# The Quest for Low Loss High Refractive Index Dielectric Materials for UV Photonic Applications
## 4 Results
### Interaction Effects  
In this section, possible interaction effects are considered by studying the near-field enhancement that can be produced between two NPs in close interaction and the consequences that this interaction may have in the heat generation process. Interaction effects are widely used when designing nanostructures for surface enhanced spectroscopic techniques [6]. A dimer made of spheres with \(R\) = 70 nm and interparticle distances (gap) of 10 and 20 nm are analyzed. TiO\({}_{2}\) and diamond are selected as representative materials of the two types of HRI dielectrics studied previously: those with losses (\(\varepsilon_{2}>0\)) above 3.5 eV (AlP, AlAs, CeO\({}_{2}\) and TiO\({}_{2}\)) and those with losses above 6 eV (AlN and diamond). The selected materials for this study (TiO\({}_{2}\) and diamond) when studied as isolated spheres, showed the highest value of \(\langle\,\mid\,E\,\mid\,/\,\mid\,E_{0}\,\mid\,\rangle\). We also present the behavior of Al NPs as a reference. Once again, a 5 nm oxide shell (Al\({}_{2}\)O\({}_{3}\)) has been considered in order to mimic common experimental situations [52]. Figure 4 shows the spectral values of \(\mid\,E\,\mid\,/\,\mid\,E_{0}\,\mid\,\) and \(\Delta T\) (temperature increase with respect to ambient temperature) at the center of the gap of dimers made of Al + Al\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\) and diamond. In all cases, the dimers are excited with a plane wave linearly polarized along the dimer axis (see inset in Figure 4a). This polarization is chosen to enhance the interaction effects [58]. By comparing the values of \(\mid\,E\,\mid\,/\,\mid\,E_{0}\,\mid\,\) for the different materials analyzed, both diamond and TiO\({}_{2}\) produce the highest values above 3 eV. In the case of the Al + Al\({}_{2}\)O\({}_{3}\) dimer, the enhancement is significantly smaller than that of either diamond or TiO\({}_{2}\) (around 4 times and one order of magnitude, respectively) and at an energy slightly below 3 eV. Also note that, for diamond and TiO\({}_{2}\), the temperature does not increase (\(\Delta T\) = 0) when excited at the photon energies of maximum enhancement. However, for Al/Al\({}_{2}\)O\({}_{3}\), the temperature is increased  
Similar results to those of TiO\({}_{2}\) are expected for AlP, AlAs and CeO\({}_{2}\): high values of \(\mid\,E\,\mid\,/\,\mid\,E_{0}\,\mid\,\) and low values of \(\Delta T\) at energies below their corresponding absorption bands (\(\approx\)3.5 eV). At higher  
Figure 4: Electric field enhancement \(\mid\,E\,\mid\,/\,\mid\,E_{0}\,\mid\,\) (blue line, left axis) and temperature difference with respect to ambient temperature \(\Delta T\) (red line, right axis) at the center of the gap of (**a**) Al + Al\({}_{2}\)O\({}_{3}\), (**b**) TiO\({}_{2}\) and (**c**) diamond dimers. The analyzed dimer configuration is shown as inset in (**a**): a dimer of spheres of \(R\) = 70 nm with gap = 10 (dashed line) and 20 nm (solid line), illuminated with a plane wave linearly polarized along the dimer axis. energies, \(\Delta T\) increases while \(\left|\left.E\right|\right./\left|E_{0}\right|\) decreases. Also, mention that for AlN, a behavior similar to diamond is expected, moderate values of \(\left|\left.E\right|\right./\left|E_{0}\right|\) with \(\Delta T=0\) for energies below 6 eV. The main difference between having a gap of 10 or 20 nm is that for the shortest one both \(\left|\left.E\right|\right./\left|E_{0}\right|\) and \(\Delta T\), are higher. This is caused by the strength of the interaction: as the distance between particles become smaller the interaction becomes stronger. Also, a red-shift of both \(\left|\left.E\right|\right./\left|E_{0}\right|\) and \(\Delta T\) spectra is produced as the gap becomes shorter [58]. Figure 5 shows the near-field enhancement (\(\left|\left.E\right|\right./\left|E_{0}\right|\)) and heat maps (\(\Delta T\)) for diamond and Al + Al\({}_{2}\)O\({}_{3}\) dimers with a gap = 20 nm.