纳米三明治结构光子超材料中电磁场振荡行为研究

对金属-介质-金属的三层纳米三明治结构中的磁性等离子体模式特性进行了时域有限差分数值计算分析,并给出该类谐振器的品质因数变化规律.基于纳米三明治结构中的磁性等离子体模式对该种结构进行设计,研究几何构型参数以及介质材料折射率对磁性等离子体模式的调控规律.以电感电容等效电路近似理论对纳米三明治结构磁性等离子体模式谐振频率的变化规律进行理论解释,理论解释与数值计算结果符合很好.同时还研究了纳米三明治振荡器对电磁能量的限制,并从热和辐射两方面分析了该类结构的品质因数变化情况,可为基于磁性等离子体模式的新型波导及激光器结构的设计提供指导.     

Selective mode suppression in microstrip differential lines by means of electric-LC (ELC) and magnetic-LC (MLC) resonators

In this paper, it is demonstrated that the so-called electric-LC (ELC) resonators, and their dual counterparts, the magnetic-LC (MLC) resonators, are useful for the selective suppression of either the differential or the common mode in microstrip differential lines. The key point to mode suppression is the alignment of the resonator with the electric (differential mode) or magnetic (common mode) wall of the line. It is shown that by simply rotating the resonators 90^° we can selectively choose the suppressed mode in the vicinity of the resonator’s fundamental resonance frequency. The theory is validated through full-wave electromagnetic simulation, the lumped element equivalent circuit models of the proposed structures and experimental data.     

Piezoelectric resonators with mechanical damping and resistance in current conduction

A novel design method for high Q piezoelectric resonators was presented and proposed using the 3-D equations of linear piezoelectricity with quasi-electrostatic approximation which include losses attributed to mechanical damping in solid and resistance in current conduction. There is currently no finite element sofware for estimating the Q of a resonator without apriori assumptions of the resonator impedance or damping. There is a necessity for better and more realistic modeling of resonators and filters due to miniaturization and the rapid advances in frequency ranges in telecommunication. We presented new three-dimensional finite element models of quartz and barium titanate resonators with mechanical damping and resistance in current conduction. Lee, Liu and Ballato’s 3-D equations of linear piezoelectricity with quasi-electrostatic approximation which include losses attributed to mechanical damping in solid and resistance in current conduction were formulated in a weak form and implemented in COMSOL. The resulting finite element model could predict the Q and other electrical parameters for any piezoelectric resonator without apriori assumptions of damping or resistance. Forced and free vibration analyses were performed and the results for the Q and other electrical parameters were obtained. Comparisons of the Q and other electrical parameters obtained from the free vibration analysis with their corresponding values from the forced vibration analysis were found to be in excellent agreement. Hence, the frequency spectra obtained from the free vibration analysis could be used for designing high Q resonators. Results for quartz thickness shear AT-cut and SC-cut resonators and thickness stretch poled barium titanate resonators were presented. An unexpected benefit of the model was the prediction of resonator Q with energy losses via the mounting supports.     

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.     

Analysis of open resonators

A multiple reflection approach is employed to examine the properties of the lowQ open resonators commonly used in high power gyrotrons. The resonant frequency, diffractionQ, and RF field profile are derived in closed forms in the lowQ limit. Formation of the eigenmode in the resonator is shown as the result of constructive wave interference. The lower limit ofQ in the context of a conventional definition ofQ is derived.     

Determination of Quality Factor for Highly Overcoupled EPR Resonators

We reported determination of the loaded quality factor (Q) of highly overcoupled (dielectric, loop-gap, and cavity) resonators used in time-domain electron paramagnetic resonance. We introduced a microwave absorber into resonators and achieved critical-coupling. Due to the deep “Q-dip” of critical-coupling, we can easily determine the loaded Q as low as 10. The loaded Q of resonators with and without the microwave absorber was examined under various overcoupling conditions. We found that the radiation Q (Q _r) can be calculated from the loaded Q of the resonator that contains the microwave absorber. We proposed a simple model that represents the loaded Q of the overcoupled resonator in terms of two parameters, Q ₀ and Q _r. Q ₀ is the effective unloaded Q of the resonator determined for the critically coupled resonator without the microwave absorber and is independent of a degree of coupling. The model can be applied to overcoupling in which the coupling parameter (Q ₀/Q _r) is in the range of 1 to ca. 20.     

