Three-dimensional crystallization inside photosensitive glasses by focused femtosecond laser

Using scanning electron microscopy, we analyze the laser fluence dependence of three-dimensional crystallized areas induced in bulk photosensitive glass (Foturan) by focused femtosecond laser pulses exposition and subsequent heat treatment. For low fluences ( F < 2 J ∕ cm 2 ) , the crystallized area is essentially determined by the energy dose distribution above a critical dose. For higher fluences ( F > 2 J ∕ cm 2 ) , the crystallization length is higher than predicted from the energy distribution due to the filamentation which occurs above a critical fluence F crit = 1.4 ± 0.3 J ∕ cm 2 . The filamentation length is found to follow the square root dependence F − F crit .