In-situ investigation of laser ablation of thin films

The probe beam deflection technique, based on the mirage effect, has been applied to monitor laser processing of polymer and metal films and to measure damage thresholds of optical coatings. The technique is described and it will be shown that its sensitivity permits distinguishing between surface effects like heating and cracking, causing normal sound waves, and plasma formation outside the surface, giving rise to shockwaves. Examples are presented for single-shot ablation with 14 ns laser pulses of 248 nm wavelength. In particular, ablation studies of Ni films of varying thicknesses disclose a quantitative correlation between threshold fluences and thermal properties like heat diffusion, melting, and evaporation. In contrast, for Cr films a simple thermodynamic model fails because the films crack before melting and vaporizing.