Femtosecond laser irradiation of titanium oxide thin films: accumulation effect under IR beam

This paper discusses the mechanisms of laser-induced periodic surface structures (LIPSS) formation using a high repetition rate femtosecond laser beam irradiation of magnetron-sputtered titanium oxide thin films (TiO1.8) grown onto SiO2/Si substrates. An Yb:YKW 500 fs linearly polarized laser emitting at a wavelength, λ, of 1030 nm, was used to irradiate the films (300 nm thickness) at a repetition rate of 100 kHz under both static and dynamic (scanning) conditions. Under static beam conditions, an incubation behavior related to materials in thin film form was established with a damage threshold of 72 mJ/cm2. Close to this fluence value and increasing the number of laser shots from 1 to 1000, micro-cracking occurred and propagated inside the beam waist diameter zone estimated close to 60 µm. In addition, using a higher fluence value of 280 mJ/cm2, i.e., well above the damage threshold, a melting occurred in an intermediate zone within the irradiated area, with a surprising ‘cure effect’ that contributes to the micro-cracks stabilization. Simultaneously, at the center of the Gaussian laser beam spot, the entire film ablation was observed. Furthermore, irradiation under dynamic mode with a scanning speed of 4 mm/s and a repetition rate of 100 kHz were achieved for the large-scale processing of the TiO1.8 films up to surface area of 25 × 25 mm2. For these irradiation conditions case that correspond to a fluence of 110 mJ/cm2 and a cumulative number of shots of 3000, 2D-LIPSS nano-cracks (200 nm length and λ/8 to λ/9 period) are obtained over the whole irradiated surface, a phenomenon that is mainly attributed to a thermo-mechanical ablation mechanism.