Analyses of surface coloration on TiO2 film irradiated with excimer laser

TiO2 film of around 850 nm in thickness was deposited on a soda-lime glass by PVD sputtering and irradiated using one pulse of krypton-fluorine (KrF) excimer laser (wavelength of 248 nm and pulse duration of 25 ns) with varying fluence. The color of the irradiated area became darker with increasing laser fluence. Irradiated surfaces were characterized using optical microscopy, scanning electron microscopy, Raman spectroscopy and atomic force microscopy. Surface undergoes thermal annealing at low laser fluence of 400 and 590 mJ/cm2. Microcracks at medium laser fluence of 1000 mJ/cm2 are attributed to surface melting and solidification. Hydrodynamic ablation is proposed to explain the formation of micropores and networks at higher laser fluence of 1100 and 1200 mJ/cm2. The darkening effect is explained in terms of trapping of light in the surface defects formed rather than anatase to rutile phase transformation as reported by others. Controlled darkening of TiO2 film might be used for adjustable filters.