Influence of ambient medium on femtosecond laser processing of silicon

Femtosecond laser radiation (800 nm, 250 fs, 1 mJ/pulse) has been used to treat a Si surface in different ambient environments, namely vacuum, air and water. We show that ablation threshold and crater characteristics (diameter “D” and shape of craters) are similar for the three media at low laser fluences, suggesting an identical radiation-related mechanism of material removal. In contrast, at high fluences the characteristic dependence of the crater size D2(F) in the semi-logarithmic scale becomes non-linear, starting from F = 10, 4 and 2 J/cm2 for vacuum, air and water, respectively, while the shape of craters becomes different for these media. The non-linear phenomena are ascribed to plasma-related ablation effects. Possible mechanisms of material removal are discussed.