Fluence dependence of femtosecond-laser-induced nanostructure formed on TiN and CrN

The fluence dependence of the nanostructure formation, which has been observed in our recent experiments on femtosecond (fs)-laser ablation, was examined in detail for hard thin films of TiN and CrN. The size D of the periodic fine structure formed with fs-laser pulses can be divided into two regions that depend on the laser fluence F, i.e. the region I where D increases rapidly with increasing F near the ablation threshold, and the region II where D increases slowly with an increase in F and almost saturates. The nanostructure has been observed only in the region I with a narrow width of F. The region II produces a periodic ripple structure whose size is 1/2-4/5 of the wavelength used. The effects of the thermal process and material composition on the nanostructure formation are discussed.