**1. Introduction**

Graphene-based absorbers are receiving a considerable interest due to their potential applications in photovoltaics [1–3], as wave modulators [4–8], biological sensors [9–11], photodetectors [12–15], etc. Achieving graphene-based perfect absorbers is quite challenging because single layer graphene has got a weak and spectrally broad absorption of 2.3% over a wideband wavelength range from Vis to Far-infrared [16–21].

In the THz and IR regions, however, high quality graphene may show strong interaction with light thanks to generation of surface plasmon polaritons (SPPs), making graphene a promising alternative to typical plasmonic materials [22–25]. This is thanks to the capability of graphene to support plasmon modes with extremely tight confinement, long lifetime, and low losses at IR and THz frequencies [26–29].

In addition, tunable graphene electromagnetic response may be achieved by chemical and/or electrical doping or by using a magnetic field [30–32].

**Citation:** Nematpour, A.; Grilli, M.L.; Lancellotti, L.; Lisi, N. Towards Perfect Absorption of Single Layer CVD Graphene in an Optical Resonant Cavity: Challenges and Experimental Achievements. *Materials* **2022**, *15*, 352. https:// doi.org/10.3390/ma15010352

Academic Editor: Fabrizio Roccaforte

Received: 28 November 2021 Accepted: 28 December 2021 Published: 4 January 2022

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In the Vis and NIR regions, on the contrary, absence of SSPs makes it necessary to couple graphene with resonant structures such as metamaterials, photonic crystals and plasmonic materials [33–39].

Many examples of perfect graphene-based absorbers have been reported in the literature basing on the critical coupling concept [40–42].

Thongrattanasiri et al. have simulated 100% absorption in doped graphene nanodisks by exploiting the critical coupling conditions [43].

Piper et al. [44] have demonstrated numerically perfect absorption in unpatterned monolayer graphene in the Vis and NIR range by means of critical coupling with guided resonances of a photonic crystal slab with a back reflector.

Liu et al. [45] have demonstrated experimentally close to total absorption (85%) in monolayer graphene based on critical coupling with guided resonances in Fano resonance photonic NIR filters, by using a structure with a back reflector.

Xiao et al. [46] have reported about a general theoretical method for tailoring the absorption bandwidth of graphene via critical coupling in the NIR range by using a simple two-port resonant structure composed of a graphene-covered two-dimensional photonic crystal slab.

Jin et al. [47] have reviewed recent advances of graphene-based architectures for perfect absorption from Vis to THz band, both narrowband and broadband, and have discussed also about criticalities in the practical implementation of the simulated structures.

Another strategy to enhance graphene absorption is to exploit the electric field enhancement by resonant cavities [48–50]. Table 1 reports the graphene absorption values obtained by many authors by exploiting the electric field enhancement inside Fabry–Perot resonant cavities.


**Table 1.** Summary of simulated and experimental absorption of graphene inside Fabry–Perot structures in the Vis-THz region, as reported in the literature.

Nulli et al. reported theoretically that the optical absorption of single-layer graphene can be enhanced up to 50% by increasing the electric field on the surface of undoped graphene by placing graphene inside symmetric Fabry–Perot structures [48]. Moreover, Ferreira et al. have calculated the same absorption result for single layer graphene by utilizing one Fabry–Perot cavity and they have simulated also that graphene optical absorption can increase up to 100% using two symmetric Fabry–Perot cavities [50].

Zand et al. have used a genetic optimization algorithm coupled to a transfer matrix code to design one-dimensional aperiodic multilayer microstructures embedding single layer graphene, where near-total absorptions at selected wavelengths is obtained by existence of critical coupling [61]. They simulated that, compared with asymmetric-Fabry– Perot-based designs, aperiodic structures may provide higher efficiency for the spatial selective localization of the resonant modes.

Furchi et al. [63] have demonstrated experimentally in 2012 a graphene-based photodetector. Absorption enhancement up to 60% at 855 nm was demonstrated by inserting exfoliated µm-sized graphene flakes inside a dielectric multilayer stack grown by combining plasma-enhanced chemical vapor deposition and molecular beam epitaxy.

Many of the theoretical studies found in the literature simulating perfect graphene absorption consider for single layer graphene a very high mobility µ of about 10.000 cm<sup>2</sup> ·V <sup>−</sup>1/s−<sup>1</sup> , which is generally much higher than the real one (non-isolated from the environment, CVD graphene), leading sometimes to too overoptimistic predictions [64,65].

Apart from the work from Furchi et al., the enhancement of graphene absorption by using an optical resonant cavity has been rarely explored experimentally due to the many experimental challenges related to embedding graphene inside metals or dielectric stacks [63].

The graphene absorption enhancement inside an optical resonant cavity is generally a narrowband and is, therefore, more suitable for application in photodetectors, sensors, or absorption filters [63,66,67]

In the present work, we summarize our recent studies [67–69] on CVD graphene absorption inside three different Fabry–Perot (FP) filters fabricated by radiofrequency sputtering. Experimental challenges related to graphene-based Fabry–Perot filters fabrication are described in detail with the aim to provide a useful recipe which can help researchers to embed graphene inside materials grown by conventional physical vapor deposition (PVD) techniques, without altering its properties.

In the optimized structure, a high absorption of 84% at 3150 nm was measured [67] in case of large area (1 inch) single layer CVD graphene, which is the highest value of absorption so far experimentally achieved for single layer graphene inside a Fabry–Perot optical cavity.
