Investigation of the photorefractive effect in lithium niobate crystals using femtosecond laser pulses

Abstract

The photorefractive effect in iron-doped lithium niobate crystals is investigated, using femtosecond pulses and cw light, both at a wavelength of 532 nm, in direct comparison. For that purpose, measurements about light-induced or holographic scattering of a single beam as well as writing of index gratings with two interfering beams are performed. We find that light-induced scattering is reduced for femtosecond pulses, and even absent for a sufficiently low Fe2+ concentration, in comparison and in contrast to cw light. Additional differences include a slower buildup time and a weaker angular selectivity for the scattering of pulses. Our observations can be attributed to the smaller temporal coherence of the pulses. When writing index gratings into as-grown lithium niobate crystals, the saturation value of the refractive index unexpectedly decreases with increasing pulse energy fluence. Furthermore, in oxidized crystals, writing with femtosecond pulses turns out to be much faster than with cw light. A model about the charge excitation, migration and trapping is proposed that explains these differences.