Micro-Modulations of Titanium Oxide Coatings Formed Using a Femtosecond Laser

Highly useful microstructures are required in order to enhance the properties of titanium oxide (TiO2) coatings, so as to substantially increase the surface and underlayer surface areas. Along with coating deposition techniques, femtosecond laser irradiation has high capability to control micro-modulations consisting of mesopores and microspheres, with no unexpected damage due to thermal expansion. In this study, phase contrast microscopy (PCM) is employed in conjunction with differential-emphasis and contrast-enhancement image-processing techniques. This approach is confirmed to allow precise identification of the refractive index modulation generated in the TiO2 coatings. The effects of the laser output per pulse (5-10 μJ) and the pulse number (1-1000) on the TiO2-coating ablation craters formed by the femtosecond laser irradiation are also investigated. The double ridges are composed of inner and outer crater edges, the clarity R of which is characterized by the difference between the inner and outer diameters. This value is distinct and increases with increasing laser output for single-pulse irradiation. For high-pulse-number irradiation, the superposition of the collapsed circles created by multiple fluctuating pulses decreases R. Differential-emphasis PCM images, which emphasize the difference in image intensity between adjacent image pixels, facilitate identification of surface periodic structures in ablation craters formed by five pulses or less. Further, contrast-enhanced PCM images are found to reveal microspheres mechanically propelled by the laser beam from the interiors to the peripheries of the ablation craters. The micro-modulations in the TiO2 coatings formed by the femtosecond laser irradiation and revealed by the PCM images are expected to modify the nano-adsorbency of this material for more effective use in water treatments, by increasing the coating surface areas.