## 4 Structure Parameters Evaluation  
This article will show the resonant behavior of the EOT, as well as the capability of amplify and concentrate electromagnetic radiation, for a metallic nanoantenna at frequencies on the visible and near-infrared spectral regions. This research work proves that it is possible to design a optical sensor using its resonant properties. The first step to develop a structure is to try to understand how its parameters can influence the output spectrum [2]. This article aimed to find new nanostructures that can work as sensors. For that reason, parameters are studied, before simulating a structure as a sensor, in order to verify how the structure optical response can be tuned, considering the same simulation environment, inputs, methods and simulation conditions. Thus, materials were modeled by the same type of fitting, boundary conditions are always the same and well defined, as well as the measurement conditions (that leads to the following spectra). Several simulations were performed in order to verify the variation of the spectral response regarding parameter sweeps (periodicity, hole diameter, nanoantenna thickness and substrate thickness) [2; 9; 11; 12; 14; 18; 22]. These sweeps are done based on a reference structure and are presented in Figure 1. It is a \(3\times 3\) squared array structure, composed by circular holes and characterized by a periodicity of \(a_{0}=0.5\,\mathrm{\SIUnitSymbolMicro m}\), a hole diameter of \(d=150\,\mathrm{nm}\), a nanoantenna thickness of \(t=100\,\mathrm{nm}\), as well as a substrate thickness of \(t_{sub}=50\,\mathrm{nm}\). In addition, the dielectric medium has a thickness of \(200\,\mathrm{nm}\); however, it is measured from the interface between the nanoantenna and the substrate in order to set the hole medium as dielectric [2]. An electric field, defined by a norm of \(|E_{0}|=1\,\mathrm{\SIUnitSymbolMicro m}^{-1}\), is generated on the top of the structure presented in Figure 1 (that is on the external boundary of the dielectric medium) and its perpendicularly incident is on the nanoantenna, having the \(z\)-axis opposite direction. Thus, on the generation port, this electric field was defined as \(|E_{x}|=|E_{y}|=\frac{1}{\sqrt{2}}\,\mathrm{\SIUnitSymbolMicro m}^{-1}\) and \(|E_{z}|=0\,\mathrm{\SIUnitSymbolMicro m}^{-1}\). Furthermore, all the external boundaries are total absorbent, meaning that there is no reflection. Moreover, all the simulations are done for 250 equidistant wavelength points, from \(250\,\mathrm{nm}\) to \(1200\,\mathrm{nm}\). The frequency analysis is done using a boundary probe (on/under the substrate) and measuring the square of the maximum ratio between the electric field norm and the incident electric field norm, \(|\frac{E}{E_{0}}|^{2}\). All the spectra that will be presented are only possible to reach due to the generation and propagation of surface plasmon polaritons, near the interfaces, as it is possible to visualize in Figure 2[2], for metal made nanoantennas and at frequencies on the visible and near-infrared regions. In this figure, it is quite noticeable that in these regions, there is field amplification, because the electric field norm is higher than the incident one [2]. In this article, a structure composed by an air dielectric medium and a quartz substrate is analyzed. The optical response of a gold and an aluminum nanoantenna is going to be shown. The complex dielectric function can be related to the complex refractive index function by \(\bar{\epsilon}=\epsilon^{\prime}+je^{\prime\prime}=\bar{n}^{2}=(n+jk)^{2}\) such as \(\epsilon^{\prime}=n^{2}-k^{2}\) and \(\epsilon^{\prime\prime}=2 nk\). On the other hand, \(\epsilon^{\prime\prime}(\omega)=\frac{\sigma(\omega)}{\epsilon_{0}\omega}\), where the conductivity of the material is given by \(\sigma(\omega)\), and so that it is possible to relate Ohmic losses with \(\epsilon^{\prime\prime}\).