Laser-induced decomposition and ablation dynamics studied by nanosecond interferometry

Abstract

A polyurethane sample film was prepared by adding phenol resin as a structural stabilizer and its laser ablation dynamics was studied upon excimer laser irradiation. The ablation threshold by 248 nm excitation was determined to be 40 mJ/cm 2 and no appreciable debris was left. The etch depth increases with the fluence and reaches 0.5 μm at the fluence of 1 J/cm 2 . Time-resolved interferometric images were measured in the nanosecond to microsecond time region under surface and internal optical alignments. The film was decomposed efficiently into debris, fragments, aggregates, and so on which are smaller than the wavelength, as observed images were not disturbed by the decomposed products. At the fluence of 310 mJ/cm 2 , etching proceeds fast during the initial half of the excitation laser pulse, but stopped at the late stage of the pulse. The direct measurement of transient absorbance change at 248 nm explains that ejected products still absorb the excitation photons and interrupt a further etching. Laser-induced expansion was not observed above the ablation threshold, while below the value an expansion forms a bump and followed by rapid contraction. The morphological behavior is quite different from that of usual photothermal expansion and contraction dynamics observed for poly(methyl methacrylate) and polystyrene, hence it is considered that laser ablation is induced photochemically.