# Implementation of Atomically Thick Graphene and Its Derivatives in Electromagnetic Absorbers
## 3 Graphene-Based Metasurface Absorbers
### Multiband and Broadband Operation for Graphene Metasurface Absorbers  
Similar to the Salisbury screen, most of the graphene metasurface absorbers also utilize metallic ground that allow almost zero transmittance. Full-wave simulations have been widely used to estimate the performance of graphene metasurface absorber. With carefully designed unit cell structures, a couple of graphene metasurface absorbers with dual resonant modes within THz frequencies have been theoretically investigated [26; 107; 108; 109]. In 2016, a quad-band THz absorber utilizing a single patterned graphene layer was presented by [PERSON] et al. The symmetrical unit cell design makes it insensitive to the polarization of EM waves and the simulation results demonstrate excellent wave absorption under an incident angle between 0\({}^{\circ}\) to 40\({}^{\circ}\). The next year, [PERSON] et al. proposed a penta-band metasurface absorber composed of metallic rings and graphene sheets operating at far-infrared frequencies, as shown in Figure 6a,b [110]. A transmission line model is derived and the calculated results agree excellently with the results achieved from full wave simulations. The absorber exhibits stable wave absorption while the incident angle varies between 0\({}^{\circ}\) to 75\({}^{\circ}\) for both TE and TM polarized waves. It is noted that, for metasurfaces supporting multiple resonant modes, if the resonant frequencies are designed close enough, the absorption peaks would partly overlap with each other and the device tends to perform as a broadband absorber. For instance, [PERSON] et al. proposed a graphene metasurface absorber utilizing unit cells consist of three metallic circular patches with different diameters, as shown  
Figure 6: (**a**) schematic of penta-band graphene metasurface absorber; (**b**) the absorption spectra achieved with full-wave simulation (green solid line) and equivalent circuit model (black dashed line), reprinted with permission from Ref. [110]. in Figure 7a [111]. From the numerical simulation results, it is easy to see that the device resonates at 33.68 THz, 35.90 THz and 39.65 THz, resulting in an enhanced effective absorption bandwidth compared with the device with a single resonant mode. In addition, thanks to the symmetric circles used in the unit cells, the metasurface also demonstrates polarization-insensitive performance over 31.2-37 THz with no less than 90% wave absorption (see Figure 7b). In addition, multiple resonant modes can also be achieved by utilizing the polarization-insensitive dual electric LC (i.e., inductance (L) and capacitance (C)) unit cells, as shown in Figure 7c [95]. Effective wave absorption is achieved within 27.78 THz to 42.16 THz under a wide incident angle (see Figure 7d,e), corresponding to a maximum fractional bandwidth of 41.12% at normal incidence. Moreover, it is also possible to achieve broadband THz absorption utilizing a single layer of periodic graphene ribbons with gradient width [112], sinusoidally-patterned graphene sheets [113] and square-patterned graphene [114]. Although asymmetric graphene sheets and ribbons are used for unit cell designs, these absorbers demonstrate polarization-insensitive absorption and small performance variation for incident angles between 0\({}^{\circ}\) to 30\({}^{\circ}\). In 2016, [PERSON] et al. proposed a broadband absorber with four gold helices buried in the dielectric for each unit cell. Graphene is placed on the top of the metasurface [115], as shown in Figure 8, to enhance the wave absorption and a relative absorption bandwidth of 160% (i.e., for wavelength 200-1792 nm) is achieved. The symmetrical unit cell design also guarantees polarization-independent wave absorption performance over near ultraviolet to near infrared frequency. Figure 7: (**a**) unit cell of broadband graphene metasurface absorber consisting of continuous graphene and three circular metallic patches; (**b**) simulated absorption spectra with various azimuth angles (\(\varphi\)), reprinted with permission from Ref.