Ultrafast pump-probe ellipsometric measurements of transient optical properties in metals during laser ablation

Ultrashort pulsed lasers offer a high potential for precise and efficient material processing. Thus, the fundamental understanding of the laser-material interaction aspects is of great importance. The reflectivity and absorption of material experience significant changes during and after being irradiated by an ultrashort laser pulse. These changes influence decisively the laser-material interaction of the processing pulse or of subsequent pulses in for example double pulse experiments. Absorption changes in metals induced by an ultrashort laser pulse near the ablation threshold are however insufficiently investigated to date. In this work, a unique pump-probe ellipsometry microscope is presented allowing the determination of the transient complex refractive index N = n - ik with a sub-ps temporal resolution (Fig. 1, left) [1]. The imaginary part k can directly be converted into the optical absorption coefficient α or into the optical penetration depth d. The setup enables measurements of laser irradiated optically thick samples below and above the threshold fluence for irreversible material modification. Thus, experiments above the ablation threshold fluence Ft
_hr
can be performed.