Photothermal conversion dynamics in femtosecond and picosecond discrete laser etching of CU-phthalocyanine amorphous film analysed by ultrafast UV-VIS absorption spectroscopy

摘要

Novel etching of Cu-phthalocyanine (CuPc) amorphous film which is characteristic of ultrashort laser irradiation was successfully confirmed by tuning Ti:Sapphire laser (780 nm) to 150 fs, 250 ps, or 100 ns, and the primary processes were investigated by fs pump-fs probe and ps pump-fs probe spectroscopic measurements. In the fs and ps laser ablation, we have found discrete laser etching in that the etch depth becomes constant and is independent of laser fluence above the ablation, we have found discrete laser etching in that the etch depth becomes constant and is independent of laser fluence above the ablation threshold, although gradual (normal) etching, in which the etch depth increase continuously with the fluence above ablation threshold, was observed inthe ns laser ablation. The transient absorption spectral measurements reveal the nonlinear photothermal conversion processes, corresponding to exciton-exciton annihilation and cyclic multiphotonic absorption. Their time evolutions during and after the excitation pulse duration were considered and elucidated to depend strongly on the excitation pulse width. On the basis of these results, we discuss an ablation mechanism for the ps and fs ablation that the temperature elevation bringing about transient high pressure is responsible for discrete etching.