Ben Geng Cai

Frequency-Controls of Electromagnetic Multi-Beam Scanning by Metasurfaces

We propose a method to control electromagnetic (EM) radiations by holographic metasurfaces, including to producing multi-beam scanning in one dimension (1D) and two dimensions (2D) with the change of frequency. The metasurfaces are composed of subwavelength metallic patches on grounded dielectric substrate. We present a combined theory of holography and leaky wave to realize the multi-beam radiations by exciting the surface interference patterns, which are generated by interference between the excitation source and required radiation waves. As the frequency changes, we show that the main lobes of EM radiation beams could accomplish 1D or 2D scans regularly by using the proposed holographic metasurfaces shaped with different interference patterns. This is the first time to realize 2D scans of antennas by changing the frequency. Full-wave simulations and experimental results validate the proposed theory and confirm the corresponding physical phenomena.

Generation of spatial Bessel beams using holographic metasurface

We propose to use backward radiations of leaky waves supported by a holographic metasurface to produce spatial Bessel beams in the microwave frequency regime. The holographic metasurface consists of a grounded dielectric slab and a series of metal patches. By changing the size of metal patches, the surface-impedance distribution of the holographic metasurface can be modulated, and hence the radiation properties of the leaky waves can be designed to realize Bessel beams. Both numerical simulations and experiments verify the features of spatial Bessel beams, which may be useful in imaging applications or wireless power transmissions with the dynamic focal-depth controls.

Total transmission and super reflection realized by anisotropic zero-index materials

We demonstrate theoretically and experimentally total transmission and super reflection which are realized by anisotropic zero-index materials (AZIMs). We show that total transmission will be observed when using a rectangular perfectly magnetic conductor (PMC)-coated object sandwiched by two AZIM slabs, which has the properties of ?rx?=?0 (in the normal direction to the wavefronts) and ?ry?=??rz?=?1 for transverse-electric polarized incident waves. When the object is coated with a perfectly electric conductor (PEC), however, the incident waves will be totally reflected by the finite-sized object in the way of an infinite PEC plane, generating a super reflection. Closed-form formulas are derived to explain the physical mechanisms of total transmission and super reflection, which agree with the full-wave numerical simulations perfectly. Experimental samples of AZIM and PMC are designed, fabricated and measured in the microwave frequency, which show good transparent and super-reflecting effects.

Simultaneous controls of surface waves and propagating waves by metasurfaces

We propose a hybrid metasurface to control surface and propagating waves simultaneously. The hybrid metasurface is composed of planar metamaterial to interact with surface waves and holographic metasurface to modulate propagating waves. As an experimental verification, we design and fabricate a special hybrid metasurface in microwave frequency, which contains a surface-wave Luneburg lens and a focusing holographic surface. Numerical and measured results show multi-functional abilities of the hybrid metasurface in controlling the surface and propagating waves simultaneously. It is expected that the proposed methodology will facilitate applications of surface waves in information processing, near-field detection, and wireless communications.

Continuous leaky-wave scanning using periodically modulated spoof plasmonic waveguide

The plasmonic waveguide made of uniform corrugated metallic strip can support and guide spoof surface plasmon polaritons (SSPPs) with high confinements. Here, we propose periodically-modulated plasmonic waveguide composed of non-uniform corrugated metallic strip to convert SSPPs to radiating waves, in which the main beam of radiations can steer continuously as the frequency changes. To increase the radiation efficiency of the periodically-modulated plasmonic waveguide at the broadside, an asymmetrical plasmonic waveguide is further presented to reduce the reflections and realize continuous leaky-wave scanning. Both numerical simulations and experimental results show that the radiation efficiency can be improved greatly and the main beam of leaky-wave radiations can steer from the backward quadrant to the forward quadrant, passing through the broadside direction, which generally is difficult to be realized by the common leaky-wave antennas.

Diffraction-free surface waves by metasurfaces.

We propose a method to design and realize planar Bessel lens using artificial metasurfaces to produce diffraction-free surface waves. The planar Bessel lens is composed of two sublenses: a half Maxwell fisheye lens which can shape the surface cylindrical waves to surface plane waves, and an inhomogeneous flat lens which can convert the surface plane waves into approximate diffraction-free surface waves in a diamond-shaped focusing area. Through the planar Bessel lens, a point source on the metasurface directly radiates the diffraction-free surface waves. In realization, we construct the inhomogeneous metasurfaces by subwavelength metallic patches printed on a grounded dielectric substrate. Simulation and experimental results have good agreements, which jointly show the formation of the diffraction-free surface waves in the microwave band.

Surface Fourier-transform lens using a metasurface

We propose a surface (or 2D) Fourier-transform lens using a gradient refractive index (GRIN) metasurface in the microwave band, which is composed of sub-wavelength quasi-periodical metallic patches on a grounded dielectric substrate. Such a metasurface supports the transverse magnetic (TM) modes of surface waves. To gradually change the size of textures, we obtain different surface refractive indices, which can be tailored to fit the required refractive-index profile of a surface Fourier-transform lens. According to the theory of spatial Fourier transformation, we make use of the proposed lens to realize surface plane-wave scanning under different feeding locations. The simulation and experimental results jointly confirm the validity of the surface Fourier-transform lens. The proposed method can also be extended to the terahertz frequency.

Spatial power combination within fan-shaped region using anisotropic zero-index metamaterials

We show that the radially anisotropic zero-index metamaterials can be used to realize spatial power combination within fan-shaped regions. When the radial component of the permittivity or permeability tensor tends to zero, the coherent omnidirectional radiations will occur when sources are within the fan-shaped metamaterial backed with perfect magnetic conductors (PMCs), which leads to spatial power combination in finite areas. As the included angle of the PMC corner reflector decreases, the powers from internal sources can still be efficiently combined, which has potential applications in radio frequency engineering.

Leaky-Wave Radiations by Modulating Surface Impedance on Subwavelength Corrugated Metal Structures.

One-dimensional (1D) subwavelength corrugated metal structures has been described to support spoof surface plasmon polaritons (SPPs). Here we demonstrate that a periodically modulated 1D subwavelength corrugated metal structure can convert spoof SPPs to propagating waves. The structure is fed at the center through a slit with a connected waveguide on the input side. The subwavelength corrugated metal structure on the output surface is regarded as metasurface and modulated periodically to realize the leaky-wave radiation at the broadside. The surface impedance of the corrugated metal structure is modulated by using cosine function and triangle-wave function, respectively, to reach the radiation effect. Full wave simulations and measuremental results are presented to validate the proposed design.

Holographic leaky-wave metasurfaces for dual-sensor imaging.

Metasurfaces have huge potentials to develop new type imaging systems due to their abilities of controlling electromagnetic waves. Here, we propose a new method for dual-sensor imaging based on cross-like holographic leaky-wave metasurfaces which are composed of hybrid isotropic and anisotropic surface impedance textures. The holographic leaky-wave radiations are generated by special impedance modulations of surface waves excited by the sensor ports. For one independent sensor, the main leaky-wave radiation beam can be scanned by frequency in one-dimensional space, while the frequency scanning in the orthogonal spatial dimension is accomplished by the other sensor. Thus, for a probed object, the imaging plane can be illuminated adequately to obtain the two-dimensional backward scattered fields by the dual-sensor for reconstructing the object. The relativity of beams under different frequencies is very low due to the frequency-scanning beam performance rather than the random beam radiations operated by frequency, and the multi-illuminations with low relativity are very appropriate for multi-mode imaging method with high resolution and anti- noise. Good reconstruction results are given to validate the proposed imaging method.