Nanoscale investigations of femtosecond laser induced nanogratings in optical glasses

Abstract

The intrinsic structure of nanogratings imprinted in silica by femtosecond laser direct writing is composed of light-aligned oblate nanopores. These nanogratings are at the origin of the strong form birefringence exploitable to write various optical devices.

Impact of Glass Free Volume on Femtosecond Laser-Written Nanograting Formation in Silica Glass

“…Modeling studies of bulk nanogratings are usually based on a low dielectric constant seed as an initial "nanovoid" [15,20]. Some studies have suggested point defects like E'; ODC; and, generally, color centers [21], as well as transient defects like self-trapped electrons (STEs) [18,45] and self-trapped holes (STHs) [22] as seeds. In discussing potential precursors, we must consider the necessity of a lower change in the refractive index to facilitate the plasma field enhancement perpendicularly to light polarization.…”

“…As suggested above, glass free volume could serve as a primary seed for the nanocavitation process. The rapid transition from nanoplasma hotspots into nanopores is driven by swift temperature transfer to phonons and local thermal expansion, which effectively imprints oblate nanopores [18]. These nanopores' shapes and orientations are largely influenced by the incident light's polarization, supporting a plasma-mediated over a thermo-mediated nanocavitation process.…”

“…These nanoplasma regions are "forced-arranged" by light into distinct periodic patterns, oriented in a direction orthogonal to the laser's polarization vector. Finally, plasma-mediated oxide decomposition occurs, resulting in nanolayers made of an assembly of oblate nanopores [17,18].…”