  Non-linear absorption phenomena induced by controlled irradiation with a
femtosecond laser beam can be used to tailor materials properties within the
bulk of substrates. One of the most successful applications of this technique
is the ability to fabricate three-dimensional micro-devices integrating
optical, mechanical or fluid handling functions in a single substrate. In this
context, amorphous SiO2 is the most widely studied material. Here we show that
short (50-fs) femtosecond pulses induce the separation of Si and O ions in SiO2
substrates, leading to the formation of micro-crystallites that we identify as
pure crystalline phase of Si. Interestingly, this polymorphic phase
transformation occurs in the absence of laser-induced confined microexplosion
and with moderate numerical aperture. These findings not only unravel a key
mechanism related to the transformation of the material and its subsequent
properties, but also pave the road for the development of three-dimensional
Si-rich structures embedded in a pure silica phase, eventually leading to novel
disruptive approaches for fabricating three- dimensional micro-devices. For
instance, one could imagine a silica-host substrate, in which arbitrary
three-dimensional silicon-based components are direct-write using a femtosecond
laser, rather than through assembly of components coming out of different
substrates or using multiple processing steps.
