Ultrafast Laser Direct Writing Nanogratings and their Engineering in Transparent Materials

Femtosecond laser direct writing is a powerful technique for fabricating micro-nano devices as it can modify the interior of transparent optical materials in a spatially selective manner through nonlinear multi-photon absorption. In this context, laser-induced nanogratings, i.e., a sub-wavelength assembly of nanolayers (approximate to 20 nm in width, approximate to 200 nm period), are ultrashort self-organized structures created by light in the bulk of transparent materials. These have been intensively explored over the last two decades opening a novel era of micro photonic devices due to their unique physicochemical properties, like orientable form birefringence, anisotropic light scattering, highly selective chemical etching, optical chirality, and extraordinary thermal stability. This review provides a throughout overview of the advances in this field, specifically focused on the formation of nanogratings, optical properties that can be exploited in various transparent solids, and the related main applications. Also, the fundamental characteristics, formation mechanism, tuning methods of nanogratings are reviewed.