Regensburg 2002 – scientific programme

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DS: Dünne Schichten

DS 20: Laserverfahren zur Schichtherstellung und Oberfl
ächenmodifizierung II

DS 20.2: Talk

Thursday, March 14, 2002, 12:00–12:15, HS 31

Surface modification of ionic crystals induced by ultra-short laser pulses — •Florenta Costache, Matthias Henyk, and Jürgen Reif — BTU-Cottbus, LS Experimentalphysik II, Universitätsplatz 3-4, 03044 Cottbus, Deutschland

The surface morphology of ionic crystals (BaF2 and CaF2) after femtosecond laser ablation was examined ex-situ by optical, scanning electron and atomic force microscopes. At laser intensities above the ablation threshold and thousand of laser shots several superimposed periodic surface structures (ripples) occurred on the same crater bottom: a coarse ripple pattern with period of 600-900 nm running parallel to the beam polarization, superimposing a fine pattern perpendicular on the electric field of the incident beam with a period of about 200-300 nm. The ripple formation and development were investigated with an increasing number of shots. However, ripple periods appear to depend on the incident intensity. Tof mass spectrometer measurements performed during the structure creation showed a strong emission of fast ions, signature of an electrostatic decomposition of the surface by Coulomb explosion, which calls into question the thermal effects as the origin of the ripples. Furthermore, the influence of interference patterns on the ablated spot was investigated by a two-beam experiment. The laser pulses from both beams were subsequently delayed in a range of 0 - 200 ps with respect to each other, in order to follow the dynamics of the surface relaxation. We observed that the crater morphology exhibits complex patterns, again suggesting self-organization mechanisms at the surface.