Dynamics of a laser-induced optical breakdown in the bulk of fused silica initiated by a sub-nanosecond laser pulse of an energy fluence as high as 8.7 kJ/cm 2 was investigated by using femtosecond time-resolved shadowgraphy. Plasma ignition, growth of the damaged region and accompanying hydrodynamic motion were recorded from the moment directly before the arrival of the driving laser pulse, in the time steps adapted to the rate of the occurring processes. The growth rate of the plasma channel, curvature radii and velocities of the wave fronts were extracted from the shadowgrams. It was found that the plasma channel develops with a supersonic velocity and the first observed shock front tends to transform itself from the initial bowl-like shape to the final spherical one characterising an acoustic wave. Appearance of multiple fronts accompanying the main shock front was registered and used in more detailed analysis of the optical breakdown dynamics in the transparent dielectrics.

It is well known that from the two fundamental thermodynamic parameters pressure p is much more efficient in transformation of the matter state than temperature T . Optical breakdown in a form of microexplosion proved to be a simple, cheap, and efficient source of transient extreme pressure and temperature. Our interest is focused on the breakdown performed with femstosecond laser pulses both on a surface and in the bulk of material. The paper delivers some examples of the transformation effects observed mostly in the transparent dielectrics under irradiation with femtosecond laser pulses.

We present characterization of structural modification triggered by tightly focused single pulses of a nanosecond laser inside single-crystal sapphire. Structural changes induced in the shock compressed region were investigated using high resolution transmission electron microscopy (HRTEM). Analysis of the zone around cavity in the bulk of sapphire reveals loss of crystalline order and formation of a mixture of amorphous/poly-crystalline structure. The properties of the laser-affected solid and possible routes of material transformation to the final state long after the pulse end is discussed. The results suggest that transformations to amorphous/poly-crystalline state occur as a result of sufficient heating of the shell region. This creates a localized molten zone which solidifies so rapidly that crystallization is by-passed.

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