Abstract
We demonstrated experimentally for the first time laser breakdown (ablation) of transparent materials through photoinonization in the intermediate regime (Keldysh paramer, r ~ 1.5) through a study of single and multi-shot femtosecond laser ablation of polymethylmethacrylate (PMMA) by using dual-color synthesized waveforms. Clear modulations in ablated areas were observed by changing the relative phase of the fundamental (ω) and second-harmonic (2ω) output of the exciting laser. Such modulation as well as the dependence of the ablation thresholds on the relative phase between the ω and 2ω beams correlate well with the theoretically predicted phase dependence of the photoionization rate. The incubation coefficient changes from 0.67 for one-color excitation (800nm or 400nm) to 0.83 for the dual-color case. This indicates that photochemical processes are not as significant for excitation by the dual-color synthesized waveform.