Optical properties of titanium induced by below-ablation-threshold irradiation

[Display omitted] •Ablation rate for Ti was investigated with pump-pulse fluence below the threshold.•Change in effective laser penetration depth by pump-pulse irradiation was examined.•The effective laser penetration depth decreased to 42% of that without pump pulse. To understand the changes in the optical properties of a titanium surface induced by irradiation with low-intensity laser light that does not cause surface peeling, we have proposed a pump-and-probe method using femtosecond laser pulses with fluences below and above the ablation threshold (BAT and AAT, respectively). By analyzing the dependence of the ablation rate on the AAT probe-pulse fluence, the dynamical changes in the effective laser penetration depth and effective ablation threshold could be estimated. The dependences of the effective laser penetration depth and effective ablation threshold on the pump-pulse fluence Fpump below the ablation threshold Fth (Fpump 0.18 Fth reduced both the effective laser penetration depth and effective ablation threshold compared with irradiation without the pump pulse. The obtained results also indicated a large reduction in the effective laser penetration depth (58% for Fpump = 0.9 Fth) and a small reduction in the effective ablation threshold (15% for Fpump = 0.9 Fth).