## 4 Conclusions  
To conclude, we have reported in this work three different tripod-based metasurfaces working as thin-film sensors in the THz band. We have shown that the higher electric field concentration on the surfaces, the larger the frequency shift, and sensitivity results. To corroborate this fact, two different metastructures have been initially studied, a solid tripod and a hollow tripod metasurface, with higher electric field confinement, obtaining average sensitivities of \(3.54\times 10^{-5}\) nm\({}^{-1}\) and \(3.72\times 10^{-5}\) nm\({}^{-1}\), respectively. Other metastructures have been designed, to improve the sensitivity, by changing the unit cell parameters, including additional elements in the metallic pattern of the unit cell, such as arms, or rotating the structure pattern, thus obtaining the best design with an average sensitivity of \(1.42\times 10^{-4}\) when measuring extremely thin analytes, meaning an improvement of 381% with respect to the initial hollow tripod structure. Finally, we have performed a comparison with other designs found in the literature, with similar complexity and dimensions, showing that our designed structure is on top of the list, improving most of the previous works by more than one order of magnitude. These results emphasize the importance of using metastructures with complex geometries for sensing applications, in which the electric field is distributed throughout the surface and not only at discrete points, such as metagometries.  
The following are available online at [[http://www.mdpi.com/2076-3417/10/18/6504/s1](http://www.mdpi.com/2076-3417/10/18/6504/s1)]([http://www.mdpi.com/2076-3417/10/18/6504/s1](http://www.mdpi.com/2076-3417/10/18/6504/s1)): Figure S1: Spectral transmission of the simple tripod metasurface plotted as a function of the polarization angle. Figure S2: Reflection coefficient for different ultrathin analyte thicknesses.  
Conceptualization, I.J.-L, B.O., V.P.-P. and M.B.; methodology, I.J.-L. and M.B.; software, I.J.-L.; validation, B.O., V.P.-P. and M.B.; formal analysis, I.J.-L, B.O., V.P.-P. and M.B.; investigation, I.J.-L, B.O., V.P.-P. and M.B.; resources, I.J.-L, B.O., V.P.-P. and M.B.; data curation, I.J.-L.; writing--original draft preparation, I.J.-L. and M.B.; writing--review and editing, I.J.-L., B.O., V.P.-P. and M.B.; visualization, I.J.-L.; supervision, B.O., V.P.-P. and M.B.; project administration, M.B.; funding acquisition, M.B. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Spanish Ministerio de Ciencia, Innovacion y Universidades, Project KIT2018-094475-B-I00 (MCIU/AEI/FEDER,UE).  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Reference** & \(h_{a}\) **(nm)** & \(n_{a}\) **(RIU)** & \(f_{0}\) **(GHz)** & \(\Delta f\) **(GHz)** & **Sensitivity (nm\(\cdot\)RIU)\({}^{-1}\)** \\ \hline Ref [22], 2018 & 2000 & 1.01 & 2250 & 16 & \(3.52\times 10^{-6}\) \\ Ref [23], 2018 & 16 000 & 1.6 & 2260 & 200 & \(3.45\times 10^{-6}\) \\ Ref [24], 2019 & 250 & 1.6 & 396 & 1 & \(6.3\times 10^{-6}\) \\ Ref [7], 2019 & 10 000 & 1.4 & 1550 & 10 & \(4.6\times 10^{-7}\) \\ Ref [12], 2019 & 7 & 8 & 1000 & 3 & \(5.35\times 10^{-5}\) \\ This work & 5 & \(\sqrt{8}\) & 604 & 0.6 & \(7\times 10^{-5}\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison of different values of sensitivity achieved in different works, calculated as \(S=(\Delta f\!/\!f_{0})/(h_{a}\!\cdot\!n_{a})\), with \(h_{a}\) the analyte thickness and \(n_{a}\) the analyte refractive index, in refractive index units (RIU), and ordered chronologically.