Design of reflective linear-circular polarization converter based on phase gradient metasurface
Authors: Zhuang Ya-Qiang  , Wang Guang-Ming  , Zhang Xiao-Kuan  , Zhang Chen-Xin  , Cai Tong  , Li Hai-Peng  ,   Air and Missile Defense College, Air Force Engineering University, Xi'an 710051, China
Published: December 2020

Manipulating the propagating direction and polarization state of electromagnetic wave is always fascinating and used in a wide field. One of the approaches to achieving this aim is typically based on steering the propagation phase of wave traveling inside an optical medium, such as dielectric lens. Nevertheless, this approach creates new problems, such as high loss, bulky volume and fabrication difficulty. Recently, metasurface was found to be a two-dimensional equivalence of metamaterial, which attracted a great deal of attention because of its unique properties and capability of manipulating and controlling electromagnetic waves on a sub-wavelength scale. So metasurface serves as an alternative approach to dealing with the loss and fabrication issues, and opens a door for bridging the gap between the fundamental research of the artificial structures and their device applications.
A reflective phase gradient metasurface (PGM) achieving the linear-to-circular (LTC) polarization conversion and anomalous reflection simultaneously is designed in this paper. Firstly, the conventional cross-shaped structure is modified for enlarging the phase range. Then, six modified cross-shaped structures are designed cautiously to serve as quarter wave-plates, and achieve 60 phase difference between adjacent structures. The reflection phase difference between x-and y-direction components is 90, and their magnitudes are both equal to 0.5. Secondly, a one-dimensional PGM is constructed by distributing six modified cross-shaped quarter wave-plates one by one. Furthermore, an LTC polarization converter with an area of 216 mm216 mm is designed by placing 366 one-dimensional PGMs periodically. The mirror reflectivity and axial ratio are simulated and measured to verify the performances of LTC polarization conversion and anomalous reflection. The measured sample is fabricated by printing circuit board technique through using FR4 substrate, and a free space method is adopted in measurement in the anechoic chamber. In addition, the operating bandwidth can be evaluated from the reflective power density spectra. The measured results of mirror reflectivity, reflective power density spectra and axial ratio characteristic are in good agreement with the corresponding simulations, which shows that the mirror reflectivity is lower than -10 dB; the axial ration is lower than 2 dB within the frequency band of 13.8-14.7 GHz. Meanwhile, the theoretical reflection angles from the generalized Snell law are consistent with the CST microwave studio simulated results and measured results. Compared with the reported LTC polarization converters, the proposed LTC polarization converter not only achieves polarization conversion, but also can manipulate the output wave direction, thereby it has an important promising application value for microwave engineering and communication system.