Xian Qi Lin

Arbitrarily elliptical–cylindrical invisible cloaking

Based on the idea of coordinate transformation (Pendry, Schurig and Smith 2006 Science 312 1780), arbitrarily elliptical–cylindrical cloaks are proposed and designed. The elliptical cloak, which is composed of inhomogeneous anisotropic metamaterials in an elliptical-shell region, will deflect incoming electromagnetic (EM) waves and guide them to propagate around the inner elliptical region. Such EM waves will return to their original propagation directions without distorting the waves outside the elliptical cloak. General formulations of the inhomogeneous and anisotropic permittivity and permeability tensors are derived for arbitrarily elliptical axis ratio k, which can also be used for the circular cloak when k = 1. Hence the elliptical cloaks can make a large range of objects invisible, from round objects (when k approaches 1) to long and thin objects (when k is either very large or very small). We also show that the material parameters in elliptical cloaking are singular at only two points, instead of on the whole inner circle for circular cloaking, which are much easier to be realized in actual applications. Full-wave simulations are given to validate the arbitrarily elliptical cloaking.

Controlling electromagnetic waves using tunable gradient dielectric metamaterial lens

We propose a metamaterial particle which is composed of a dielectric block with a thin metallic rod screwed inside. By adjusting the height of rod inside the dielectric block, we can control the effective medium parameters of a periodic structure composed of the particles. An experiment is presented to retrieve the effective medium parameters, which have good agreements with those from simulation results. Using the unique property of the tunable particles, gradient metamaterial lenses are easily designed to deflect and focus the incident plane waves.

Design and Analysis of Super-Wide Bandpass Filters Using a Novel Compact Meta-Structure

This paper presents a novel compact meta-structure, which is characterized by left-handed properties. In the novel structure, two linear arrays of metallic vias and short-stub inductors are used to generate shunt inductors besides the series interdigital capacitors. Compared with conventional composite right/left-handed transmission-line structures, the new structure has weaker coupling to other components and is easily controlled by adjusting the shunt inductance and series capacitance to produce different left-handed properties. Due to the isolation of two via arrays, the proposed structure can also be transplanted directly from the microstrip system to the coplanar-waveguide system. We have proposed an efficient procedure to retrieve the equivalent propagating modes for both forward and backward waves based on the matrix pencil method, which provides an easy way to study the propagation characteristics. Using such a meta-structure, a series of super-wide bandpass filters are designed and fabricated at different frequency bands. Good agreements between simulation and experiment results have been achieved, and good performance in passbands and stopbands has been observed with a relative 3-dB bandwidth larger than 70%. For easy design of such super-wide bandpass filters, an experienced formula is given between the physical size and the center frequency of the 3-dB passband, which can be used to redesign any filters at other frequency bands

Compact Diplexer With High Isolation Using the Dual-Mode Substrate Integrated Waveguide Resonator

This letter presents a substrate integrated waveguide (SIW) diplexer using the dual-mode resonator. By enhancing the perturbation strength, the resonant frequencies of the dual-mode square cavity can be assigned to required frequencies. Thus, the cavity can be employed as the common resonator to replace the T-junction. The coupling aperture between the common resonator and the triangular resonator is parallel to one resonant modes magnetic field and perpendicular to the others. As a result, the isolation between the two channels is increased. Furthermore, the cavities are vertically stacked to realize a compact size. A diplexer with lower and upper channels centered at 8 and 9 GHz, respectively, is designed to verify the proposed concept. The simulated and measured results agree with each other.

A Novel Miniaturized Printed Planar Antenna Using Split-Ring Resonator

In this letter, we present a novel miniaturized resonant antenna based on the design of metamaterial unit cell. The antenna is similar to the split-ring resonator (SRR) in geometry, but can be easily integrated in practice due to its printed planar structure. The SRR-based antenna, which has low profiles while maintaining satisfactory antenna performances, is demonstrated theoretically and experimentally. The metal vias are integrated in the structure to provide sufficient capacitance and inductance for the whole antenna system. In general, the presented structure is a good candidate for the future miniaturized antenna systems.

