## 6 Conclusions  
The discovery of the extraordinary optical transmission questioned all the classical theories of diffraction of the electromagnetic field. None of them explain the new phenomenology, because it is assumed the existence of ideal materials, namely metals. In doing so, the generation and propagation of surface plasmon polaritons and of creeping or leaky waves is not considered and, consequently, the classical theories fail. The consideration of these effects demand the consideration of the electrical and optical properties of real materials, through a correct modeling of the dielectric function and refractive index. Then, the propagation of surface plasmon polaritons can be theoretically analyzed from Maxwell's equations. Their generation and propagation, combined with the propagation of creeping waves, especially out of the visible spectral region, allow to describe the extraordinary optical transmission phenomenon. Moreover, if the nanoantenna is not metal made or if the incident field has a frequency away from the visible region, other wave phenomena (for example, the propagation of creeping and leaky waves) become more important than the polaritons ones, that may not even be generated, although EOT can occur. Thus, EOT can be observed using metallic nanoantennas. This kind of structure allows the concentration and amplification of electromagnetic energy. Using a Finite Element Tool, it was created a 3D simulation environment to model and simulate a metallic nanoantenna, obtaining its optical response at frequencies on the visible and near-infrared regions. The optical response is dependent on the used materials, on the nanoantenna and also on the surrounding materials. Furthermore, it is possible to verify that for the gold nanostructure, when the structure periodicity increases, the maximum intensity value decreases, keeping the same peak wavelength value. On the aluminum spectra, it is concluded the periodicity affects the flatter region, the region that can have gain. In relation to the hole diameter sweep, it is possible to verify that the larger the hole is, the higher the intensity peak is. However, in this case, a careful conclusion should be drawn, because it is also known that the hole diameter should be much smaller than the incident wavelength. In addition to this effect, red shifts will occur as the diameter increases. The nanoantenna thickness is also analyzed. Considering the previous simulations, it is possible to conclude that the thicker the nanoantenna is, the less the peak intensity will be. In addition, blue shifts will occur in this situation and, specifically on the aluminum response, it seems that the flat region becomes flatter. The substrate thickness leads to the idea of adjusting losses \begin{table}
\begin{tabular}{c c c c c} \hline  & Liver & Colon Mucosa & Colon Submucosa & Colon Serosa \\ \hline Liver & - & 0.0954 & 0.1124 & 0.0913 \\ Colon Mucosa & 0.0954 & - & 0.0170 & 0.0041 \\ Colon Submucosa & 0.1124 & 0.0170 & - & 0.0211 \\ Colon Serosa & 0.0913 & 0.0041 & 0.0211 & - \\ \hline \end{tabular}
\end{table} 
Table 5: Variation (sensibility) of the gain at 700 nm. and consequently gains without changing the spectrum shape. The thicker the substrate is, the higher the losses are, keeping the optical response shape. The simulations' results also lead to the conclusion that the spectral response can be tuned by varying the topology and the parameters of the structure. However, in order to create rigorous analytical models, a huge simulation database has to be created, requiring huge computational efforts. Furthermore, all the simulations show high amplification and concentration of the electromagnetic field near the interfaces. This is a clear evidence of the generation and propagation of surface plasmon polaritons. The nanostructure of gold was simulated as a temperature sensor, using models for the complex dielectric function including the temperature. A nanostructure like this can behave as a temperature sensor with a critical temperature.