Yongqiang Pang

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.

Wideband, wide-angle coding phase gradient metasurfaces based on Pancharatnam-Berry phase

A new concept of the coding phase gradient metasurface (CPGM) is proposed, which is constructed using the phase gradient metasurface as the coding elements. Different from the previous coding metasurface (CM), both the coding sequences and gradient phases in the coding elements are designed to manipulate the electromagnetic (EM) wave for the CPGMs, and thus the manipulation will be more flexible. As an example, wide-band, wide-angle CPGMs with zero and non-zero phase gradient based on Pancharatnam-Berry (PB) phase are achieved using the co-polarization reflection unit cells under circularly polarized (CP) wave incidence. Both theoretically calculated and numerically simulated scattering patterns of the designed CPGMs demonstrate the expected manipulations. Additionally, two kinds of random CPGMs with different phase gradients are designed for radar cross section (RCS) reduction, and the measured RCS reveals a good accordance with the simulation.

Spatial k -dispersion engineering of spoof surface plasmon polaritons for customized absorption

Absorption of electromagnetic waves in a medium is generally manipulated by controlling the frequency dispersion of constitutive parameters. However, it is still challenging to gain the desired constitutive parameters for customized absorption over a broad frequency range. Here, by virtue of spoof surface plasmonic polaritons (SPPs), we demonstrate capabilities of the spatial k-dispersion engineering for producing the customized broadband absorption. Incident waves can be efficiently converted to the spoof SPPs by plasmonic arrays, and their propagation and/or absorption can be controlled by engineering the spatial dispersion of k-vector. Based on this feature, we show how such concept is employed to achieve broadband as well as frequency-selective broadband absorptions as examples. It is expected that the proposed concept can be extended to other manipulations of propagating electromagnetic waves over a broad frequency range.

A Miniaturized Dual-Band FSS With Second-Order Response and Large Band Separation

In this letter, a miniaturized dual-band frequency selective surfaces with second-order band-pass response at each operation band is presented. The design is implemented by cascading a two-dimensional periodic array of double square loops and an array of wire grids. The proposed structure composed of three metal and two dielectric layers acts as a spatial dual band microwave filter with large band separation. The predicted FSS has the merits of broadband response, excellent stability for different incident angles, and sharp roll-off at X- and Ka-band, respectively. The simulation and measurement are carried out and further discussed. A good agreement between simulated and measured results verifies the design of the dual-band FSS.

Symmetry-based coding method and synthesis topology optimization design of ultra-wideband polarization conversion metasurfaces

In this letter, we propose the synthesis topology optimization method of designing ultra-wideband polarization conversion metasurface for linearly polarized waves. The general design principle of polarization conversion metasurfaces is derived theoretically. Symmetry-based coding, with shorter coding length and better optimization efficiency, is then proposed. As an example, a topological metasurface is demonstrated with an ultra-wideband polarization conversion property. The results of both simulations and experiments show that the metasurface can convert linearly polarized waves into cross-polarized waves in 8.0–30.0u2009GHz, obtaining the property of ultra-wideband polarization conversion based on metasurfaces, and hence validating the synthesis design method. The proposed method combines the merits of topology optimization and symmetry-based coding method, which provides an efficient tool for the design of high-performance polarization conversion metasurfaces.

Microwave birefringent metamaterials for polarization conversion based on spoof surface plasmon polariton modes.

We propose the design of wideband birefringent metamaterials based on spoof surface plasmon polaritons (SSPPs). Spatial k-dispersion design of SSPP modes in metamaterials is adopted to achieve high-efficiency transmission of electromagnetic waves through the metamaterial layer. By anisotropic design, the transmission phase accumulation in metamaterials can be independently modulated for x- and y-polarized components of incident waves. Since the dispersion curve of SSPPs is nonlinear, frequency-dependent phase differences can be obtained between the two orthogonal components of transmitted waves. As an example, we demonstrate a microwave birefringent metamaterials composed of fishbone structures. The full-polarization-state conversions on the zero-longitude line of Poincaré sphere can be fulfilled twice in 6–20u2009GHz for both linearly polarized (LP) and circularly polarized (CP) waves incidence. Besides, at a given frequency, the full-polarization-state conversion can be achieved by changing the polarization angle of the incident LP waves. Both the simulation and experiment results verify the high-efficiency polarization conversion functions of the birefringent metamaterial, including circular-to-circular, circular-to-linear(linear-to-circular), linear-to-linear polarization conversions.