Controlling the Bandwidth of Split Ring Resonators

A method to control the half-power relative band- widths of split ring resonators (SRRs) coupled to a transmission-line system is presented. An equivalent circuit model is proposed to analyze the main factors that effect the relative band- widths of SRRs. Based on the analysis, we propose an efficient way to control the relative bandwidth of SRR, which has been verified by both full-wave simulations and experiments. The proposed analysis can be used to design novel resonators and other microwave components.

Advanced parameter retrievals for metamaterial slabs using an inhomogeneous model

The S-parameter retrieval has proved to be an efficient approach to obtain electromagnetic parameters of metamaterials from reflection and transmission coefficients, where a slab of metamaterial with finite thickness is regarded as a homogeneous medium slab with the same thickness [D. R. Smith and S. Schultz, Phys. Rev. B 65, 195104 (2002)]. However, metamaterial structures composed of subwavelength unit cells are different from homogeneous materials, and the conventional retrieval method is, under certain circumstances, not accurate enough. In this paper, we propose an advanced parameter retrieval method for metamaterial slabs using an inhomogeneous model. Due to the coupling effects of unit cells in a metamaterial slab, the roles of edge and inner cells in the slab are different. Hence, the corresponding equivalent medium parameters are different, which results in the inhomogeneous property of the metamaterial slab. We propose the retrievals of medium parameters for edge and inner cells from S parameter...

Frequency Selective Surface Designed Using Electric Resonant Structures in Terahertz Frequency Bands

Electric resonant structures are primary unit cells to construct single negative permittivity metamaterials, which are also widely used in the design of novel microwave components. In this paper, the relative equivalent circuit model is first studied, the resonant characteristics of such structure is analyzed, the factors effected on the distribution of zero and pole transmission points are presented, based on which a realizable way to design filters with narrow transition-band from stop-band to pass-band is elicited. Then, a frequency selective surface (FSS) is designed with –3.9 dB attenuation in 40 GHz near the working frequency of 0.7 THz. The S parameters are also compared with those of a conventional electric resonant structure, from which we observe that the later has larger size besides wider transition bandwidth. On the other hand, the novel FSS can increase the utilization factor of frequency spectrum. In order to further decrease the transition bandwidth, three layers of the FSS structures are arrayed. By tuning the distance between each two layers, much narrower bandwidth is easily achieved which can be reached to 10 GHz.

Resonant crystal band gap metamaterials in the microwave regime and their exotic amplification of evanescent waves

The authors realize resonant crystal band gap metamaterials (CBMs) using circuit structures in the microwave regime, which behave quite differently from the conventional photonic band gap structures. They show that a periodic array of series inductor and shunt capacitor is equivalent to a CBM similar to a magnetic plasma at higher frequency band, and a periodic array of series capacitor and shunt inductor behaves like a CBM similar to an electric plasma at lower frequency band. The equivalent medium parameters have been derived in both cases, which show the existence of conjugate loss in imaginary parts. When two such structures are cascaded together, the authors demonstrate that evanescent waves are amplified and transmitted in a very narrow frequency band under the antimatching condition. Such a feature can be used to design extremely narrow band filters. Theoretical predictions, circuit simulations, and experimental results are presented to validate their conclusions.

Experimental observation of evanescent-wave amplification and propagation in microwave regime

Surface plasmon can lead to wave propagation not only along the surface as a dielectric waveguide but also in the normal direction to the surface because of the evanescent-wave amplification by metamaterial. In this letter, the authors design a surface plasmon structure to implement the electric and magnetic plasma resonances in microwave regime. Near the plasma resonant frequency, they directly observe the strong surface plasmon by experiments. The evanescent waves can be transmitted in an exponentially increasing and decreasing distribution, which is the evanescent-mode propagation by the tunneling effect.