Dongxing Wang

Application of coordinate transformation in bent waveguides

We apply coordinate transformations in the design of waveguides with bent geometry to reduce reflection at the incident port. It was found that in the case of metallic waveguides, by applying the transformation to the medium inside a right-angle bent waveguide, low reflection can be obtained. For the case of dielectric waveguides, this can also be done by applying the same transformation to the bent region containing both the dielectric core and its claddings. Our proposed technique provides an alternative method for designing bent waveguides with low insertion loss.

Active left-handed material collaborated with microwave varactors

In the present letter, a design for active left-handed material collaborated with microwave varactors is proposed and experimentally realized. Both the results of simulation and experiment verify the tunable left-handed properties of the structure in microwave band. A controllable beam steering is then observed experimentally by a prism fabricated by such metamaterial. The simplicity of the active design and its tunable negative refraction indices make it useful for various applications.

Experimental validation of negative refraction of metamaterial composed of single side paired S-ring resonators

A one-dimensional metamaterial structure is realized by printing subwavelength paired S-shaped metallic strips only on one side of a dielectric substrate. The simultaneously negative effective permittivity and permeability of such metamaterial are retrieved from the scattering parameters yielded in the numerical simulation. Experiments aimed to verify the transmission and refraction properties of the metamaterial are conducted, and the results show the existence of the negative refraction within the same frequency band, as indicated in the simulation. The proposed structure design makes it easy to fabricate and provides feasibilities for making controllable metamaterial by incorporating itself with lumped active elements.

Metamaterial with randomized patterns for negative refraction of electromagnetic waves

Artificial metamaterials made to date are all periodic in structure. Here we show that by randomizing contour patterns deposited lithographically on circuit board materials, a metamaterial characterized by a constitutive relation with negative constitutive parameters is produced in solid-state form. We clearly demonstrate the phenomenon of negative refraction to show that it is not produced by periodicity. This underlines the importance of using constitutive relations for media characterization in electromagnetic theory and suggests that metamaterials could be realized with composite materials or fabricated with various techniques by using versatile hosting materials.

Second-harmonic generation and spectrum modulation by an active nonlinear metamaterial

The nonlinear properties of a metamaterial sample composed of double-layer metallic patterns and voltage controllable diodes are experimentally investigated. Second harmonics and spectrum modulations are clearly observed in a wide band of microwave frequencies, showing that this kind of metamaterial is not only tunable by low dc bias voltage but also behaves strong nonlinear property under a small power incidence. These properties are difficult to be found in normal naturally occurring materials.

Reconfigurable cloak for multiple operating frequencies

An invisible cloak based on active metamaterials whose permittivity can be controlled is proposed, and the performances of the cloak are verified in numerical simulations. Because of the tunability of the active metamaterials, the working frequency of the invisible cloak is reconfigurable, which is different from the traditional cloak designs based on passive metamaterials whose working frequencies are fixed.

Multi-frequency resonator based on dual-band S-shaped left-handed material

In this paper, we experimentally realize a one-dimensional RHM (Right-handed Material)-LHM (Left-handed Material) multi-frequency resonator that consists of a dual-negative-band LHM and air arranged in an X-band waveguide. Multi-resonant frequencies are observed within two left-handed bands of the LHM. The effects of the loss and the hyperbolic dispersion relation of LHM layer are discussed. The incorporation of such a LHM into the resonator design allows more flexibility to realize multi-resonance.

Measurement of Dielectric Constant of Thin Film Materials at Microwave Frequencies

To determine the dielectric constant of thin film materials with thickness around one or several microns at microwave frequencies, a novel method based on modified cavity perturbation theory is presented. Corresponding measurement fixture is composed of Agilent 8720ES VNA, a rectangular cavity with small slots, and crystal slices used as the carriers of thin films. By measuring the shift in resonant frequency, the dielectric constant of thin film can be determined. 810 nm thick (Mg0.95Ca0.05)TiO3 thin film deposited by RF magnetron sputtering was used as sample to verify the feasibility of this technique, and the measured dielectric constant is 21.1, which is close to that of the bulk material. The measurement error of this method is about 6%, which mainly comes from the measurement errors of resonant frequencies and the thickness of thin film. This error can be reduced by using a cavity with higher Q, or by using thin film samples with larger dielectric constant or with thickness bigger than 1 micron. Experimental results show that the method presented in this paper is feasible and has good accuracy for dielectric thin film.

Measurement of negative permittivity and permeability from experimental transmission and reflection with effects of cell misalignment

The measurement of the negative effective permittivity and permeability of a material is a direct way to verify its left-handed properties. In this paper, a transmission experiment for a left-handed material (LHM) sample is performed in a parallel plate waveguide. The effective frequency dispersive permittivity and permeability are retrieved from the experimental scattering data and found to be simultaneously negative in a specific frequency range. The simulation results also show that the misalignments of the metallic inclusions of the LHM introduce some dips to the retrieved curves, which prevent us from obtaining ideal curves following the Lorentz or Drude model.

EFFECTS OF DIFFERENT TRANSFORMATIONS ON THE PERFORMANCE OF CYLINDRICAL CLOAKS

The cylindrical cloak created with nonlinear transformation is studied based on the analytical scattering theory. The equivalent surface displacement current at the inner boundary of an ideal cloak is shown to be independent of the transformation function. For the nonideal cloak with perturbation at the inner boundary, we show that the performance of the cloak with low loss can be greatly improved by using certain nonlinear transformations.