Optical diagnostics of the laser-induced phase transformations in thin germanium films on silicon, sapphire, and fused silica

The in-situ procedure is used to study the modification of thin (200–600 nm) germanium films induced by nanosecond pulses of a ruby laser. The films are produced using the ion-beam or magnetron sputtering on single-crystalline silicon (Si), sapphire (Al$_2$O$_3$), and fused silica ($\alpha$-SiO$_2$) substrates. The results on the dynamics of the laser-induced processes are obtained using the optical probing of the irradiated region at wavelengths of $\lambda$ = 0.53 and 1.06 $\mu$m. The results of probing make it possible to determine the threshold laser energy densities that correspond to the Ge and Si melting and the generation of the Ge ablation plasma versus the amount of deposited Ge and thermophysical parameters of the substrate. The reflection oscillograms are used to obtain the dependences of the melt lifetime on the laser-pulse energy density.