Timescale dependent sign of amorphous titanium dioxide thermo-optic coefficient

We report on the thermo-optic properties of electron-beam evaporated amorphous titanium dioxide (TiO2) at different timescales. We investigate the thermo-optic response of TiO2 from static regime down to the micro-second regime by applying Joule heating on hybrid metallo-dielectric integrated Mach-Zehnder interferometers. We show that amorphous TiO2 exhibits a very large negative thermo-optical coefficient in the range of-6.5x10-4 K-1 at 1550 nm at typical timescales of a few seconds. Such a slow thermo-optic response is consistent with an organic origin of amorphous TiO2 negative thermo-optic coefficient. However, when observed at the micro-second timescale, we show that the same amorphous TiO2 has a positive thermo-optic coefficient, just like many other materials. Based on our results, TiO2 can be conveniently deployed in energy-effective integrated optic devices by taking into account the specific multi-timescale thermo-optic properties of this material. (c) 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement