Polarization-Insensitive Metasurface for Harvesting Electromagnetic Energy with High Efficiency and Frequency Stability over Wide Range of Incidence Angles  
[PERSON]  
1 Key Laboratory of Specialty Fiber Optics and Optical Access Networks, Joint International Research Laboratory of Specialty Fiber Optics and Advanced Communication, Shanghai Institute of Advanced Communication and Data Science, Shanghai University, Shanghai 200444, China; [EMAIL_ADDRESS] (F.Y.); [PERSON]DRESS] (J.D.)  
1 Key Laboratory of Specialty Fiber Optics and Optical Access Networks, Joint International Research Laboratory of Specialty Fiber Optics and Advanced Communication, Shanghai Institute of Advanced Communication and Data Science, Shanghai University, Shanghai 2004444, China; [EMAIL_ADDRESS] (F.Y.); [PERSON]DRESS] (J.D.)  
1 Key Laboratory of Specialty Fiber Optics and Optical Access Networks, Joint International Research Laboratory of Specialty Fiber Optics and Advanced Communication, Shanghai Institute of Advanced Communication and Data Science, Shanghai University, Shanghai 200444, China; [EMAIL_ADDRESS] (F.Y.); [PERSON]DRESS] (J.D.)
without reflection. Meanwhile, the harvester should not only receive EM waves, but also effectively transfer the energy to a harvesting load for further energy recycling. Compared with the conventional rectenna, the harvester could receive EM waves with a higher efficiency over a wide range of incidence angles.  
In 2012, [PERSON] et al. proposed harvesting EM energy by the metasurface first [12]. Afterward, the research on metasurfaces was mainly based on split-ring resonators (SRRs) [12; 13] and complementary split-ring resonators (CSRRs) [14; 15], and metasurfaces with the characteristics of a wide range of incidence angles [16; 17; 18; 19; 20], broadband [12; 16; 21], multiband [15; 22], and polarization insensitivity [12; 13; 14; 15; 16; 23; 24] were investigated. There are two main methods for designing and analyzing metasurfaces: the artificial intelligence [22; 25] and equivalent circuit approach [26; 27; 28]. However, there exists a defect in those metasurfaces; the harvesting efficiency decreases significantly at a fixed frequency as the incidence angle increases. That is to say, the operation frequency of the peak efficiency changes with the incidence angle, although the harvester operates on a wide angle range. A broadband metasurface can avoid the influence of frequency shifting. However, the reported broadband metasurfaces could only remain at half of the peak efficiency [12; 16; 21], and the band with an efficiency of 90% was maintained only around the center frequency in the broadside direction.  
An absorber with a circular sector structure was proven to have a stable operation frequency under oblique incidence [10; 11]. In this paper, a polarization-insensitive metasurface with frequency variation of only 0.19% and up to a 75\({}^{\circ}\) incidence angle is proposed. The harvesting load is introduced in the eight circular sectors at a proper position to make the metasurface insensitive to polarization, and it is applied to collect EM energy. A square, ring-shaped metal via is added around the metasurface unit cell to reduce the mutual coupling for further improvement of frequency stability. Table 1 gives a comparison of the frequency deviations of the published works, with the incidence angle range and the maximum harvesting efficiencies at the original frequency on the broadside and at a 60\({}^{\circ}\) incidence angle. It is obvious that the proposed metasurface can maintain a higher efficiency over a wide, oblique incidence angle at the fixed operation frequency than those previously published.