## 4 Discussion
It shows a reflective band of 260 nm that is almost half of the bandwidth achieved in this manuscript in \(\pi\)-polarisation using 11.5 layers of SiN\({}_{\text{x}}\)/SiO\({}_{\text{y}}\). In our study, the number of layers is about halved for the same reflectivity because the refractive index difference between our layers is almost 20% higher than in the mentioned study. This optimisation compared with previous work is beneficial in terms of increment of tuning speed and FSR of the MEMS devices due to the reduction in mass and in cavity length [12; 27]. Besides obtaining a reduction in the top electrode mass and cavity length due to changes in the MEMS composition, their geometry also plays an important role in the optimisation of these characteristics. As Table 2 shows, the length of the cavity increases as the width and the length of their arms do. This table also illustrates the results on [PERSON]'s modulus product of the finite element simulation of every architecture. These results are lower compared with [11] where SiH\({}_{4}\)/NH\({}_{3}\)/N\({}_{2}\) were used as precursor gases and a Young's modulus in the range between 240 and 320 GPa was calculated by membrane point deflection with a substrate temperature of 300 \({}^{\circ}\)C. Such high values would lead to excessively large \(L_{\text{c}av}\) and thus reduced free spectral range. Results in the same range applying nanoindentation methods and using SiH\({}_{4}\)/NH\({}_{3}\)/N\({}_{2}\) as precursor gases was obtained in [15], a study that also demonstrates how the value of \(E\) is affected by the substrate temperature. The nanoindentation method was also applied in [8] where an \(E\) of 97 GPa at 125 \({}^{\circ}\)C of deposition was obtained. To our knowledge, there is no study in \(E\) where SiH\({}_{4}\)/N\({}_{2}\) were used as precursor gases at temperatures inferior to the already cited. Taking into account the mentioned studies, the increase in Young's modulus with the substrate temperature and with the increase in H-N bonds that the use of NH\({}_{3}\) introduces, the results of our study are reasonable [28]. Figure 5 shows the electro-thermal tunability characterisation of one of the fabricated architectures. By comparing to Figure 1, we actually expect a single peak. The further resonances besides the main resonance appear most likely because of imperfect coupling to the resonator, exciting undesired (off-axis) modes or with light outside the central hole of the main cavity. Their amplitude can be reduced by improved coupling schemes, exactly matching the window size and required NA of the filter. This goes beyond the investigations in this manuscript. For applications in a laser, only the dominant mode will survive as long as there is sufficient gain. In the following, we estimate the maximum tuning range limited by the rupture limit of the device at 80 mA current. The main peak reaches the maximum tuning wavelength of the CTL by applying a current of 60 mA through the top electrode. The investigated device shows a tuning range of 86.7 nm as compared to the case without any tuning current. As the heat delivered to the MEMS mirror scales quadratically with current, we expect a quadratic temperature and thus length change in the cavity, resulting in a maximum tuning range of 156 nm, corresponding to a maximum wavelength of 1698 nm where the reflection is still \(R>94\%\). However, in previous experiments with MEMS-tuned VCSELs [29], we see a super-quadratic increase, possibly caused by the interplay of heat dissipation and the fact that the MEMS is curved, i.e., a linear increase in circumference results in a non-linear change in cavity length. In the results presented in Figure 5, we see an empiric tuning with a cubic current dependence, resulting in a maximum tuning range of 208 nm. The resulting maximum wavelength of 1750 nm is, however, outside the block band of the MEMS, which would likely result in a mode hop.