Laser ablation driven by femtosecond optical and XUV pulses

I investigated the behavior of different materials irradiated with intense femtosecond radiation with wavelengths in near infrared and XUV range and fluences close to the ablation threshold. Expansion of the surface irradiated with optical radiation shows distinctively different behavior for fluences below and above the ablation threshld. Few percent below the threshold, large long lasting expansions of material are observed for a wide range of materials. Contrary to the expansion induced by low fluences, these expansions can not be explained by the existing thermo-acoustical model, and they give validation to models of pressure driven spallation mechanism of material decomposition