Sub-damage-threshold femtosecond laser ablation from crystalline Si: surface nanostructures and phase transformation

Surface structures and structural transformations are investigated upon femtosecond laser ablation (800 nm, 120 fs) from crystalline silicon (100) targets placed under ultra-high vacuum. After repetitive illumination with several thousand laser pulses at intensities below the single shot damage threshold, at normal incidence, the crater morphology indicates the development of periodic structures at the crater bottom, with the orientation depending on the laser beam polarization. Periods of 200 nm and 600-700 nm, respectively, are shorter than the laser wavelength and appear as a result of surface instability. The ablation dynamics monitored by time-of-flight mass spectrometry shows the emission of positive silicon ions and clusters with kinetic energies of about 7 eV. Raman spectroscopy reveals phase transformations in the irradiated spot from Si-I to the polymorphs Si-III, Si-IV, Si-XII, and amorphous silicon as well as a stable, uncommon phase of hexagonal Si-wurzite.