Genesis of nanogratings in silica bulk via multipulse interplay of ultrafast photo-excitation and hydrodynamics

International audienceStructuring below diffraction limit is key to developing new laser processing technologies as well as to understanding light-induced processes on mesoscopic scales, notably self-organization. Here, an advanced numerical perspective on the generation of embedded self-arranged sub-wavelength periodic patterns is developed, describing multipulse ultrafast laser interaction with bulk silica glass. Combining light and material dynamics, the approach couples self-consistently nonlinear propagation, electronic excitation, and fluid dynamics resulting in irreversible phase transitions and localized damage. With increasing the number of applied pulses, the modification changes from localized nanovoids and elongated random nanopatterns towards regular void nanogratings dominantly covering the spot of the focused laser beam. Driven by local and collective scattering events, the order imposed by electric field patterns is then amplified and stabilized by the material response. The model predicts the gradual evolution of the optical properties considering the complex interplay between material arrangement and the electromagnetic field distribution. It allows thus to define light transport optical functions optimizing losses and anisotropic effects