Delay times for ablation rate suppression by femtosecond laser irradiation with a two-color double-pulse beam

We investigated the delay time & UDelta;t for suppressing the ablation rate of titanium with a two-color double-pulse laser beam at & UDelta;t = 0-600 ps. The double-pulse laser beam consisted of cross-polarized pulses with 150 and > 150-fs pulse widths and 800 and 400-nm wavelengths, respectively. The ablation rate was suppressed most at & UDelta;t & AP; 60 ps for a 400-nm second pulse but at & UDelta;t & AP; 200 ps for an 800-nm second pulse. This difference suggests that the ablation rate was effectively suppressed when the expanding surface plasma produced by the first pulse was near the critical density for the second pulse.