Excellent device performance as low insertion loss,high Q factor and single-mode are achieved by carefully selecting the metallic electrode thickness,cavity length and acoustic aperture.Prior to fabrication,the coupling of modes(COM)model was performed for device simulation to determine the optimal design parameters.The fabricated single-mode SAW resonator at operation frequency of 300 MHz range exhibits matched insertion loss of~6.5 dB and loaded Q factor in the 3000 range.Using the fabricated resonator as the feedback element,a dualresonator-oscillator with excellent frequency stability(0.1 ppm)was developed and evaluated experimentally,and it is significant for performance improvement of SAW gas sensor.     

Thermal tunability in terahertz metamaterials fabricated on strontium titanate single-crystal substrates

We report an experimental demonstration of thermal tuning of resonance frequency in a planar terahertz metamaterial consisting of a gold split-ring resonator array fabricated on a bulk single-crystal strontium titanate (SrTiO?) substrate. Cooling the metamaterial starting from 409 K down to 150 K causes about a 43% shift in resonance frequency, and there is very little variation in resonance strength. The resonance shift is due to the temperature-dependent dielectric constant of the strontium titanate. The experiment opens up avenues for designing tunable terahertz devices by exploiting the temperature-sensitive characteristic of high dielectric constant substrates and complex metal oxide materials.     

温控太赫兹调制器多频带调制特性

基于开口谐振环结构设计了多频带太赫兹波调制器,并在谐振环的开口处及两侧均填充温敏介质锑化铟(InSb),研究了锑化铟的电磁性质随温度的变化、等效电感的组数对共振频带数目的影响以及锑化铟不同的填充方式对太赫兹波调制特性随温度的变化规律。研究结果表明,当环境温度从160 K上升到350 K时,锑化铟的载流子浓度和等离子体频率逐渐增大,然而等效介电常数却不断减小;每增加一组等效电感,太赫兹波调制器都会相应的增加一个共振频带;在调制器开口处和两侧均填充锑化铟时,当环境温度在160～350 K变化时,温度对太赫兹波的共振频率和共振幅度的调制效果比仅在开口处或者两侧填充锑化铟时更明显,且随着温度的升高,每个共振频带所对应的共振频率均明显增大。     

Optical response of resonator induced plasmon filters: Nanometric diatomic structures

We investigate the optical response of plasmon filters, which are composed of a diatomic chain of metallic nanoclusters along which a resonator, composed of one or two metallic nanoclusters, is coupled vertically. Taking into account the resonator, we show that the transmission amplitude T$T$ of the electromagnetic radiation may display dips when the geometrical parameters are chosen properly. The presence of a resonator composed of one metallic nanocluster yields a dip at the cluster resonance frequency. When the resonator is composed of two nanoclusters, then if the nanoclusters are of the same material, two dips emerge as a consequence of the splitting of the dip of the one-nanocluster resonator. If the resonator is of two different materials, then we obtain two dips near the resonance frequencies of the nanoclusters. These dips appear when the separations between nanoclusters are properly adjusted. Such a device may be used to transfer directionally the electromagnetic radiation. In the limit of equal atomic nanoclusters we reproduce the monoatomic chain results.     

Ultra-Compact, Subwavelength and Single-Mode Cavity Resonator

We propose an ultra-compact subwavelength cavity resonator that is composed of a bilayer of metamaterials, one with only negative dielectric constant and the other one with only negative permeability. It is shown that the total thickness of the resonator can be much smaller than the resonant wavelength. In addition, this resonator is always single-mode.     

Strip dielectric wave guide antenna-for the measurement of dielectric constant of low-loss materials

The value of dielectric constant are the most important parameters in material science technology. In micro-wave and millimeter wave circuits using dielectric materials the values of this parameters should be known accurately. It is observed that the number of methods are reported in litrature, however these methods impose difficulties in experimentation and are not very accurate. In this paper a novel approach to the measurement of the dielectric constant of low loss materials at micro-wave and millimeter wave frequencies has been discussed. In this method by using antenna theory, a metallic strip dielectric guide is taken in to constideration and band reject phenomenon of dielectric antenna is used. Frequency response of an antenna in band reject mode is a function of the dimensional parameters, such as the metallic strip period, the profile of the metallic strip and the dielectric constant of the material used. Hence if one measure the frequency responce of the antenna in band reject mode, the dielectric constant of the material is determined provided all other parameters are known. This method gives a direct measure of dielectric constant and is quite accurate as computer techniques are used for evaluating the dielectric constant. This method verified experimentally also.     

Waveguide resonator with high quality factor excited through the subwavelength slit

The excitation of standing TM waves by a subwavelength slit in the single-mode waveguide resonator with the metallic borders is considered. In this structure, the input energy through the slit strongly depends on the incident wavelength. Consequently, the Q-factor of the system computed by the finite element method reaches the values of 10⁴ in the near infrared region. The significant change of the scattered wave energy is achieved by the changing of dielectric constant value by 5 ×10^?4 or the thickness of resonator by 0.5 nm. The sensitivity of system to the parameters of structure allows one to use the system in various applications such as devices of optical bistability, modulators, and optical sensors of vibration, displacement and temperature.     

