Time- and space-resolved dynamics of melting, ablation, and solidification phenomena induced by femtosecond laser pulses in germanium

Femtosecond time-resolved microscopy has been used to analyze the structural transformation dynamics (melting, ablation, and solidification phenomena) induced by intense $130\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ laser pulses in single-crystalline (100)-germanium wafers on a time scale from $\ensuremath{\sim}100\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ up to $10\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$. Complementary information on longer time scales $(350\phantom{\rule{0.3em}{0ex}}\mathrm{ps}--1.4\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s})$ has been obtained by means of simultaneous streak camera and photodiode measurements of the sample surface reflectivity. In the ablative regime, transient surface reflectivity patterns are observed by fs microscopy on a ps to ns time scale as a consequence of the complex spatial density structure of the ablating material. Complementing point-probing streak camera measurements allow one to characterize the temporal evolution in real time up to $40\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$ after the fs-laser pulse excitation. Fs microscopy reveals additional reflectivity patterns for fluences below the ablation threshold of the germanium. It is shown that these patterns are originating from the selective removal of the native oxide layer at the wafer surface within a certain fluence range. After solidification, and in contrast to other semiconductors, surface amorphization has not been observed in (100)-germanium upon femtosecond laser pulse irradiation in the studied fluence range.