Multiscale electronic and thermomechanical dynamics in ultrafast nanoscale laser structuring of bulk fused silica

International audienceWe describe the evolution of ultrafast-laser-excited bulk fused silica over the entire relaxation range in one-dimensional geometries fixed by non-diffractive beams. Irradiation drives local embedded modifications of the refractive index in the form of index increase in densified glass or in the form of nanoscale voids. A dual spectroscopic and imaging investigation procedure is proposed, coupling electronic excitation and thermodynamic relaxation. Specific sub-ps and ns plasma decay times are respectively correlated to these index-related electronic and thermomechanical transformations. for the void formation stages, based on time-resolved spectral imaging, we first observe a dense transient plasma phase that departs from the case of a rarefied gas, and we indicate achievable temperatures in the excited matter in the 4,000-5,500 K range, extending for tens of ns. High-resolution speckle-free microscopy is then used to image optical signatures associated to structural transformations until the evolution stops. Multiscale imaging indicates characteristic timescales for plasma decay, heat diffusion, and void cavitation, pointing out key mechanisms of material transformation on the nanoscale in a range of processing conditions. If glass densification is driven by sub-ps electronic decay, for nanoscale structuring we advocate the passage through a long-living dense ionized phase that decomposes on tens of ns, triggering cavitation. Laser modification of bulk dielectric materials is the source of a significant range of applications, from the design of optical materials resistant to radiation to the fabrication of embedded optical devices and functions. Particularly refractive index engineering using ultrafast laser radiation is at the base of the development of three dimensional optical circuits capable of transporting and manipulating light 1,2. To render the process efficient, the current challenge is to achieve index control in contrast and size with utmost precision. Recently, the utilization of non-diffractive irradiation concepts allowed to achieve characteristic processing sizes on the nanoscale 3,4. The remarkable feature size below 100 nm for irradiation wavelengths in the near-infrared and the apparent bypass of the diffraction limit are related to a material response, notably a cavitation phenomenon 4 in the presence of strong pressure gradients induced by the laser excitation. The existence of a material response enables thus to overcome the usual optical limits. Hence, material rupture scale defines the ultimate resolution, with sizes below the diffraction limit. Relying on the extreme achievable scales, structures with high aspect ratio and nanoscale sections were used to fabricate hybrid micro and nanoscale features in bulk optical materials helping to design embedded systems with performant optical functions. Nanoscale features are for example useful to create read-out centers required to sample electrical fields in optical circuits or to generate strong optical resonances for Bragg sensors 5-7 , but equally they can serve as initiation centers for accurate cleavage of optical materials of technological importance 8-10. Confined interactions were at the same time advocated for generating extreme conditions of pressure and temperature as the birthplace of novel material phases 11,12. The achievement of high pressure levels and shock beyond the material mechanical resistance limit (tens of GPa in fused silica for example) is usually in competition with ope