## 3 Results and Discussion  
The numerical results of the reflection coefficient for the designed metasensors are shown in Figure 2. As shown in the figure (black curves), the response of the analyte-free metastructures exhibits a dip in the reflection coefficient at 551 GHz (hollow tripod, Figure 2b) and 620 GHz (solid tripod, Figure 2a). Before coating the surfaces with different analyte thicknesses, a study of the electric field distribution over the surface of the analyte-free structure was carried out, and the results are shown as insets in the same figure. As shown in Figure 2a,b, there are apparent differences between both metastructures. While in the solid tripod metastructure (Figure 2a) the electric field is confined only at the tips of the metallic blades, in the hollow tripod structure, the electric field is also confined inside the hollow blades, due to the capacitance generated between the metallic strips (see Figure 2b). Figure 1: Front view of the designed tripod metasurfaces unit cells. Metallization is represented in grey and polypropylene (PP) substrate in red. When using this type of unit cells for sensing applications, it becomes essential to have the maximum possible electric field concentration to increase the sensitivity to dielectric changes in the vicinity of its surface. The sensing performance of both metastructures was studied by coating them with a dielectric thin-film acting as an analyte, with dielectric permittivity \(\varepsilon_{a}\) = 8, and thicknesses ranging from \(h_{a}\) = 200 nm to \(h_{a}\) = 800 nm. The reflection spectra for the different analyte thicknesses are plotted in Figure 2a,c. As observed, a redshift of the resonance dip occurs when the analyte thickness increases, as expected. For completeness, this frequency shift as a function of the analyte thickness is plotted in Figure 2d,e where the redshift is clearly shown. To obtain a quantitative estimation of the sensing performance, we calculate the sensitivity of each structure, defined as \(S\) = (\(\Delta\)/\(\overline{f}\)\({}_{0}\))/\(h_{a}\) and measured in nm\({}^{-1}\), where \(\Delta\)/ = \(f\) - \(f_{0}\), with \(f\) the resonance frequency for each analyte thickness, \(h_{a}\), and \(f_{0}\) the resonance frequency without the analyte. With this definition, we obtain an average sensitivity of \(3.54\times 10^{-5}\) nm\({}^{-1}\) for the solid tripod structure, and an average sensitivity of \(3.72\times 10^{-5}\) nm\({}^{-1}\) for the hollow tripod structure. These results translate into an improvement of the hollow structure of approximately 5% with respect to the solid tripod. As mentioned above, this can be because the electric field confinement is more distributed along the surface of the hollow tripod, making it more sensitive to changes in the refractive index around the surface of the structure. Figure 2: Reflection coefficient for different analyte thicknesses: 0 nm (black line), 200 nm (red line), 400 nm (blue line), 600 nm (cyan line), and 800 nm (dark pink line), for the solid tripod structure (**a**), and the hollow tripod structures (**b,c**). Insets: electric field magnitude over the empty (analyte-free) tripod metasurfaces. Frequency shift as a function of the analyte thickness, for extremely thin analytes with average sensitivity for the solid tripod structure (**d**), and the hollow tripod structures (**e,f**). The previous results suggest that one of the most critical factors for a good sensing device is to have large electric field confinement. To verify this, new metastructures were designed, based on the hollow tripod. First, we modify the strip width \(g\) (see dimensions in Figure 1) and the gap between strips, \(a\), to optimize these parameters, and to achieve as deep a resonance as possible.