Femtosecond laser interaction with photo-refractive crystals: High frequency field and internal field contributions to the refractive index changes

The interaction of single femtosecond laser pulses tightly focused in the bulk of LiNbO
3
doped with Fe (400 ppm doping) and LiTaO
3
crystals was recently studied [1,2]. Single shots of 150 fs, 800 nm laser pulses, 3–50 nJ per pulse, were focused inside a crystal to the depth of 50 microns to the focal spot with diameter of 1.8 μm; the intensity in the interaction region varied from 1.0 TW/cm
²
to 16.7 TW/cm
²
. The threshold for optically detectable change of refractive index has been found to be 3.8±0.5 nJ in Fe: LiNbO
3
and 5.2±0.5 nJ in pure LiNbO
3
crystal (intensity TW/cm
²
and 1.37 TW/cm
²
respectively). The permanent modification of LiTaO
3
was observed at 32±5 nJ (∼10 TW/cm
²
) that might be considered as a result of breakdown. It has been found that the laser-induced material transformation is fully reversible at the energy per pulse of 14.5 nJ (3.8 TW/cm
²
).