Localized Phase Transition of TiO₂ Thin Films Induced by Sub-bandgap Laser Irradiation

Laser-induced localized phase transitions of sputtered titanium dioxide (TiO₂) thin films are reported in this study. Laser-irradiation under vacuum with a 532 nm sub-bandgap continuous wave (CW) laser results in an anatase-to-rutile phase transition. Irradiating the rutile region in air changes a portion of the domain back to anatase. Raman spectroscopy and micro-Raman mapping were used to detect phase transformations and study the spatial intensity distribution of Raman-active modes. A Raman map of anatase mode E_g (144 cm^-1) and rutile mode A_g (608 cm^-1) revealed the formation of microstructures due to the laser treatment. The results suggest that irradiated photons are absorbed by defects, resulting in localized electronic excitation that leads to a phase transition in the treated regions.