Interferometry and femtosecond imaging setup for multiscale pump probe experiments

Femtosecond time-resolved microscopy has been used to analyze the structural transformation dynamics induced by intense 130 fs laser pulses in single-crystalline (100)-Germanium wafers, including melting, ablation and solidification phenomena which have been studied on a timescale from ~100 fs up to 10 ns. Complementary information on the solidification dynamics has been obtained on longer timescales (350 ps - 1.4 Ás) 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 timescale due to the formation of a rarefaction wave. Complementary point-probing streak camera measurements allow to characterize their temporal evolution in real time up to 40 ns after the fs-laser pulse excitation. It is shown that the native oxide layer at the wafer surface can be selectively removed at fluences below the ablation threshold. In contrast to other semiconductors, surface amorphization has not been detected in (100)-Germanium upon femtosecond laser pulse irradiation in the studied fluence range.