Imaging of plasma formation, ablation and phase explosion upon femtosecond laser irradiation of crystalline Si and Ge

In this work we have applied femtosecond pump-probe microscopy to study the regime of strong excitation, previously not reported on with imaging techniques. The ability of this technique to simultaneously measure reflectivity changes with fs temporal resolution at locations of strong and weak excitation enables a direct comparison of mechanisms occurring in adjacent regions. For the experiments we have used Ti:sapphire amplifier delivering 100 fs pulses, at 800 nm, operating in single pulse mode. In order to enhance the excitation of the material by the laser we have used frequency doubling (400 nm), thus reducing the optical penetration depth of the laser light.