Morphology of ablation craters generated by fs laser pulses in LiNbO3

The surface morphology of the ablation craters generated in LiNbO3 by 130 fs laser pulses at 800 nm has been investigated by AFM/SNOM microscopy. The single pulse fluence corresponding to the ablation threshold has been estimated to be ≈1.8 J/cm2. A complex structure including random cone-shaped protrusions is observed inside the ablated crater. The scale of the protrusion spacing is in the submicron range and the heights are typically of a few tens of nanometers. At and outside the crater rim a novel quasi-periodic wave-like topography pattern is observed in both types of microscopy techniques. The average wavelength, that is slightly dependent on pulse fluence, is (500-800 nm) comparable to the light wavelength. This novel topography feature keeps a close similarity with a Fresnel diffraction pattern by an absorbing circular obstacle or impact wave pattern produced by a combination of heat and shock wave (resemble that of impact crater). It is proposed that the obstacle is associated to the strongly nonlinear multiphoton absorption at the peak of the pulse profile. The energy deposited by nonlinear absorption of such profile causes ablation of both the crater and the rippled structure.