Spontaneous periodic ordering on the surface and in the bulk of dielectrics irradiated by ultrafast laser: a shared electromagnetic origin

International audiencePeriodic self-organization of matter beyond the diffraction limit is a puzzling phenomenon, typicalboth for surface and bulk ultrashort laser processing. Here we compare the mechanisms of periodicnanostructure formation on the surface and in the bulk of fused silica. We show that volumenanogratings and surface nanoripples having subwavelength periodicity and oriented perpendicularto the laser polarization share the same electromagnetic origin. The nanostructure orientation isdefined by the near-field local enhancement in the vicinity of the inhomogeneous scattering centers.The periodicity is attributed to the coherent superposition of the waves scattered at inhomogeneities.Numerical calculations also support the multipulse accumulation nature of nanogratings formationon the surface and inside fused silica. Laser surface processing by multiple laser pulses promotes thetransition from the high spatial frequency perpendicularly oriented nanoripples to the low spatialfrequency ripples, parallel or perpendicular to the laser polarization. The latter structures also share theelectromagnetic origin, but are related to the incident field interference with the scattered far-field ofrough non-metallic or transiently metallic surfaces. The characteristic ripple appearances are predictedby combined electromagnetic and thermo-mechanical approaches and supported by SEM images of thefinal surface morphology and by time-resolved pump-probe diffraction measurements