Control of ablation anisotropy in fluoride crystal by a linearly polarized femtosecond laser

We experimentally showed that the pi/2-period oscillation of femtosecond laser ablation threshold with laser polarization orientation can be observed in calcium fluoride (CaF2). While the lowest single pulse ablation threshold, 5.3 J/cm(2), was observed when laser polarization is along the 010-crystal axis, the highest one, 5.8 +/- 0.1 J/cm(2), was found when angle between polarization and the 010-crystal axis were pi/4 or 3 pi/4. The pi/2-period oscillation of ablation threshold tends to be eliminated (yet still detectable) at high pulse fluence. The anisotropic ablation is possibly resulted from the different rate of avalanche ionization in different crystal axes.