Dynamics and mechanism of discrete etching of organic materials by femtosecond laser excitation

Laser ablation and etching of microcrystalline Cu-phthalocyanine thin films were examined by changing pulse duration (170 fs, 250 p5, 100 ns) of a 780 nm Ti:sapphire laser. Above fs (40 mJ/cm²) and ps (50 mJ/cm²) ablation thresholds, the etch depth becomes constant and is almost independent of laser fluence, and further increase in the fs fluence results in complete removal of the film. We name the unique ablation phenomenon discrete etching. On the other hand, the depth etched by ns laser excitation increases gradually with the fluence above its ablation threshold (80 mJ/cm²). In order to reveal the difference between the fs and ns etching behaviors, we measured directly excitation energy relaxation and surface morphology change with time-resolved absorption spectroscopy and time-resolved surface scattering imaging, respectively. The fs discrete etching phenomenon and its mechanism were considered in view of time evolutions from highly intense fs laser excitation to the step-wise etching. On the basis of the results, we propose a fs laser ablation model that ultrafast stress increase brings about mechanical disruption leading to the discrete etching behavior.