一种高增益低雷达散射截面的新型圆极化微带天线设计

设计了一种基于人工电磁材料覆层的高增益低雷达散射截面(radar cross section, RCS)圆极化微带天线. 人工电磁材料覆层是由介质板及其两侧的人工周期表面构成, 上表面是加载集总电阻的方环贴片, 具有宽带吸波特性; 下表面是开条带缝和圆环缝的金属贴片, 具有部分反射特性. 将其加载到圆极化微带天线上方, 通过覆层上表面的电阻可吸收入射的雷达波, 结合下表面与接地板构成Fabry-Perot谐振腔的多次反射, 可实现圆极化微带天线辐射和散射性能的同时改善. 实测结果表明: 加载人工电磁材料覆层后, 天线的相对轴比带宽由5.9%扩展为7.1%; 天线增益在整个工作频带内都得到了提升, 最大提高了6.61 dB; 天线RCS在宽频带宽角域内实现了明显的减缩, 在天线工作频带内也实现了3 dB以上减缩. 实测结果与仿真结果符合较好.     

The narrow band THz filter in metallic photonic crystal slab framework: Design and investigation

A band pass THz filter has been presented by the aid of coupling between the waveguides and ring resonator based on metallic photonic crystal slab. The rows of rods are missed and remaining rods in the rows play as a reflector then the position of central rods are rearranged as a ring, the radius of reflector rods and ring resonator is modified in order to optimize the resonance frequency and Q-factor. In addition, 3D-FDTD method is used for simulation results. One of the predominant features of the proposed structure for THz filter is its capability of obtaining a Q-factor of about 333 in the 1 THz resonance frequency with the normalized peak transmission of higher than 0.7, because of high Q-factor cavity based on ring resonator.     

基于小型化结构的各向同性负磁导率材料与左手材料

提出了一种双面环各向同性结构单元模型, 理论与实验研究了其微波电磁谐振行为. 结果表明: 在电磁波平行入射和垂直入射条件下, 该结构可在同一频段实现磁谐振, 且在谐振频段磁导率为负; 当电磁波以不同角度斜入射时, 其产生负磁导率频段也保持不变, 即该结构的电磁特性不依赖于入射角度, 双面环结构具有各向同性的特点. 将双面环结构与金属线结构组合, 该组合结构具有负折射率. 另外,双面环结构还具有小型化的优点, 在不增加结构单元几何尺寸的情况下, 通过在结构单元中引入金属化过孔的方法, 增加结构单元的电长度, 可使谐振频率大幅度地向低频方向移动, 使其在低频工作时 仍保持小型化的优点. 在双面环结构中引入金属化过孔技术可使谐振单元的几何尺寸减小50%, 在微波器件、滤波器、天线等领域有广阔的应用前景.     

Hybrid photonic surface-plasmon-polariton ring resonators for sensing applications

We introduce a hybrid photonic surface plasmon ring resonator which consists of a silicon nitride (Si₃N₄) dielectric traveling-wave ring resonator vertically coupled to a thin layer of metallic strip ring resonator made of Silver (Ag) on top. The cladding is assumed to be porous alumina on top of the metal layer, which provides more surface area for the adsorption of target molecules and their efficient interaction with the surface plasmon wave excited at the metal-cladding interface. Simulations show that this hybrid structure has a large refractive index sensitivity due to the excitation of surface plasmon waves and also a relatively narrow resonance linewidth due to the large quality factor of the photonic ring resonator. The Finite Element method is used to systematically design the hybrid structure and to investigate the performance of the hybrid resonator as a refractive index sensor. The proposed structure is very compact and can be implemented on a chip in an integrated platform. Thus, it can be used for lab-on-a-chip sensing applications and is capable of being spectrally and spatially multiplexed for muti-analyte sensing.     

Oscillation characteristics of a coupled unidirectional ring resonators with photorefractive crystals

For a coupled unidirectional photorefractive ring resonator (UPRR), the oscillation characteristics have been studied in details in terms of the photoconductive and dielectric constant of the photorefractive (PR) crystals under the assumption of the plane-wave approximation based on non-degenerate two-wave mixing in the photorefractive materials. It has been found that the steady oscillations are possible when the two resonators oscillate independently. Using the plane-wave approximation and steady state oscillation conditions, the effect of the frequency detuning, photoconductivity and dielectric constant of the PR crystals on the relative intensity and frequency of oscillation of the secondary resonator in the coupled UPRR have been studied. It has been found that the relative oscillation frequency of the secondary resonator could be enhanced by selecting PR crystal A of higher absorption strength relative to PR crystal B and the higher photoconductivity of the crystals B as compared to that of the crystal A. Due to the non-reciprocal energy transfer between the oscillating beams and the additional PR phase-shift in the PR crystals A and B, the magnitude of the relative oscillation frequency of the secondary resonator could be controlled by the absorption strength, dielectric constant and photoconductivity of the two crystals.