*3.1. Graphene Absorption Simulation*

Figure 5a shows the simulated absorption of graphene inside the three FP filters while Figure 5b shows the FPs' absorption without the single layer graphene. The wavelengthdependent absorption A(λ) was inferred from the wavelength-dependent transmittance T(λ) and reflectance R(λ) (A(λ) = 1 − R(λ) − T(λ)) [71,72], by using the Wave Optics Module of Comsol Multiphysics software. Graphene optical properties were simulated by a wavelength-dependent complex refractive index *<sup>n</sup>g*(*ω*) = <sup>q</sup> *εg*(*ω*) [73,74], where the single-layer permittivity graphene was [75–77]:

$$\varepsilon\_{\mathcal{S}}(\omega) = \varepsilon\_b + \frac{i\sigma(\omega)}{t\_G\omega\varepsilon\_0}$$

being *ε<sup>b</sup>* (2.5) [78,79], *t*<sup>G</sup> (0.35 nm) [80,81], and *σ*(*ω*) the intrinsic contribution to the graphene relative permittivity, the thickness of single layer graphene and the graphene optical conductivity, respectively.

The graphene optical conductivity was considered as the sum of interband transitions and Drude-like intraband conductivity [82–85]:

$$\begin{array}{l} \sigma(\omega) = \frac{\sigma\_0}{2} \big[ \tanh(\frac{\hbar \omega + 2E\_f}{4k\_B T}) + \tanh(\frac{\hbar \omega - 2E\_f}{4k\_B T}) \big] \\\ -\frac{i\sigma\_0}{2\pi} \log(\frac{(\hbar \omega + 2E\_f)^2}{\left(\hbar \omega - 2E\_f\right)^2 + \left(2k\_B T\right)^2}) \\\ +i\frac{4\sigma\_0}{\pi} \frac{E\_f}{\hbar \omega + i\hbar \gamma} \end{array}$$

with *σ*<sup>0</sup> = ( *<sup>e</sup>* 2 4ℏ ) [82,86], *E<sup>f</sup>* the Fermi energy, and *γ* the intraband scattering rate. In the simulation, ℏ*<sup>γ</sup>* = 40 meV [83] and *<sup>E</sup><sup>f</sup>* = 190 meV corresponding to slightly doped graphene as reported elsewhere [83,87].

Single layer permittivity was simulated also as a function of the wavelength and Fermi level, as shown in Figure 6.

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**Figure 5.** (**a**) Simulated absorption curves of SLG inside the Fabry–Perot filters in case of symmetric FP1 (A1), asymmetric FP2 (A2), and asymmetric reflective FP3 (A3). (**b**) Simulated absorption of FP filters without graphene insertion.

**Figure 6.** *Cont*.

**Figure 6.** Simulated SLG permittivity as a function of wavelength and Fermi level. (**a**) Real and (**b**) imaginary parts.
