Xin Mi Yang

Design of arbitrarily shaped concentrators based on conformally optical transformation of nonuniform rational B-spline surfaces

We study the design of arbitrarily shaped electromagnetic (EM) concentrators and their potential applications. To obtain closed-form formulas of EM parameters for an arbitrarily shaped concentrator, we employ nonuniform rational B-spline (NURBS) to represent the geometrical boundary. Using the conformally optical transformation of NURBS surfaces, we propose the analytical design of arbitrarily shaped concentrators, which are composed of anisotropic and inhomogeneous metamaterials with closed-form constitutive tensors. The designed concentrators are numerically validated by full-wave simulations, which show perfectly directed EM behaviors. As one of the potential applications, we demonstrate a way to amplify plane waves using a rectangular concentrator, which is much more efficient and easier than the existing techniques. Using NURBS expands the generality of the transformation optics and could lead toward making a very general tool that would interface with commercial softwares such as 3D STUDIOMAX and MAYA.

Invisibility cloak without singularity

An elliptical invisible cloak is proposed using a coordinate transformation in the elliptical-cylindrical coordinate system, which crushes the cloaked object to a line segment instead of a point. The elliptical cloak is reduced to a nearly circular cloak if the elliptical focus becomes very small. The advantage of the proposed invisibility cloak is that none of the parameters is singular and the changing range of all parameters is relatively small.

Compact-sized and broadband carpet cloak and free-space cloak

Recently, invisible cloaks have attracted much attention due to their exciting property of invisibility, which are based on a solid theory of transformation optics and quasi-conformal mapping. Two kinds of cloaks have been proposed: free-space cloaks, which can render objects in free space invisible to incident radiation, and carpet cloaks (or ground-plane cloaks), which can hide objects under the conducting ground. The first free-space and carpet cloaks were realized in the microwave frequencies using metamaterials. The free-space cloak was composed of resonant metamaterials, and hence had restriction of narrow bandwidth and high loss; the carpet cloak was made of non-resonant metamaterials, which have broad bandwidth and low loss. However, the carpet cloak has a severe restriction of large size compared to the cloaked object. The above restrictions become the bottlenecks to the real applications of free-space and carpet cloaks. Here we report the first experimental demonstration of broadband and low-loss directive free-space cloak and compact-sized carpet cloak based on a recent theoretical study. Both cloaks are realized using non-resonant metamaterials in the microwave frequency, and good invisibility properties have been observed in experiments. This approach represents a major step towards the real applications of invisibility cloaks.

Experiments on high-performance beam-scanning antennas made of gradient-index metamaterials

A planar lens made of gradient index metamaterials can transform cylindrical waves to plane waves, and the beam direction of plane waves is controlled by adjusting the refractive-index distributions of the lens. Based on such properties, we present high-performance beam-scanning antennas experimentally using the gradient-index planar lens and horn antenna. The lens is carefully designed with metamaterials to achieve different refractive indices and good matching of impedance. The near-field distributions of antennas are measured using a two-dimensional near-field microwave scanning apparatus, and the radiation patterns are presented to show the high directivity and low sidelobe.

Shrinking an arbitrary object as one desires using metamaterials

Based on transformation optics, we present a shrinking device, which can transform an arbitrary object virtually into a small-size object with different material parameters as one desires. Such an illusion device will confuse the detectors or the viewers, and hence the real size and material parameters of the enclosed object cannot be perceived. We fabricated and measured a shrinking device by using metamaterials, which works at the nonresonant frequency and has low loss. The device has been validated by both numerical simulations and experiments on circular and square objects. Good shrinking performance has been demonstrated.

Layered high-gain lens antennas via discrete optical transformation

In reality, the optical-transformation media with complicated and continuous electromagnetic parameters are difficult to realize. In this work, we propose a discrete embedded optical transformation from which a layered high-gain lens antenna is designed. All layers of the lens antenna are composed of homogeneous and uniaxially anisotropic metamaterials, which are simple and realizable. When the layered lens is embedded in a horn antenna, the lens antenna provides a high-directivity radiation beam. We also use the discrete optical transformation to design a multibeam high-gain antenna.

Gradient index circuit by waveguided metamaterials

Metamaterials are artificially structured materials that provide considerable flexibility for control of electromagnetic waves. The metamaterial concept can also be applied to the design of planar waveguiding structures. Here, we illustrate this design approach with the development of two-dimensional (2D) planar gradient index (GRIN) circuits. To form the structure, we make use of a 2D complementary split ring resonator, which exhibits an electric response to guided transverse-electric waves. To confirm the properties of the planar GRIN structure predicted from numerical simulations, we present experimental results for a beam-steering and a focusing GRIN circuit. These examples illustrate the versatility of the metamaterial approach in the design of complex waveguiding structures.

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.

Increasing the Bandwidth of Microstrip Patch Antenna by Loading Compact Artificial Magneto-Dielectrics

We realize artificial magneto-dielectric loading for microstrip patch antenna by etching embedded meander-line (EML) array in the ground plane under the patch. The related artificial magneto-dielectric medium belongs to the waveguided metamaterial proposed previously. Both simulation and measurement results show that the proposed patch antenna with the EML array has wider impedance bandwidth than the conventional patch antenna with the same size. Though we have to increase the antenna profile by attaching an additional shield metal plate to suppress the back radiation, the proposed magneto-dielectric loading method requires lower fabrication complexity and lower cost than the existing techniques.

Design of multibeam scanning antennas with high gains and low sidelobes using gradient-index metamaterials

Metamaterials, composed of artificial structures with subwavelength unit cells, can be fabricated to achieve gradually refractive indices. By designing the refractive indices properly, a gradient-index planar slab will transform spherical waves to plane waves. Using such a property, we propose a beam-scanning antenna with high gain and low sidelobes based on the gradient-index planar lens and a horn antenna, where the beam direction is controlled by the refractive-index distribution. When such four lenses are fixed together, we can get a four-beam scanning antenna with high gain and low sidelobes, in which each beam can be controlled independently. The gradient-index planar lenses can be realized using metamaterial structures.