# Towards Mirror-Less Graphene-Based Perfect Absorbers
## 3 Graphene Perfect Absorbers without External Mirror
### 'One-Port Mimicking Absorber' Scheme  
As aforementioned, the mirror-less GPA is extremely desirable for circumventing numerous limitations associated with the use of the external mirror such as DBR or metal reflector. As an alternative, one-port mimicking absorber scheme is conceptually to mimic a single-mode/mirror system by introducing the internal (or virtual) mirror in a two-port system, and thus the relevant absorption efficiency can be approximately described as Equation (1). In 2017, [PERSON] et al. [46] proposed a novel device structure for the perfect absorption of single-sided illumination, which consists of a high-contrast grating (HCG) with a broadband reflection spectrum, a slab separated by a gap region, and monolayer graphene placed just below the slab (Figure 9). In the CMT analysis, the authors treated the HCG as a lossless resonator with two nondegenerate resonance modes with high leakage rates \(\gamma_{1}\) and \(\gamma_{2}\) and resonance frequencies \(\omega_{1}\) and \(\omega_{2}\), respectively, while the slab with a graphene layer is treated as a lossy single-mode resonator with a low leakage rate \(\gamma_{3}\), a loss rate \(\gamma_{loss}\), and a resonance frequency \(\omega_{3}\) (Figure 9b). The proposed scheme can be dubbed triple-mode absorber'. The perfect absorption is obtained through proper direct  
Figure 8: GPA based on all-pass filter, which consists of two coupled grating resonators with mirror inversion–symmetry and graphene absorbing layers. (**a**) Schematic of the GPA. (**b**) Theoretical model of the coupled resonators for coupled mode theory analysis. (**c**) Transmission (solid line) and reflection (dashed line) spectra for different \(E_{f}\). (**d**) Transmission (solid line) and phase shift (dashed line) for different \(E_{f}\). Reproduced with permission from [88], © 2023 Springer Nature. and indirect coupling between the HCG and slab, by which the HCG (as a 100% internal mirror) and slab (as a single-mode resonator) act as single-mode/mirror system, that is, one-port mimicking. Regardless of the existence of graphene layer, transmission becomes zero. The proposed absorber provides excellent tolerance to structural parameters and graphene quality due to the strong field enhancement in the slab region (Figure 9d). Also, absorption spectrum tuning over a wider wavelength range of \(\sim\)300 nm is possible, keeping significantly high maximum absorption (\(>\)95%). These results mean that the proposed scheme significantly relieve the complexity of absorber design compared to the degenerate critical coupling absorber (or dual-mode absorber) [80] because the frequency degeneracy condition is no longer required. Recently, [PERSON] et al. [47] proposed one-port mimicking scheme in an asymmetric single resonator supporting two degenerate resonant modes (Figure 10). Although the two coupled resonator-based PA scheme mimicking the one-port system has many advantages [46], in general, a single resonator-based structure is preferred in terms of fabrication simplicity if no specific structural symmetry is required [47; 48; 49]. In the designed GPA with undoped monolayer graphene placed on a slab-waveguide grating (SWG), only one of the guided-mode resonance modes (GMR\({}_{21}\)) is responsible for absorption, while the other (GMR\({}_{13}\)) plays as an internal 100% mirror in conjunction with the Fabry-Perot-like background scattering. The operation concept was confirmed through CMT analysis (Figure 10b). Almost perfect absorption (\(A>99.95\%\)) was achieved at the resonant wavelength of the high-Q GMR\({}_{21}\) mode. Unlike the degenerate critical coupling absorber [80], the proposed SWG structure is vertically asymmetric, and thus, two degenerate resonant modes (that is, GMR\({}_{13}\) and GMR\({}_{21}\)) are indirectly (not directly) coupled due to partial reflection in the internal wave propagation channel (Figure 10c).