Phase explosion and ablation in fused silica initiated by an ultrashort-pulse, tunable mid-infrared free-electron laser

Ultrashort laser pulses interacting with brittle dielectrics in the mid-infrared region of the spectrum produce a number of novel effects which are potentially useful in materials processing and analysis. These include the texturing of the surface, the generation of hydrodynamic instabilities, and a surprisingly efficient and gentle ablation behavior. Nevertheless, the mechanism of infrared laser ablation remains somewhat mysterious. Here we present evidence for a mechanism of explosive vaporization in fused silica, initiated by picosecond pulses from a tunable free-electron laser operating in the wavelength region from 2-10 mu m. The unusual pulse structure of the free-electron laser - which produces 1-ps micropulses at intervals of 350 ps in a macropulse lasting up to 4 ps - makes it possible to test separately the effects of intensity and fluence. We show in particular that thermal descriptions of the ablation process fail in the regime where there is high vibrational excitation density in the solid due to resonant absorption of mid-infrared laser light.