Suppression of photorefractive damage with aid of steady-state temperature gradient in nominally pure LiNbO3 crystals

Abstract

Photorefractive damage (PRD) in as-grown, oxidized, and slightly reduced nominally pure LiNbO3, and iron-doped crystals with different compositions has been studied with the closed-aperture Z-scan and pseudo-Z-scan techniques at uniform temperature distribution, as well as at a steady-state temperature gradient ∇T, created by two external thermoelectric elements in the transverse direction to the light beam trajectory. The most important experimental finding consists of the demonstration of the possibility of a full PRD suppression in nominally pure crystals with the aid of a relatively small temperature gradient (12.5–84 K/cm), if ∇T is above a threshold value specific for each crystal studied. The threshold ∇Tth decreases significantly with the increase in the so-called bipolaron absorption band (center at 470–500 nm) in the optical spectra, and it is not correlated with the composition of the crystals within the studied range of compositions ([Li]/[Nb]=0.946–0.983). No any partial suppression of PRD was observed in iron-doped ([Fe]≥0.01 wt %) lithium niobate crystals, even at the largest temperature gradient (85 K/cm) used in our study. To explain these experimental results, we use theoretical model taking into account local changes of spontaneous polarization (i.e., polarization charge field) at photoionization of intrinsic defects (polarons, bipolarons, and hole polarons). It has been shown that a steady-state temperature gradient may induce a local thermoelectric current of light-induced charge carriers and their instant recombination, resulting in a decrease in the polarization space-charge field and hence, a light-induced refractive index change. The latter mechanism may induce the full PRD suppression, if macroscopic charge separation (space-charge field effect) gives insignificant contribution to the total light-induced electric field, as in as-grown and reduced nominally pure LiNbO3 crystals.