XRD studies on the femtosecond laser ablated single-crystal germanium in air

Ultrashort laser ablation of single-crystal germanium has been performed in air with femtosecond laser pulses (150 fs, 1 kHz) of 810 nm in the laser fluence range of 0.7-35.4 J/cm2. Ablation depth dependence on the laser fluence shows that there are two different processes, which are explained in terms of electronic heating process and the optical penetration one. Structure of ablated region is characterized by means of two different XRD techniques. With increasing the laser fluence higher than 10.2 J/cm2, the laser-processed region of germanium exhibits poly-crystalline diffraction peaks in a wide-angle (θ/2θ) scan and a split of diffraction peak of (4 0 0) plane in the rocking curve, which are absent in the lower laser fluence. These observations could be explained in terms of structural changes induced by ultrashort laser irradiation at the higher laser fluence.