When a high power laser (1012 W/cm2)
irradiates a target, it induces a shock wave, which reaches
the (free) rear surface. The free surface is accelerated
and the shock wave is back-reflected as a rarefaction wave.
In the shock wave pressure regime involved here, melting
of the target during the shock or during the rarefaction
may occur. An optically recording velocity interferometric
system (ORVIS) has been developed to measure the time evolution
of the change in the reflectivity of the free surface.
Shock waves of the order of hundreds of kilobars are produced
in 50–125 μm thick Sn and Al foils, by a Nd:YAG
laser system with a wavelength of 1.06 μm, pulse width
of 7 ns (FWHM), and irradiance in the range
The changes in the reflectivity occur along two time scales:
a slow one, more than 17 ns in Al and more than 30 ns in Sn,
and a rapid one, less than 2.5 ns, in both materials. A
possible explanation for the sharp decreases in the time scale
is that melting occurs during the release of the free surface.