*2.2. Fabry–Perot Filter: Fabrication*

λ λ The Fabry–Perot filters consisted of alternate layers of Si and SiO<sup>2</sup> with optical thickness equal to a quarter wavelength (λ/4, where λ is the central wavelength of the FP filter). Si and SiO<sup>2</sup> layers were fabricated by radio frequency sputtering in MRC (Material Research Corporation) systems, starting from a 99.999% purity Si and/or SiO<sup>2</sup> targets, as described elsewhere [67–69]. For FP1 and FP2, two different sputtering conditions were chosen: in case of all the layers except for the first 30 nm of the SiO<sup>2</sup> layer covering graphene, a radiofrequency sputtering of 200 W and a sputtering pressure of 1 Pa were used. For the first 30 nm of SiO<sup>2</sup> layer deposited onto graphene, a milder sputtering condition was used: i.e., sputtering power of 50 W and sputtering pressure of 3 Pa. For FP3 filter, grown in a different system, the same sputtering conditions (sputtering power of 200 W and sputtering

pressure of 0.53 Pa) were used for all SiO<sup>2</sup> layers, due to the fact that such system geometry allowed a milder sputtering process.

MgF<sup>2</sup> layer was carefully evaporated in a Balzer BAE 250 evaporation system, and the evaporation rate was controlled by a quartz crystal microbalance.
