Ablation area quasiperiodic oscillations in semiconductors with femtosecond laser double-pulse delay

A surprising repeatable phenomenon regarding semiconductor ablation area changes has been discovered. Irradiated by femtosecond double pulses, the ablation area quasiperiodically oscillates as the pulse delay increases from 0 to 1 ps at a material-dependent fluence range. In contrast, the ablation area monotonically decreases as the pulse delay increases beyond 1 ps or if the total fluence increases close to or beyond the single-shot threshold. Similar unexpected patterns of area quasiperiodic oscillations with the double-pulse delay are observed in various semiconductors, including Ge, Si, GaAs, and ZnO. The comparison study shows the same phenomenon in Au-plated ZnO. Yet, its oscillation periods are shorter and more stable than those in bulk ZnO, which implies that the localized carrier density is the key factor in oscillation periods. (C) 2014 Optical Society of America