Athermalization of a self-assembled rolled-up TiO2 microtube ring resonator through incorporation of a positive thermo-optic coefficient material in planar bilayers

For the first time, this paper theoretically and experimentally investigates the thermal stability of optical filters based on self-assembled TiO2 rolled-up microtube ring resonators (RUMRs) by incorporating positive thermo-optic coefficient (TOC) materials (e.g., SiO2 and/or Si3N4). The influence of the TOC, refractive index, and thickness of the positive TOC materials on the filtering performance of the TiO2 RUMR is theoretically studied. The results illustrate that an increase in temperature leads to a blueshift in the resonant wavelength of the RUMR-based optical filter, which changes at a rate of - 33.3 pm/K owing to the negative TOC of TiO2 (- 4.9 +/- 0.5 x 10(-5)/K). By increasing the thickness of SiO2 or Si3N4 as a positive TOC material together with TiO2, the temperature-induced resonant shifts of TiO2/SiO2 and/or TiO2/Si3N4 RUMRs are theoretically obtained. The TIRS varies between - 40 pm/K (- 22 pm/K) and about 30 pm/K (22 pm/K) for TiO2/SiO2 (TiO2/Si3N4) RUMRs. It is shown that thermal stability occurs when the thickness of the SiO2 (Si3N4) layer is similar to 16 nm (12 nm). At the end of this study, as a proof of concept, an experiment is demonstrated by fabricating an RUMR based on TiO2/SiO2 on the flat silicon wafer. The experimental results show that the athermalization of the system is experimentally achieved by selecting the appricated thickness ratio of TiO2/SiO2. This novel approach for athermalization of the resonators opens up interesting perspectives on the implementation of vertical and multi-routing coupling between photonic and optoelectronic layers and more specifically in a three-integration fashion.