Optical waveguiding characteristics of TiO2 sol–gel thin films for photonic devices: effects of thermal annealing

We have investigated the effects of thermal annealing (450°C, 500°C, and 550°C) on the titanium dioxide (TiO2) thin film physical and optical waveguiding properties using x-ray diffraction, Raman spectroscopy, scanning electron microscopy, atomic force microscopy, UV–visible spectrophotometry, and m-lines spectroscopy. The results show that the as-deposited film is amorphous, whereas the annealed films are polycrystalline and crystallize only in the anatase tetragonal structure with a preferential orientation of (101). The grain size, morphology, and surface roughness of the TiO2 films are significantly affected by the thermal annealing. The as-deposited TiO2 thin films reveal high transparency with an average transmittance >80  %   in the visible region. Moreover, a decrease in the optical transmission and energy bandgap with increasing annealing temperature is also observed. All the TiO2 planar waveguides exhibit only single-mode confinement regardless of the polarization. Both transverse electric and transverse magnetic refractive indices are found to increase upon annealing. Moreover, the as-deposited TiO2 planar waveguide shows a remarkable propagation loss of as low as 0.8  ±  0.1  dB  /  cm at a 632.8 nm wavelength, which suggests that the fabricated TiO2 films are promising for photonic applications.