Suppressed ablation rate of titanium surface by two-color double pulse femtosecond laser

The material processing with a femtosecond double-pulse laser beam has been demonstrated since 1990s. Only a few papers have discussed the possibility of ablation suppression mechanism. The mechanism of laser ablation with the double pulse beam is still open question due to two pulses consist of same laser wavelength because it is difficult to distinguish between the effects of the first and second pulses. In this study, the ablation rate has been investigated for titanium with a two-color double-pulse laser beam in the delay time (Delta t) from 0 to 600 ps. The double pulse laser beam consisted of 800 nm with 150 fs pulse and 400 nm with > 150 fs pulse in cross polarization. The fluence of the first pulse was kept above the ablation threshold, while the fluence of the second pulse was kept below the ablation threshold. The ablation rate was clearly suppressed at the delay time of Delta t similar to 60 ps in case of second pulse of 400 nm. On the other hand, in case of second pulse of 800 nm, the ablation rate was suppressed at the delay time of Delta t similar to 200 ps. The delay time was approximately three times difference for both irradiation case. The difference of the delay time might be suggested that the ablation rate was effectively suppressed when the expanding surface plasma produced by first laser pulse should be close to the critical density for the second laser pulse.