# Towards Mirror-Less Graphene-Based Perfect Absorbers
## 5 Conclusions and Perspectives  
In this topical review, we briefly summarized the recent advancements of the GPAs with external mirror and without external mirror, focusing mainly on their operation principle and design requirement. Graphene is a very attractive material for optoelectronic devices due to its remarkable carrier mobility and broad absorption band. However, because its potential use is limited by low absorption of graphene, GPA is the first step to approach practical application. Until now, numerous studies for enhancing graphene absorption have been reported, but there is still considerable room to develop the perfect graphene absorbers based on the different schemes investigated in this review paper. Table 1 presents a comprehensive summary of the key parameters governing the absorption performance of representative GPAs, including the state-of-the-art technology GPAs. Actually, experimental achievements have been limited to the GPAs with an external mirror. For example, as shown in Figure 4b, for monolayer graphene coupled with subwavelength PMMA gratings on top of a back gold mirror, peak absorptions over 99% at a wavelength of \(\sim\)1500 nm with FWHM about 20 nm were measured, which are in excellent quantitative agreement with the simulation results [29]. In 2021, [PERSON] et al. [107] demonstrated experimentally for the first time a great enhancement in absorption in large-area (1 in diameter) monolayer CVD graphene by exploiting the electric field inside an asymmetric Fabry-Perot resonator fabricated by RF sputtering. The measured absorption of 84% that peaked at 3150 nm with a bandwidth of 44 nm is in very good agreement with COMSOL Multiphysics calculations. In 2023, [PERSON] et al. [108] proposed the photodetector structure using Tamm plasmon by combining a metal film and a DBR, where graphene was transferred onto the DBR, and a metal film was fabricated on the graphene layer. A strong localized field is generated by the structure and detected by graphene, which converted to the photocurrent response. So, the responsivity of the photodetector can be affected by the absorption of the structure. In detail, a maximum absorption of \(\sim\)60% and responsivity of \(\sim\)330 \(\mu\)A/W are measured at an incident angle of 50 degrees and a wavelength of \(\sim\)850 nm. The difference in reflectance (or absorption) between the experiment and simulation can be observed, which is caused by the error in the thickness of the metal film in the fabrication process. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Structure** & **Wavelength** & **Absorption** & **FWM** & **Fractional** & **Sim. ** & **Ref. ** \\  & **or Frequency** & **Efficiency (\%)** & **FWM** & **Bandwidth (\%)** & **or Exp. ** & **Ref. ** \\ \hline  & \(\sim\)1500 nm & 99.6 & \(\sim\)5.5 nm & — & Sim. & [24] \\  & 1306 nm & 99.4 & \(\sim\)0.5 nm & — & Sim. & [39] \\  & 1586 nm & 95.52 & \(\sim\)35 nm & — & Sim. & [54] \\  & \(\sim\)1606 nm & \(\sim\)95.8 & \(\sim\)0.018 nm & — & Sim. Among various absorbers discussed in this review, the mirror-less GPAs are expected to be very promising due to their having two key advantages over the conventional absorbers using an external mirror. Firstly, they simplify the design and fabrication of absorber structure due to the absence of DBR, which requires a sophisticated growth technique. Secondly, they improve net absorption in the graphene layer because the parasitic absorption such as ohmic loss in metal mirror is minimized. In particular, it is noteworthy that the mirror-less absorbers based on one-port mimicking scheme do not require strict structural symmetry, and thus, the limitation on material index choice is greatly relieved. Adopting a novel all-dielectric Huygens' metasurface, which offers easy individual tuning of the relevant resonant modes by adjusting geometrical parameters of each element, would offer a new route for mirror-less GPAs. Another prospective research direction is to introduce mechanical flexibility in GPAs by utilizing flexible substrate.