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

Abstract

We 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.

Multiscale Laser Written Photonic Structures in Bulk Chalcogenide Glasses for Infrared Light Transport and Extraction

“…It has to be noticed that a strong plastic deformation appears at the 400 ns snapshot. This kind of deformation is not observed in fused silica [80], and is probably due to an accumulation of mechanical constraints, which are then relaxed in a very fast timescale (<1 µs), as evidenced by the subsequent snapshots. The subsequent slow opening of the void (white trace) suggests that a liquid form is formed, undergoing cavitation.…”

“…The observed dynamics in ns-µ s timescales is globally similar to that observed in fused silica and described in ref. [80]. Both Type I and II dynamics indicate the occurrence via laser excitation of a transient heat source, diffusion, and cooling.…”

“…The subsequent slow opening of the void (white trace) suggests that a liquid form is formed, undergoing cavitation. The final permanent void structure being the result of a competition between the slow cavitation opening and the cooling process, can have a characteristic transverse size well below the diffraction limit [80], because optics is no more involved. This indicates the importance of a material response in achieving structuring sizes significantly smaller than the wavelength and the capability of beam engineering in triggering and controlling such response and the associated relaxation dynamics.…”

Thermal and mechanical limitations to processing resolution in volume non-diffractive ultrafast laser structuring

“…The formation of an energy deposition channel in the bulk that relaxes laterally determines the generation of a void in a narrow region that seems decorrelated from the incident wavelength. The current scenarios indicate as mechanisms pressure or stresstriggered micro explosion and cavitation in either solid or soft phases [15,20,30] with a radial relaxation geometry. A mechanical material rupture defines the characteristic nm-range.…”

Picosecond Bessel Beam Fabricated Pure, Gold-Coated Silver Nanostructures for Trace-Level Sensing of Multiple Explosives and Hazardous Molecules

“…BB has been found to accomplish extensive and exciting photonic applications in various modern age fields, such as materials processing [16][17][18][19][20][21][22][23][24][25][26], photopolymerization [27], and microactuator fabrication [28]. BBs have also been extensively implemented for particle trapping [29], particle handling applications [30], or acoustic radiation force strategies in liquids [31].…”