# Tuning of Graphene-Based Optical Devices Operating in the Near-Infrared
## 3 Graphene-Based Optical Devices and Tuning  
In this section, we discuss the graphene-based optical device configurations in order to understand the possible options that can be exploited for wavelength tuning. As mentioned above, several types of device configurations can be distinguished; among them, the modulators can be classified in: Mach-Zehnder graphene modulator (M-ZM) [8; 9; 18; 67], planar photonic crystal nanocavity-based modulator (CNM) [7; 10; 19; 20] (which represent a graphene FET transistor) and micro-ring resonator (MRR) [24; 25; 26; 26]. Figure 7 shows some examples of these configurations. Mach-Zehnder interferometer (MZI)-based modulators suffer from a large footprint, typically a few millimeters, while ring or disk modulators exhibit a narrow optical bandwidth, making them difficult to control and sensitive to temperature variations or fabrication errors [14]. The use of a CNM with an air gap with highly restricted modes in the central air gap improves the overlap between graphene and resonant cavity modes. These devices can be characterized as a waveguide combined with a capacitor structure (GIW or GIG configurations) and contacts connected to GEs for voltage supply. It should be borne in mind that there are important differences among different configurations. Assuming quasi-TM polarization, GIG-Si devices exhibit almost double the absorption with respect to that of GIW-Si devices. For TE polarization, the absorption in Si-based devices is significantly lower (up to 0.12 and 0.08 dB/\(\upmu\)m for GIG-Si and GIW-Si devices, respectively) [14]. This is due to the interaction with the strong longitudinal component of the electric field at the upper boundary of the silicon waveguides (note that the graphene sheet does not interact with the field components outside the plane [57]). In Si\({}_{3}\)N\({}_{4}\) devices, quasi-TE-polarized modes exhibit stronger interactions with the graphene film than quasi-TM modes. The GIG-Si\({}_{3}\)N\({}_{4}\) has lower absorption than the GIW-Si device, but by embedding the capacitor in the Si\({}_{3}\)N\({}_{4}\) waveguide, the absorption level of the GIG-Si\({}_{3}\)N\({}_{4}\)-Emb can be increased again, almost to that of the GIG-Si, although the waveguide is large [66]. The modulators discussed were fabricated using standard semiconductor manufacturing processes. This work uses photolithography [7; 8; 9; 10; 25] or electron beam lithography in combination with dry etching [9; 11; 12; 13]. While silicon photonics is very promising for optical routing, a complete optical network also requires generation, modulation and light detection--which is difficult to achieve on a completely monolithic platform. Growing materials is a technologically challenging task on a silicon substrate due to the mismatch of the lattice constants and thermal expansion coefficients. Defects arising from imperfections during crystal growth also tend to limit the optical and electrical performance of such devices. One solution to this problem is to grow materials on a compatible substrate and then transfer them onto silicon [69]. In this way, it is not necessary to match lattice constants or thermal expansion coefficients as required in direct growth processes. Two-dimensional (2D) materials are a class of crystals that naturally lend themselves to this type of transfer process. Because these materials are covalently bonded in-plane and held together out-of-plane by Van der Waals forces, individual atomic planes can be mechanically separated from the bulk crystal and placed onto arbitrary substrates [70]. Since the discovery of the first isolated graphene layer prepared by mechanical exfoliation of graphite crystals, many chemical approaches to synthesize large-scale graphene have been developed, including epitaxial growth on silicon carbide and ruthenium, as well as two-dimensional assembly of reduced graphene oxides and exfoliated graphene sheets [71]. Several transfer processes are known that can be classified as \"wet\" or \"dry\" [69].