# Towards Mirror-Less Graphene-Based Perfect Absorbers
## 2 Graphene Perfect Absorbers with External Mirror
### Performance Improvement of Absorbers
#### 2.2.2 Ultra-Broadband Absorption  
In general, a fundamental trade-off exists between the bandwidth and amplitude of attainable absorption. To achieve both broadband and high absorption, it is crucial to employ multiple resonances. In this subsection, we focus on ultra-broadband absorber structures, wherein various resonant modes are superimposed upon one another. The graphene-based ultra-broadband absorbers are highly desirable for some applications such as photovoltaics, photodetectors, and thermal emitters. Firstly, monolayer graphene-based ultra-broadband absorbers are reviewed. One effective method is to use unpatterned monolayer graphene sandwiched between a periodic structure consisting of dielectric or metal (for example, 1D gratings with isosceles trapezoid cross section [63], bricks arrays [64], elliptic cylinder arrays [65], and muti-circular gold patches of different radii [66], snowflake Koch Fractal (SKF) [67]) and a dielectric spacer on top of a metal reflector. In particular, in ref. [63] (Figure 6a), due to the coupling between Mie resonances with graphene plasmon resonance, a flat-top absorption spectrum (in detail, above 99% absorption covers the frequency ranges of 0.66-1.21 THz, and the fractional bandwidth reaches about 60%) is demonstrated. In 2019, [PERSON] et al. [67] introduced an ultra-wideband THz metamaterial absorber based on SKF dielectric loaded on a graphene sheet. For both TE and TM polarizations, ~160% fractional bandwidth is achieved while attaining over 90% absorption. Another alternative method to accomplish broadband absorption could be the utilization of multi-resonances of patterned monolayer graphene. For example, by applying the graphene concentric double rings [68], graphene ribbons with gradient width [69], sinusoidal-patterned graphene [70] (Figure 6b), complementary cross-oval-shaped graphene [71], and slotted-square graphene meta-rings [72], several or continuous graphene plasmon resonances can be excited over a broadband wavelength  
Figure 5: Angle-insensitive absorption based on GPAs using unpatterned graphene and periodic metal nanocavity. (**a**) Top: schematic of a GPA consisting of monolayer ENZ graphene embedded in metal nanocavity with silver ribbons. Bottom: absorption spectra as a function of incident angle for TM polarization at \(E_{f}=0.7\) eV [23]. (**b**) Top: schematic of a GPA consisting of monolayer dielectric graphene embedded in metal nanocavity with square silver plates. Bottom: absorption spectra as a function of incident angle for TM polarization [31]. Reproduced with permission from [23; 31], © 2023 Optical Society of America; © 2023 Springer Nature. range. In 2023, [PERSON] et al. [72] proposed the polarization-insensitive, broadband THz absorber comprising a simple meta-square ring of graphene, which possesses different slots in its structure to induce multiple plasmonic resonances. Above 95% absorption covers the frequency ranges of 2.2-4.6 THz, and thus, the fractional bandwidth reaches \(\sim\)70%. Secondly, mutli-layer graphene can be used to obtain ultra-broadband absorption. In 2019, [PERSON] et al. [73] experimentally demonstrated a 90 nm thick graphene metamaterial with grating, which consists of alternating graphene and dielectric layers (Figure 6c). The grating couples the light sideways into waveguide modes that propagate along the surface, leading to large absorption in the metamaterial. The absorber has broad-bandwidth absorption of well over 80% of non-polarized light over almost the entire solar spectrum (300-2500 nm). [PERSON] et al. [74] showed that the absorption efficiency of the proposed absorber can be as high as more than 90% over 2.10 THz (from 6.98 to 9.10 THz). From top to bottom, the  
Figure 6: GPAs with ultra-broadband absorption. **(a)** Left: schematic of a GPA consisting of unpatterned monolayer graphene and 1D gratings with isosceles trapezoid cross section. Right: absorption spectra at \(E_{f}=0.3\) eV. Inset show magnetic field amplitude for six representative frequencies in the 99% absorption band [63].