## 4 Enabling Technologies for THz-FSO Communication
### Advanced Materials  
Recent innovations in material thickness have acted as the driving foundation of the fabrication of THz-FSO group communication systems. These materials facilitate critical operations like high-speed signal processing, enhanced sensitivity, and low power consumption, which form the framework for building efficient THz-FSO networks [55; 68; 69].  
Graphene and 2D materials: THz-FSO systems have been enriched with THz waves' excellent features, which are embodied by graphene or other 2D materials [43]. The client on-chip THz measurement capability of graphene is attributed to its high carrier mobility and ultra-fast carrier dynamics for THz signal modulation and detection. Its tunable conductivity, as governed by the Drude model, allows for adaptive device functionality, enhancing the bandwidth and energy efficiency of THz devices:  
\[\sigma(\omega)=\frac{\mathrm{e}^{2}}{\pi\hbar}\frac{\mu}{1+(\omega\tau)^{2}} \tag{14}\]  
Graphene devices exhibit harmonious integration with compact and flexible designs, making them suitable for portable THz-FSO systems. However, a problem exists regarding the scalability of graphene production and its compatibility with present-day device structures. There is also the development of problems such as inadequate electrical conductivity and the need for more attractive, lighter, or cheaper materials that can be formed through chemical vapor deposition (CVD).  
Metamaterials: THz metamaterials with engineered nanoscale structures feature promising capabilities in terms of the phase, amplitude, polarization, direction, and frequency of THz waves [70]. They are indispensable in controlling the beam, wavefront, and polarization in THz-FSO systems, modeling refractive indices that are negative and tunable.  
Figure 6: Enabling technologies for THz-FSO communication.  
The relationship between reflectance, transmittance, and absorption can be represented as follows:  
\[\mathrm{R+T=1-A} \tag{15}\]  
These materials have been used to fabricate high-accuracy THz lenses and waveguides with low loss and superior positioning accuracy. However, the high cost and difficulty of mass producing metamaterials are still the main issues. Subsequent innovations in optimizing the nanoparticle have also boosted the effectiveness of metamaterial devices, cutting down on light loss and representing a general upgrade of the host systems.  
Nanoparticles and hybrid materials: the incorporation of nanoparticles into photonic fibers has been demonstrated to minimize the dispersion and light loss in THz-FSO systems [35; 71]. Optically dense nanoparticles can help improve spot size localization and can permit changes on the wavefront within a fly's time. In the same way, new compounds, including ITO/SeO\({}_{2}\), are characterized by enhanced optical modulation characteristics that can increase the information transfer rate and ensure the process's reliability.  
ITO/SeO\({}_{2}\) interfaces: The material that has most recently been found suitable for use in the fabrication of optical modulators for THz-FSO systems is indium tin oxide (ITO)-selenium dioxide (SeO\({}_{2}\)). These interfaces cause this link to experience losses and, at the same time, allow for high modulation depths [1; 72]. Pertaining to both designs and simulations, their modulation stability is considered as a strength, while their non-linearity effects are minimal. Nevertheless, their potential to obtain a uniform material property over large areas is still a challenge.  
Table 9 presents details on the constituents and use of high-performance THz-FSO system enablers such as graphene, metamaterials, and hybrid nanoparticles. These materials offer specific approaches to problems associated with dispersion, light loss, and wavefront control while exposing the more minor problems that are still left to be solved, such as affordable fabrication and integration.