# Towards Mirror-Less Graphene-Based Perfect Absorbers
## 4 Applications  
Until now, practical applications of GPAs have yet to reach a state of maturity. In this section, we briefly introduce a few optoelectronic applications of GPA structures using external mirrors. Figure 12a indicates the schematic depiction of the possible applications, such as high-efficiency modulators [14; 15; 36; 93; 94; 95], photodetectors [12; 13; 16], tunable polarizers [96; 97; 98], switches [99; 100; 101; 102; 103], and sensors [104; 105; 106]. To create the optical modulator based on an electrically tunable GPA, in 2014, [PERSON] et al. [14] proposed a widely tunable metasurface composed of optical antennas on graphene, which is incorporated into an ultrathin (<\(\lambda_{0}\)/10) optical cavity (Figure 12b). By switching the GPA in and out of the critical coupling condition via the gate voltage applied on graphene, a maximum modulation depth over 95% and modulation speed up to 20 GHz over a mid-IR range can be achieved. The operation wavelength can be scaled from the near-IR to THz ranges due to the great flexibility in tailoring the response of metasurfaces combined with the broadband optical response of graphene. In 2012, [PERSON] et al. [16] reported that by monolithically integrating graphene with a Fabry-Perot microcavity with a high finesse, compared to the \(\sim\)2.3% absorption of free-standing graphene, the optical absorption is 26-fold enhanced at \(\sim\)850 nm wavelength, as shown in Figure 12c. The absorbing graphene layer is sandwiched between two DBR mirrors. A buffer layer ensures that the maximum of the field amplitude occurs right at the position where the graphene sheet is placed. In particular, for bilayer graphene, a maximum responsivity of 21 mA/W is achieved. Figure 11: GPA based on one-port mimicking, which consists of a monolayer graphene placed on a SWG that supports both quasi-BIC and GMR. (**a**) Schematic of the GPA. (**b**) Electric field distribution at optimal condition. (**c**) Reflection spectra as a function of incident angle without graphene. (**d**) Absorption spectra as a function of incident angle with graphene. Perfect absorption point is marked by the white open circle. Reproduced with permission from [49], © 2023 Springer Nature. In 2020, [PERSON] et al. [96] experimentally demonstrated an electrically tunable perfect absorber at 0.43 THz by integrating a metallic grating into a graphene-based Salisbury screen structure (Figure 12d). In particular, polarization-dependent reflection of the hybrid absorber was investigated. The measured results reveal that the reflection remains nearly 1 under different gate voltages when the polarization of the incident THz field is parallel to the gate grating, while the reflection can be electrically tuned from 0.5 to nearly 0 for the crossing polarization. Therefore, the proposed graphene-grating hybrid metasurface is highly anisotropic and able to function as an electrically tunable THz polarizer with an extinction ratio up to 23 dB. In particular, the GPAs based on dielectric resonant structures have the potential for sensing or switching applications because they can minimize the undesired ohmic loss in metals, and even ultra-narrow (\(Q>\) 10,000) graphene perfect absorption can be achieved [28; 33; 102; 103]. For example, in 2020, [PERSON] et al. [103] numerically demonstrated  
Figure 12: Application of GPA. (**a**) Schematic of the possible applications of GPA using external mirrors. (**b**) Top: schematic of the ultrathin optical modulator based on a tunable metasurface absorber backed by Al substrate. Bottom: modulation depth achieved experimentally at different wavelength and corresponding insertion loss [14]. (**c**) Top: schematic of a graphene microcavity photodetector using two DBRs. The incident light is trapped in the cavity and passes multiple times through the graphene. The graphene sheet is shown in red, and the metal contacts are in yellow. Bottom: spectral response of the monolayer graphene device.