Extraordinary transmission of electromagnetic waves through sub-wavelength slot arrays mediated by spoof surface plasmon polaritons

One-dimensional gratings consisting of sub-wavelength metallic slot arrays have been widely applied in the design of novel devices due to their polarization-selective characteristics. When the incident electric field is polarized along the slot direction, the slot arrays are opaque, behaving like a metal surface. Here we propose a scheme of making slot arrays transparent for electromagnetic (EM) waves, which is achieved by the incorporation of corrugated metal strip arrays. Incident waves are first converted into spoof surface plasmonpolaritons (SSPPs) propagating along the strips. Since SSPPs confine EM fields in sub-wavelength scales, EM waves can penetrate through the sub-wavelength slots. High transmission was thus obtained, with an efficiency as high as 95%. Moreover, position and bandwidth of the transmission band can be tailored by adjusting the groove depth and the slot width, respectively. It is expected that the design may find potential applications in the multifunctional devices with frequency- and polarization-selective features.

High-efficiency tri-band quasi-continuous phase gradient metamaterials based on spoof surface plasmon polaritons

A high-efficiency tri-band quasi-continuous phase gradient metamaterial is designed and demonstrated based on spoof surface plasmon polaritons (SSPPs). High-efficiency polarizaiton conversion transmission is firstly achieved via tailoring phase differece between the transmisive SSPP and the space wave in orthogonal directions. As an example, a tri-band circular-to-circular (CTC) polarization conversion metamateiral (PCM) was designed by a nonlinearly dispersive phase difference. Using such PCM unit cell, a tri-band quasi-continuous phase gradient metamaterial (PGM) was then realized by virtue of the Pancharatnam-Berry phase. The distribution of the cross-polarization transmission phase along the x-direction is continuous except for two infinitely small intervals near the phases 0° and 360°, and thus the phase gradient has definition at any point along the x-direction. The simulated normalized polarization conversion transmission spectrums together with the electric field distributions for circularly polarized wave and linearly polarized wave demonstrated the high-efficiency anomalous refraction of the quasi-continuous PGM. The experimental verification for the linearly polarized incidence was also provided.

Retro-reflective metasurfaces for backscattering enhancement under oblique incidence

In this letter, we propose the design of retro-reflective metasurfaces for enhancing backscattering under oblique incidence. Through reflective phase profile design along the surface, an equivalent wave-vector can be generated, with double magnitude but opposite direction to the parallel component k|| of the wave-vector k0 of incident waves. Due to this artificial parallel wave-vector, the main lobe of reflective waves can be reoriented backwards. As an example, we demonstrate an X-band retro-reflective metasurface that can operate effectively under oblique incidence θ=20°. A modified metallic square loop structure is used to achieve the required phase profile under oblique incidence. Through the phase gradient design, the metasurface can reflect incident waves backwards at 9.8GHz under transverse electric (TE) polarizations. A prototype was fabricated and measured. Both the simulation and experiment results verify the good retro-reflection performance of the metasurface.

Thermally Tunable Ultra-wideband Metamaterial Absorbers based on Three-dimensional Water-substrate construction

Distilled water has frequency dispersive characteristic and high value of imaginary part in permittivity, which can be seen as a good candidate of broadband metamaterial absorbers(MAs) in microwave. Here, an interesting idea based on the combination of water-substrate and metallic metamaterial in the three-dimensional construction is proposed, which can achieve outstanding broadband absorption. As a proof, the distilled water is filled into the dielectric reservoir as ultra-thin water-substrate, and then the water-substrates are arranged on the metal backplane periodically as three-dimensional water-substrate array(TWA). Simulation shows that the TWA achieves broadband absorption with the efficiency more than 90% from 8.3 to 21.0u2009GHz. Then, the trigonal metallic fishbone structure is introduced here between the water-substrate and the dielectric reservoir periodically as three-dimensional water-substrate metamaterial absorber(TWMA). The proposed TWMA could achieve ultra-broadband absorption from 2.6 to 16.8u2009GHz, which has increase by 64.8% in relative absorption bandwidth. Meanwhile, due to the participation of distilled water, the thermally tunable property also deserves to be discussed here. In view of the outstanding performance, it is worth to expect a wide range of applications to emerge inspired from the proposed construction.