Melt front propagation in dielectrics upon femtosecond laser irradiation: Formation dynamics of a heat-affected layer

Abstract

Several studies in dielectrics have reported the presence of a thin heat-affected layer underneath the ablation crater produced by femtosecond laser irradiation. In this work, we present a time-resolved microscopy technique that is capable of monitoring the formation dynamics of this layer and apply it to the study of a phosphate glass exposed to single pulses below the ablation threshold. A few nanoseconds after laser excitation, a melt front interface can be detected, which propagates into the bulk, gradually slowing down its speed. By means of image analysis combined with optical modeling, we are able to determine the temporal evolution of the layer thickness and its refractive index. Initially, a strong transient decrease in the refractive index is observed, which partially recovers afterwards. The layer resolidifies after approximately 1 μs after excitation, featuring a maximum thickness of several hundreds of nanometers.

Simultaneous time-space resolved reflectivity and interferometric measurements of dielectrics excited with femtosecond laser pulses

“…Obviously, additional mechanisms would need to be taken into account to fully describe the material behavior in the indicated time window, including, most likely, possible bandgap renormalization/shrinking effects prior to carrierlattice thermalization, thermionic emission [60], as well as carrier diffusion [61]. Yet the similarity of the behavior observed here with that in numerous other dielectrics, even at subablative fluence [62], and the near-zero reflectivity in this delay window in metals and semiconductors suggest a common transient state, which could be that of a black body.…”

Assessing the limits of determinism and precision in ultrafast laser ablation

“…Characterization of the ablated surface was performed by means of confocal microscopy (Leica DCM3D, 460 nm illumination, 150x objective lens). This methodology has an advantage over other microscopy techniques in that surfaces can be characterized based on their topography rather than on changes in contrast, which may depend on the type of modifications achieved in different interaction regimes 15 . The use of topographic images together with automated software analyses (Mountains 8) allowed us to monitor the onset of ablation and properly determine the ablated area.…”

Key stages of material expansion in dielectrics upon femtosecond laser ablation revealed by double-color illumination time-resolved microscopy

“…Even though propagation speed values close to 350 m/s for such a melt-front were reported in silver [31], those experiments cannot be directly compared with the present data, because of the different material type (metal vs dielectric) and the configuration in form of a thin film. In a different dielectric, such a phosphate glass, our group has recently reported the observation of a Newton ring pattern related to propagation of a melt-in front [10]. The results allowed to stablish the formation of a melt-in front with a gradually slowing down propagation speed (1 m/s at 5 ns after excitation), leaving behind a layer with homogenous refractive index.…”

“…This discovery illustrates the high potential of time-resolved imaging techniques to capture a variety of transient physical processes taking place in laser-excited transparent materials. This fact is demonstrated by the recent characterization of exciting phenomena, such as the formation of a heat-affected-layer [10] or light amplification in dielectrics upon excitation [11], and accompanied by the development of new imaging strategies [12][13][14].…”