Xiao Yang Zhou

Cylindrical-to-plane-wave conversion via embedded optical transformation

We investigate the conversion from cylindrical waves to plane waves in a short range through a metamaterial layer which has a circular shape in the inner outline and a square shape in the outer outline. Based on an embedded optical transformation, analytical formulas of the permittivity and permeability tensors are presented for the metamaterial layer which converts the cylindrical waves to plane waves. The designed conversion materials are validated by full-wave simulations using the finite-element method. The proposed structure can be used either as a four-beam antenna or a compact range for near-field measurement of plane waves.

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.

Reduction of Mutual Coupling Between Closely Packed Patch Antennas Using Waveguided Metamaterials

The reduction of mutual coupling between closely spaced antenna elements is attractive in the electromagnetic and antenna community. An efficient approach to suppress the mutual coupling between microstrip patch antennas is proposed using waveguided metamaterials. The waveguided metamaterials are designed and realized by crossed-meander-line slits, which exhibit magnetic resonances and further the band-gap property. By inserting the waveguided metamaterials between two H-plane coupled rectangular patch antennas with the edge-to-edge distance less than λ0/8, about 6 dB reduction of mutual coupling throughout the -10-dB bandwidth has been achieved, which is verified by measurement results.

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.

Convolution Operations on Coding Metasurface to Reach Flexible and Continuous Controls of Terahertz Beams

The concept of coding metasurface makes a link between physically metamaterial particles and digital codes, and hence it is possible to perform digital signal processing on the coding metasurface to realize unusual physical phenomena. Here, this study presents to perform Fourier operations on coding metasurfaces and proposes a principle called as scattering‐pattern shift using the convolution theorem, which allows steering of the scattering pattern to an arbitrarily predesigned direction. Owing to the constant reflection amplitude of coding particles, the required coding pattern can be simply achieved by the modulus of two coding matrices. This study demonstrates that the scattering patterns that are directly calculated from the coding pattern using the Fourier transform have excellent agreements to the numerical simulations based on realistic coding structures, providing an efficient method in optimizing coding patterns to achieve predesigned scattering beams. The most important advantage of this approach over the previous schemes in producing anomalous single‐beam scattering is its flexible and continuous controls to arbitrary directions. This work opens a new route to study metamaterial from a fully digital perspective, predicting the possibility of combining conventional theorems in digital signal processing with the coding metasurface to realize more powerful manipulations of electromagnetic waves.

Diffuse reflections by randomly gradient index metamaterials

We have demonstrated that a thin planar coating with randomly distributed index gradients can create diffuse reflections in front of a planar conducting sheet. This scheme is verified by two-dimensional near-field scanning measurement, in which the randomly gradient index coating is implemented by periodic crossed-I-shaped array whose unit cells dimension varies spatially. Some qualitative principles of choosing the distribution mode of random index gradient are presented.

Thermally tunable water-substrate broadband metamaterial absorbers

The naturally occurring water has frequency dispersive permittivity at microwave frequencies and thus is a promising constituent material for broadband absorbers. Here, we develop water as the dielectric spacer in the substrate of metal-backed metamaterial (MM) absorbers. The designed substrate is a hybrid of water and a low-permittivity dielectric material. Such a design allows tight packaging of water and easy fabrication of the absorber. We obtain broadband absorption at temperatures of interest by designing the hybrid substrate and MM inclusions. Additionally, the absorption performance of the water-substrate MM absorbers could be tunable according to the environment temperature. We experimentally demonstrate the broadband and thermally tunable absorption performance. We expect that water could replace dielectric layers in other structural MM absorbers to achieve the broadband and thermally tunable absorption performance.

Fast 3D-ISAR Image Simulation of Targets at Arbitrary Aspect Angles Through Nonuniform Fast Fourier Transform (NUFFT)

We present a method to generate three-dimensional (3D) inverse synthetic aperture radar (ISAR) images of a target at arbitrary aspect angles using the shooting and bouncing ray (SBR) method. We have derived a 3D-image-domain ray-tube integration formula based on the SBR technique. The imaging formula is in a form of convolution, then a nonuniform fast Fourier transform (NUFFT) is induced to achieve the convolution, which can be used to generate 3D ISAR images rapidly. Compared to the conventional algorithm where the ISAR images are obtained by inverse Fourier transforming the computed scattered fields over a range of frequencies and a range of aspect angles, the new formula needs only one-time ray tracing and physical optics (PO) integration over a small angle span. Hence the computational time is decreased tremendously. The ISAR images of a missile target and an armored car for several aspect angles are presented to demonstrate the efficiency and accuracy of the proposed method.

A New Method to Analyze Broadband Antenna-Radome Interactions in Time-Domain

Based on the new time-domain finite integration theorem (TDFIT), a fast and accurate method is proposed to analyze broadband antenna-radome interactions in time domain. Due to the time-domain feature, broadband characteristic is obtained in a single simulation. In numerical simulations, the antenna array is placed inside an enclosed or semi-closed radome by using a presented local waveguide port. The staircase error in traditional finite-difference time-domain method is reduced by a novel conformal technique. An illustrating broadband antenna array with radome, operating at millimeter wave frequency is analyzed and optimized to show the high performance of our method. A new broadband transmittance of the antenna-radome system is proposed to incorporate the side lobe effect, and it is treated as the objective function to optimize the parameters of the antenna-radome system. Furthermore, the full wave simulation, combined with an artificial neural network (ANN) algorithm, is utilized to accelerate the antenna-radome optimization. About half of the time for computation has been reduced.

Full-State Controls of Terahertz Waves Using Tensor Coding Metasurfaces

Coding metasurfaces allow us to study metamaterials from a fully digital perspective, enabling many exotic functionalities, such as anomalous reflections, broadband diffusions, and polarization conversion. Here, we propose a tensor coding metasurface at terahertz (THz) frequency that could take full-state controls of an electromagnetic wave in terms of its polarization state, phase and amplitude distributions, and wave-vector mode. Owing to the off-diagonal elements that dominant in the reflection matrix, each coding particle could reflect the normally incident wave to its cross-polarization with controllable phases, resulting in different coding digits. A 3-bit tensor coding metasurface with three coding sequences is taken as an example to show its full-state controls in reflecting a normally incident THz beam to anomalous directions with cross-polarizations and making a spatially propagating wave (PW) to surface wave (SW) conversion at the THz frequency. We show that the proposed PW-SW convertor based on the tensor coding metasurface supports both x- and y-polarized normal incidences, producing cross-polarized transverse-magnetic and transverse-electric modes of THz SWs, respectively.