Ultrafast microscopy of laser ablation of refractory materials: Ultra low threshold stress-induced ablation

The fundamental mechanisms of photothermal laser ablation of thin films of refractory materials on glass are investigated, using time-resolved microscopy with variable duration ablation pulses (1ns-10μs). A fundamental understanding of ablation mechanisms can be used to develop designer materials with ultra efficient ablation, i.e. materials that can be ablated using economical low power laser sources. Refractory Ti and TiN thin film coatings on glass substrates were studied, with the intent of finding materials that can store and then suddenly release a great deal of thermoelastic stress energy that is produced by heating with relatively low intensity laser pulses. Threshold fluences as low as J_th≈50mJ/cm² were realized. Time-resolved microscopy shows that brittle TiN coatings ablate by violent cracking on all time scales. Similar behavior is observed for Ti, at 1ns and 110ns, but at 10μs the Ti melts but does not ablate. A thermal conduction model is used to show that the temperatures in the coatings at ablation threshold are a factor of 2 (Ti) or 3 (TiN) below the melting point. Calculations of the thermoelastic stress in the coatings shows that stress-assisted ablation occurs when the stored stress energy exceeds the coating adhesion to the substrate by about one order of magnitude. The photothermal stress energy in the coating can be converted almost completely into kinetic energy of the ablated material.