## 3 Results and Discussion
This information is clearly visible in the maps of Figure 3b-d, i.e., the displacement of the same order \(N\) along the structure was related to a change in the cross-section or in the size of the resonant cavity. Then, stronger changes in the size of the cavity were observed as discontinuities in the optical resonant modes' evolution maps. In region (ii), which is what we called the transition region (Figure 5a), the modes associated with the quasi-WG were more intense and better defined than in the first region, blurring the resonances associated with WGM1 (Figure 5b). This could be related to the roughness of the structure in this region, as can be seen in the SEM image of Figure 5a. The larger roughness caused both changes in the thickness and reflection angle, favoring the confinement of the quasi-WG. Then, in the TM and TE spectra, there were two sets of peaks, making it difficult to distinguish between the WG and the quasi-WG modes. Furthermore, the dispersion of the optical path length gave rise to a shift and/or broadening of the resonance peaks that, as can be appreciated in Figure 5b, were not so well separated [21]. Consequently, in this region, no accurate calculation of the order of the resonance or the refractive index was possible. The third region (iii) corresponds to the thinner part of the rod (Figure 6a). It had a hexagonal cross-section that tapered down toward the end of the rod. As can be seen in Figure 6, the thinner the rod (i.e., the smaller the size of the cavity), the sharper and more separated the resonant peaks. Figure 5: (a) SEM image and (b) \(\upmu\)-PL spectra for all the polarization conditions of the transition region. The polarized spectra show how the modes associated with the quasi-WG were more prominent. Compared to the observations in the first region (i), in this region (iii) of the rod, the difference between the positions of a given resonance order for both polarizations was larger (see the spectra in Figure 6b) due to the smaller cross-section. The peaks were also sharper than in region (i). We have already mentioned the possible influence of the roughness when describing the transition region (ii). It is likely that the roughness was also behind these differences. Similar calculations were performed with spectra at the selected point (with a side length \"a\" of 0.72 \(\upmu\)m), and the order \(N\) and refractive index were obtained (Figure 6c,d, respectively). It can be seen that the obtained values of the refractive index were quite similar to those calculated for region (i). The second kind of morphology investigated shows more variations in the optical modes observed along the length of the structure (Figure 7). As described above, it consisted of long nails, typically with a variable cross-section and a hexagonal head. Figure 7a shows a general view of one of these nails, where the different cross-section sizes along the nail are clearly seen. However, in the optical resonances map (Figure 7b), the situation was a little bit more complicated. Thanks to this map, regions where some defects were inside the structure that were not visible in the SEM image could be identified, where these were affecting the performance of the optical cavities. For example, in the middle of region (ii), and in region (iii), there was a discontinuity in the map, indicating that at those points, no optical resonant modes were established. Then, we could conclude that the optical resonant map was useful for identifying not only changes in the size of the cavity but also points where some kinds of defects were reducing the performance of the optical cavity. Let us now provide a deeper description of the optical resonances observed in the regions marked in Figure 7. Region (i) corresponds to the hexagonal head. The cross-section was quite uneven, both in shape and size, as observed in the close-up image shown in Figure 8a.