# Implementation of Atomically Thick Graphene and Its Derivatives in Electromagnetic Absorbers
## 2 Graphene-Based Quarter-Wavelength Absorbers
### Graphene-Based Salisbury and Jaumann Absorbers
Finally, the PMMA is removed with acetone, leaving only graphene on the new substrate. To reduce the sheet resistance (\(R_{s}\)) of the graphene, they utilize multilayer graphene in the absorber by stacking the graphene-PMMA film on the top of monolayer graphene synthesized on Cu/SiO\({}_{2}\)/Si and repeating the etch-and-transfer step. As the thickness of multilayer graphene is negligible, the differences between mono- and multilayer graphene in this work is reflected by the transmittance of optical light, as shown in Figure 1d. The Salisbury screen absorber, utilizing \(d=1.3\) mm and \(\epsilon_{r}=3.8\) quartz spacer, resonates at a fundamental frequency near 29.6 GHz. The corresponding reflection loss varies with the number of stacked graphene layers at the 5 th harmonic frequency as shown in Figure 1e. To enhance its operation bandwidth, a few graphene/quartz pairs are also stacked on the top of each other to realize the Jaumann absorber and the \(n=5\) device demonstrates effective wave absorption over a wide frequency range (125-165 GHz), as shown in Figure 1f. Except increasing the number of graphene layers, [PERSON] et al. also proposed a graphene-based Salisbury screen for THz frequencies, where chemical doping is implemented to reduce the sheet resistance of their multilayer graphene [38]. As the millimeter thick dielectric spacers are used in the Salisbury and Jaumann absorbers mentioned above, it is difficult to apply electrostatic doping to the graphene layer. Hence, these absorbers operate with fixed absorption and frequencies. To achieve a tunable Salisbury screen, ionic liquid (or gel) is utilized to provide more effective gate control over the traditional dielectric gates in a graphene capacitor structure as shown in Figure 2a,b. For the ionic liquid sandwiched between two graphene/dielectric layers, when a bias voltage is applied, the electrons inside the liquid tend to move and accumulate on the anode graphene layer. The ions with positive charges, on the other hand, move in the opposite way and accumulate on the cathode graphene layer. Thus, the graphene layers on both sides are doped and \(R_{s}\) can vary significantly when a small bias voltage is applied. The thickness of ionic liquid is usually within a few tens of micrometers (e.g., 50 \(\upmu\)m) and a bias voltage as low as \(\pm\)3.5 V can be used for effective control [39]. By replacing the resistive layer with flexible graphene capacitor, tunable Salisbury screen absorbers operating at microwave [39] and THz [40] frequencies have been experimentally investigated. Under normal incidence, the graphene Salisbury screen in [39] demonstrates tunable wave absorption between 3 dB to 60 dB at 10.5 GHz while the bias voltage varies between 0 V to \(-\)1.5 V, as shown in Figure 2c. In Ref. [40], one of the graphene electrodes is replaced by the gold ground and a 20 \(\upmu\)m thick porous membrane with ionic liquid soaked inside is sandwiched between the electrodes. Thanks to the reduced thickness of the dielectric spacer (i.e., porous membrane), switchable absorption performance is achieved at 2.83 THz under an incident angle of 30\({}^{\circ}\) when the bias voltage varies between 0 V to 2 V, as shown in Figure 2d,e. In addition, the ionic liquid can also be used in a top-gate biased absorber operating at infrared frequencies, as shown in Figure 2c [41]. The device demonstrates a maximum normalized differential reflectance (i.e., \(\Delta R/R\)) of 0.042, where \(R\) is the reflectance at \(E_{F}=0\) and \(\Delta R\) is the reflectance variation when \(E_{F}\
eq 0\). This graphene Salisbury screen demonstrates poor absorbing performance at infrared frequencies due to the Drude-like behavior of free electrons in graphene [42]. Although the tunable graphene Salisbury screen absorbers have demonstrated excellent switching performance at microwave and THz frequencies, the volatile ionic liquid used for electrostatic doping has to be properly packaged to avoid evaporation.