Mode analysis in microstrip resonators part II

In this contribution, the transmission line model for a printed circuit microstrip resonator is used to theoretically investigate the modes supported by simple structures. Computer analysis yields useful information on the harmonic response of resonators that is relevant to the design of printed antennas and harmonic generators.     

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     

An active resonator system for CW-ESR measurement operating at 700 MHz

An active resonator system operating at 700 MHz, which can attain a high Q for CW-ESR measurements of a high loss sample, was developed. This system consisted of a loop-gap resonator (LGR), a receiver coil, an excitation coil, and a phase tunable amplifier. A part of the RF power at the LGR was picked up by the receiver coil, amplified, and irradiated to the LGR again by the excitation coil, which made up a feedback circuit. Because the feedback circuit provided the energy that canceled the loss in the resonator, the Q of the active resonator system increased. When a sample tube (inner diameter, 20 mm; axial length, 31 mm) containing a nitroxide radical and physiological saline solution was placed in the resonator, the Q could be varied from 55 to 4000. It was possible to obtain a Q of the active resonator system with sample that was higher than that of the value of the LGR without a sample in a no-feedback condition. The ESR signal intensity increased up to 7 times with the increase in Q. The sensitivity increased up to 50%, which was a much smaller advance than that of the Q, because the noise level also increased with the increase in signal intensity.     

Engineering of the metamaterial-based cut-band filter

Metamaterial-based cut-band filters are realized by combining either split-ring resonators (SRRs) close to a microstrip line or complementary split-ring resonators under this line. Indeed, their use reduces the size of conventional microwave filters. The study of this paper focuses on the influence of the association of different arrays of SRR especially on the bandwidth of the cut-band filter.     

Design and Cold Tests of Asymmetric Quasioptical Resonators

This paper presents the design and test of asymmetric quasioptical resonators for gyrotron applications, where most of the diffraction output coupling comes from the smaller of two mirrors. A computer code based on scalar diffraction theory is used to calculate the resonator losses and the electric field near the mirrors. Resonators with mirror separations approaching confocal have reduced side lobes in the diffraction patterns, which should allow for better power transmission and focusing. Cold tests have been performed at 94 and 141 GHz to measure the quality factor of the asymmetric resonators as a function of mirror separation and mirror misalignment. Typical resonator parameters include 65 cm mirror separation, 1-3% output coupling and 85% of the losses from the small mirror. A half-symmetric resonator with one flat and one curved mirror has also been investigated. The calculated values of Q are in reasonably good agreement with the cold test measurements.     

A tunable hybrid graphene-metal metamaterial absorber for sensing in the THz regime

In this paper, a graphene-based metamaterial absorber is proposed and investigated numerically, in which the interaction between a split ring resonator (SRR) and graphene results in a high-Q absorption. To make a better understanding of the resonance mechanism, the electric and the magnetic fields, and the surface currents at the resonance frequency are investigated. In order to ease the analysis of the structure, an equivalent circuit model is introduced using the transmission line theory, and the accuracy of the proposed model is verified by the full-wave simulation. Finally, different aspects of the designed metamaterial are discussed as a potential label-free sensor for chemical and biomedical sensing. It is shown that by using this structure, a sensor with a sensitivity of 597 GHz/RIU can be achieved.     

ESR spectroscopy at 75 GHz by using a whispering gallery mode dielectric resonator and an electromagnet: Part I

Disc-shaped Whispering Gallery Mode (WGM) dielectric resonators were used to obtain ESR spectra of powder samples at 75 GHz. A mechanically and electronically controlled Gunn diode was used while the static magnetic fieldB ₀ was obtained by modifying a Bruker electromagnet. WGM resonators having a diameter of 30 mm were prevalently used. The samples were contained in polyethylene holders suitably designed so that the resonator could retain its original circular symmetry and the sample could stay as close as possible to the curved outer rim of the resonator.Q-factors of the order of 10⁴ were obtained at room temperature when single crystal quartz was used for the disc resonator. Anyway, the value ofQ could be easily controlled by using different materials for the WGM disc. The gained experience allows one to foresee that WGM resonators can be used with similar results also at frequencies up to 150 and 300 GHz. ESR spectra of many organic and inorganic samples will be reported in Part II of this paper. Here, an evaluation is made of the intensity of the electromagnetic field on a selected sample. Suitable holders for liquid samples are under study.     

