Effect of femtosecond laser-induced high pressure on fused silica polymorphism

High-pressure studies in silica are of great interest to earth and planetary sciences. Highpressure phases are rarely found in nature as a result of shock events like meteorite impacts or volcanic events in nature. Recreating these phases remains a challenging task in lab conditions and can synthesize only very limited volume using static loading techniques like diamond anvil cell.

In this thesis, we aim at recreating high pressure phases in silica using shock waves generated from a femtosecond laser. We put forth three different approaches for the high pressure generation in the bulk of the silica matrix. The frst method being a double beam exposure method; it potentially offers a means to create arbitrary patterns of laser-induced high-pressure impacted zones by scanning the two beams across the specimen volume. Tightly focused femtosecond laser-beam in the non-ablative regime can induce a shockwave suffciently intense to reach local pressures in the giga-Pascal range or more. In a single beam confguration, the location of the highest-pressure zone is nested within the laser-focus area, making it diffcult to differentiate the effect of the shockwave pressure from photo-induced and plasma relaxation effects. To circumvent this diffculty, we consider two spatially separated focused beams individually acting as quasi-simultaneous pressure-wave emitters. The zone in between the two laser beams where both shockwaves superpose forms a region of extreme pressure range, physically separated from the regions where the plasma formed. Here, we present a detailed material investigation of pressured-induced densifcation in fused silica between the foci of two laser beams.

Further, we investigated the effect of ultrashort-pulsed laser (50 fs) on material transformation by the shockwave emitted by the pulses and the repeated hammering effect of cumulative pulse exposure. This was done by characterizing the ablated particles after oblique writing on a fused silica substrate. Finally, we used a simultaneous spatial temporal focusing technique (SSTF) to achieve higher energy confnement inside a limited volume within the substrate, thereby inducing high pressure in the material. All materials were characterized using a Raman spectroscope to evaluate the phase transformation introduced by the dynamic laser shock loading. Although this study focused on fused silica due to its relevance for both geophysical and engineering investigations, the methodology used is generic and can be implemented in a variety of other transparent substrates for high-pressure physics studies.