# Key Roles of Plasmonics in Wireless THz Nanocommunications--A Survey
## 5 Plasmonic THz Wireless Nanoscale Link Components
### Plasmonic THz Antennas
#### 5.1.1 Design Issues
These metal nanoparticle dispersive properties response of different metals at THz range are also described by the Drude-Lorentz model by relative complex permittivity of metal nanoantennas as a function of frequency as seen in Figure 8 [106], with \(\varepsilon 1\) the real part of the relative permittivity, and \(\varepsilon 2\) is the imaginary part of it. Different metal nanoantennas like gold, silver, copper and aluminum are considered. An approximate 20% increase in absorption performance is obtained. The frequency that corresponds to the maximum absorption is dependent on the nanoparticle material. The maximum absorption is observed in gold nanoparticle material, as seen in Figure 9 [106]. The frequency of the maximum absorption moves toward lower frequencies as physical dimensions of the nanoparticles increase. Hence, the choice of the best plasmonic material for a given application is a subject of discussion and research. It has been also proved that the radiation frequency of the graphene nanoantenna can be tuned in a wide spectral range by adjusting the dimensions and particularly its length. According to [107], the absorption cross section increases up to a given limit with the substrate size, and hence, a larger substrate improves the performance of the graphene nano-antenna. Moreover, the absorption cross section increases as the graphene patch is located closer to the side of the substrate, while the resonant frequency becomes higher when the patch is farther from the center [107]. Figure 8: Complex permittivity of different metal nanoantennas at the THz range. (**a**) real part and (**b**) imaginary part of relative permittivity. Figure 7: Nanoantenna resonance characteristics as a function of the chemical potential. Therefore, the optimal location for on chip graphene nano-antennas may be near the edge of the substrate, in order to maximize their efficiency. As it concerns chemical potential, there is a tradeoff between the amount of power the graphene nano-antenna can absorb and its resonant frequency. Specifically, graphene nano-antennas with zero chemical potential, resonate at a low frequency but with a small absorption cross section, thus limiting their radiation efficiency. On the other hand, nano-antennas with a higher chemical potential experience greater absorption performances with an increased resonant frequency. Thus, there exists a compromise between these two parameters to be taken into consideration. In [108] there is an exhausting investigation and comparison of the performance in transmission and reception of metallic nano-dipole antennas, implemented via various metals such as Cu, Al, Ag and Au. Taking into account each metal property, such as its dynamic complex conductivity and permittivity, the propagation properties of SPP waves and the antenna geometrical features, as length and radius, a mathematical framework is developed in order to analytically derive critical transmission and reception performance parameters such as the generated plasmonic current in reception and the total radiated power and efficiency in transmission. As far as concerns the THz antenna size future requirements, and given that the available bandwidth is inversely proportional to the antenna size, it requires only a few micrometers in size, in order to build an appropriate nanoantenna, which is almost two orders of magnitude below the dimensions of current on-chip antennas. Hence, the approach of integrating one antenna per core seems rather unfeasible for CMOS technology, especially as the core sizes continue to shrink to a few hundreds of micrometers [109]. Silicon integrated antennas could be considered as a mature option for on chip antennas, but their size ranges from a few to ten mm's, such as the zig-zag monopole antenna of axial length 1-2 mm proposed in [110], or the demonstration of a miniature on-chip antenna operating at the range of 100-500 GHz in [111]. CNT based antennas have been also considered as an alternative approach for implementing THz antennas.