Effects of oblique incidence on terahertz responses of planar split-ring resonators

Effects of oblique incidence of terahertz waves on the response of planar split-ring resonators are investigated, both experimentally and by simulation. It is found that the incident angle dependent phase delay and coupling conditions of neighboring split-ring resonator(SRR) units play important roles and greatly change both the transmission and reflection spectra for the resonant feature of linear charge oscillations. Our results show that the SRR structure-supported magnetoelectric couplings at oblique excitation are trivial and can be ignored. A highly symmetric response is found in the cross-polarization effects, which may manifest the bianisotropic properties of the SRR system but this needs further study.     

Whispering gallery microcavity lasers

Whispering gallery mode (WGM) optical microresonators have attracted intense interests in the past decades. The combination of high quality factors (Q) and small mode volumes of modes in WGM resonators significantly enhances the light‐matter interactions, making them excellent cavities for achieving low threshold and narrow linewidth lasers. In this Review, the progress in WGM microcavity lasers is summarized, and the laser performance considering resonator geometries and materials as well as lasing mechanisms is discussed. Label‐free detection using WGM resonators has emerged as highly sensitive detection schemes. However, the resolution is mainly limited by the cavity Q factor which determines the mode linewidth. Microcavity lasers, due to their narrow laser spectral width, could greatly improve the detection resolution. Some recent developments in sensing using microcavity lasers are discussed.     

Ultra high Q crystalline microcavities

We report on the fabrication technique of ultra high Q optical crystalline whispering gallery mode microresonators and discuss their properties. The technique is suitable for the majority of available optical crystals and for production of resonators with small size. To validate the method, we made CaF₂ resonators with Q factors exceeding 4 × 10⁸ and a diameter smaller than 100 μm. A single mode resonator has also been fabricated. Possible utilization of these new resonators in quantum optics is discussed.     

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.     

Development of a Resonator with Automatic Tuning and Coupling Capability to Minimize Sample Motion Noise for in Vivo EPR Spectroscopy

EPR spectroscopy has been applied to measure free radicals in vivo; however, respiratory, cardiac, and other movements of living animals are a major source of noise and spectral distortion. Sample motions result in changes in resonator frequency, Q, and coupling. These instabilities limit the applications that can be performed and the quality of data that can be obtained. Therefore, it is of great importance to develop resonators with automatic tuning and automatic coupling capability. We report the development of automatic tuning and automatic coupling provisions for a 750-MHz transversely oriented electric field reentrant resonator using two electronically tunable high Q hyperabrupt varactor diodes and feedback loops. In both moving phantoms and living mice, these automatic coupling control and automatic tuning control provisions resulted in an 8- to 10-fold increase in signal-to-noise ratio.     

Automated synthesis of transmission lines loaded with complementary split ring resonators (CSRRs) and open complementary split ring resonators (OCSRRs) through aggressive space mapping (ASM)

This paper is focused on the application of space mapping optimization to the automated synthesis of transmission lines loaded with complementary split ring resonators (CSRRs) and open complementary split ring resonators (OCSRRs). These structures are of interest for the implementation of resonant-type metamaterial transmission lines and for the design of planar microwave circuits based on such complementary resonators. The paper presents a method to generate the layouts of CSRR- and OCSRR-loaded microstrip lines from the elements of their equivalent circuit models. Using the so-called aggressive space mapping, a specific implementation that uses quasi-Newton type iteration, we have developed synthesis algorithms that are able to provide the topology of these CSRR- and OCSRR-loaded lines in few steps. The most relevant aspect, however, is that this synthesis process is completely automatic, i.e., it does not require any action from the designers, other than initiating the algorithm. Moreover, this technique can be translated to other electrically small planar elements described by lumped element equivalent circuit models.     

Sensitivity of surface resistance measurement of HTS thin films by cavity resonator, dielectric resonator and microstrip line resonator

Microwave surface resistance (R _s) of silver-doped YBa₂Cu₃O_7-δ (YBCO) thin film, deposited by laser ablation technique on 10 mm × 10 mm LaAlO₃ substrate, has been measured by resonant techniques in the frequency range from 5 GHz to 20 GHz. The geometrical factor of the sample and the resonator has been determined theoretically by the knowledge of the electromagnetic field distribution in the resonators. The microwave surface resistance of the superconducting sample is then extracted from the measured Q value as a function of temperature. The sensitivity of the R _s measurement, that is, the relative change in the Q value with the change in the R _s value is determined for each resonator.