Achieving different and arbitrary phase distributions for left-handed (LH) and right-handed (RH) circularly polarized (CP) waves, especially in transmission cases, is very important in designing CP lenses. Here, a strategy to control the propagating and geometrical phases simultaneously and independently is proposed by using a four-layered element. The element is a perfect transparent half wavelength plate, which controls the propagating phase by changing the structural size at both orthogonal polarizations and geometrical phase via rotating the structures. To verify the effectiveness of the modulation, we design three linear deflectors by manipulating only propagating phases, only geometrical phases and mix of them to achieve identical, symmetry and asymmetric refractions, respectively. Finally, we simulate and measure lens1 with refractions in different planes and lens2 with pencil-shape and vortex-shape beams. Our findings break the fixed relations of orthogonal CP waves for sole geometrical or propagating metasurface and then offer an effective strategy to achieve arbitrary bifunctional CP devices.
Phase gradient metasurfaces (PGMS) offer a fascinating ability to control the amplitude and phase of the electromagnetic (EM) waves on a subwavelength scale, resulting in new applications of designing novel microwave devices with improved performances. In this paper, a reflective symmetrical element, consisting of orthogonally I-shaped structures, has been demonstrated with an approximately parallel phase response from 15 GHz to 22 GHz, which results in an interesting wideband property. For practical design, a planar antenna is implemented by a well-optimized focusing metasurface and excited by a self-designed Vivaldi antenna at the focus. Numerical and experimental results coincide well. The planar antenna has a series of merits such as a wide 3-dB gain bandwidth of 15-22 GHz, an average gain enhancement of 16 dB, a comparable aperture efficiency of better than 45% at 18 GHz, and also a simple fabrication process. The proposed reflective metasurface opens up a new avenue to design wideband microwave devices.
For potential applications of metasurfaces in lens technologies, we propose a cross circularly polarized focusing metasurface which is capable of transforming a circularly polarized wave into cross-polarized wave and simultaneously focusing electromagnetic wave. A helicity-dependent phase change is introduced into the transmission metasurface cell, which is a single layer with a thickness of 1.5 mm and can be engineered by assembling along the spatial orientation of each Pancharatnam-Berry phase element. The phase change of the Pancharatnam-Berry phase element is analyzed theoretically, and the efficiency of the designed element is simulated under the irradiation of differently polarized waves. A phase gradient metasurface with a phase difference of 60 between neighbouring cells is designed. When simulated in CST Microwave Studio, the gradient metasurface is observed to have a ability to refract right-hand circularly polarized waves in +x direction and left-hand circularly polarized waves in -x direction but with an identical refraction angle of 33.8, which is in good accordance with the angle calculated from the general refraction law. Then we design a focusing metasurface with a size of 90 mm90 mm and 1515 cells. When the focusing metasurface lens is irradiated by left-hand circularly polarized wave, the refracted right-hand circularly polarized wave is focused at a point 40 mm away from the lens center. However, when the metasurface lens is impinged by the right-hand circularly polarized wave, the refracted left-hand circularly polarized wave is diffracted. This ultimately accords with different phase responses under different polarized waves when the metasurface cell is rotated. Furthermore, the metasurface lens diffracts the incident wave when impinged by right-hand circularly polarized wave, which validates the design principle. The beam-width at the focal spot and the focal depth are also calculated. The simulation results indicate that the beam-width at the focal spot is approximately equal to three quarters of the operating wavelength. Therefore, the circularly polarized wave refraction focusing metasurface has a good performance for focusing the refracted waves. In addition, the proposed focusing metasurface is simulated separately at f=14 GHz and f=16 GHz, and the results show a good focusing effect, which demonstrates the bandwidth characteristic of the focusing metasurface lens. This designed metasurface lens is thin, single-layered, and highly effective, and it is also convenient to fabricate. Moreover, the metasurface lens has an advantage over the conventional lens, which has potential applications in manipulating electromagnetic waves and improves the performance of lens.
Recently, metasurfaces (MSs) have continuously drawn significant attentions in the area of enhancing the performances of the conventional antennas. Thereinto, focusing MSs with hyperbolic phase distributions can be used for designing high-gain antennas. In this chapter, we first design a new reflected MS and use a spiral antenna as the feeding source to achieve a wideband high-gain antenna. On this basis, we propose a bi-layer reflected MS to simultaneously enhance the gain and transform the linear polarization to circular polarization of the Vivaldi antenna. Then, we proposed a multilayer transmitted MS and use it to enhance the gain of a patch antenna. This kind of high-gain antenna eliminates the feed-block effect of the reflected ones but suffer from multilayer fabrication. To conquer this problem, we finally propose a single-layer transmitted focusing MS by grouping two different kinds of elements and use it to successfully design a low-profile high-gain antenna.
The phase gradient metasurface has strong abilities to manipulate electromagnetic waves on a subwavelength scale and has a potential to enhance the antenna gain. Based on the single multi-resonance metallic patch srtucture, we propose a new kind of ultra-thin broadband unit cell to manipulate electromagnetic waves and enhance the gain. It has been demonstrated that anomalous reflection can be achieved by utilizing the magnetic resonance between metallic patch and ground plane. Moreover, it is believed that resonance with low quality factor (Q factor) is useful in extending the working bandwidth. In order to extend the bandwidth of phase modulation, it is necessary to design a kind of low-Q unit cell. Besides, we need to extend the phase shift to cover the entire range [0, 360] to achieve the focusing effect. Thus we design a suitable symmetrical unit cell composed of ring and cross metallic patterns to control the phase of reflected waves. The symmetrical structure is useful for decreasing the Q factor so as to get a kind of low-Q unit cell. Theoretically, ring and cross metallic patch can be regarded as multi-resonance unit cells, which can cover the entire scope [0, 360]. The unit cell operates at 15-18 GHz with a thickness of 1 mm and the sides of 0.3 0( 0=20 mm). Furthermore, we design a phase gradient metasurface composed of the designed unit cell to verify the broadband anomalous reflection and focusing effects in CST Microwave Studio; the effect can be clearly illustrated in the simulation results obtained at 15-18 GHz. Due to the successful conversion from plane wave to quasi-spherical wave, we can place the Vivaldi antenna at the focal point of the metasurface as a feed source to transform the quasi-spherical wave to plane wave to enhance antenna gain. The simulation results are in good agreement with the theoretical analysis. Meanwhile, the designed metasurface and Vivaldi antenna have been fabricated and applied to enhance the gain of Vivaldi antenna. Both simulation and test results show that the peak gain has been averagely enhanced by 11 dB during the -1 dB gain bandwidth of 15-18 GHz and the fractional bandwidth is 18.2%. Moreover, due to the thin thickness, light weight and broad band, the designed unit cell may open up a new route for the applications of phase gradient metasurfaces in the microwave band region, and may also used as an alternative of high-gain antenna.
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