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Femtosecond laser-shockwave induced densification in fused silica

Tightly focused femtosecond laser-beam in the non-ablative regime can induce a shock-wave enough to reach locally pressures in the giga-Pascal range or more. In a single beam configuration, the location of the highest-pressure zone is nested within the laser-focus zone, making it difficult to differentiate the effect of the shock-wave pressure from photo-induced and plasma relaxation effect. To circumvent this difficulty, we consider two spatially separated focused beams that individually act as quasi-simultaneous pressure-wave emitters. The zone where both shock-waves interfere constructively forms a region of extreme pressure range, physically separated from the regions under direct laser exposure. Here, we present evidences of pressured-induced densification in fused silica in between the foci of the two beams, which can be exclusively attributed to the superposition of the pressure waves emitted by each focused laser-beam. Specifically, we show how the beams gap and pulses time-delay affect the structural properties of fused silica using Raman characterization, beam deflection technique, and selective etching techniques. The method is generic and can be implemented in a variety of transparent substrates for high-pressure physics studies and, unlike classical methods, such as the use of diamond anvils, offers a means to create arbitrary-shaped laser-induced high-pressure impacted zones by scanning the two beams across the specimen volume