  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
