# Implementation of Atomically Thick Graphene and Its Derivatives in Electromagnetic Absorbers
## 3 Graphene-Based Metasurface Absorbers
### Reconfigurability of Graphene Metasurface Absorbers  
Thanks to the tunable conductivity of graphene, a large amount of theoretical investigations on tunable graphene metasurface absorbers have been presented in the past few years. By combining the continuous graphene layer with patterned metallic unit cell structures, EM wave absorbers with reconfigurable operation frequency [20; 29; 31; 124] or bandwidth [95; 105] can be achieved. The devices with two independently-tunable resonant frequencies at THz [125; 126] and mid-infrared [127] frequencies are also explored with full-wave simulations, demonstrating excellent stability on one resonant frequency while the other frequency is tuned. In addition, by varying the chemical potential of graphene, some metasurface absorbers also exhibit switching performance and operate as either fine absorbers or reflectors [104; 113; 114; 128; 129]. It is worth mentioning that the thick dielectric layer in the range of micrometers used in some of these theoretical calculations are impractical for electrostatic doping [22; 27; 28; 113]. Furthermore, in some works, the effects of interconnecting wires between unit cells for the implementation of external bias voltage have not been taken into consideration, resulting in great uncertainty in the actual performance of the absorbers [22; 28; 108; 111; 129; 130; 131; 132]. Beside the theoretical explorations, [PERSON] et al. have also presented their experimental results on metasurface absorbers with square-patterned graphene on PET [133]. By utilizing samples with different \(R_{s}\) or changing the number of graphene/PET layers, the mechanically reconfigurable absorber utilizing tuning surface resistance (TSR) and stacking graphene metasurface (SGM) approaches are experimentally investigated, as shown in Figure 11a-d. Although both TSR- and SGM-type absorbers exhibit narrowband and poor microwave absorptions for normal incident waves, they exhibit mechanically tunable resonant frequency from 12.5 GHz to 13.3 GHz and 12.3 GHz to 13.5 GHz,  
Figure 10: **(a)** image of the fabricated flexible multilayer (reduced graphene oxide) rGO. The upper inset illustrates the rGO based frequency selective surface (FSS) sandwiched between two dielectric layers and the lower inset demonstrates the dimensional of a unpatterned multilayer rGO; **(b)** the measured RL spectra without rGO (blue), with rGO in full coverage (black) and with rGO-FSS (red), reprinted with permission from Ref. [123]. respectively. From the comparisons between simulation and experiment results, it is also noted that the gaps resulting from imperfect stacking process increase the resonant frequency of the absorber. In addition, [PERSON] et al. proposed a graphene metasurface absorber operating at microwave frequencies as shown in Figure 12a,d [134]. They sandwich the electrolyte between two PVC supported graphene FSSs and tend to vary the sheet resistance of graphene through electrostatic doping. Although excellent agreement have been achieved between measurement and simulation results for samples with \(R_{s}\) = 20, 50 and 70 \(\Omega\)/sq under normal incidence, none of their samples exhibit reconfigurable absorption performance that has been predicted by full-wave simulations. The authors attribute the failure of their sample to the hot lamination process that reduces the gap between the graphene layers. Recently, [PERSON] et al. presented their experimental results on a THz absorber utilizing patterned graphene as unit cells [135]. A narrowband absorption from 1.7 THz to 1.97 THz is experimentally achieved under a bias voltage of 30 V for normal incident wave.