<title>Investigation of thermal annealing by gamma irradiation at room temperature in LiNbO<formula><inf><roman>3 </roman></inf></formula>crystals</title>

Abstract

An interesting phenomenon of thermal annealing in gamma irradiated undoped, and photorefractive Cu-and Fe-doped, Zoriented LiNbO 3 crystals has been observed. Prior and after each gamma irradiation the crystals were thermally annealed in the air at 800'C for a couple of hours. Optical homogeneity was investigated on the entire areas of LiNbO 3 wafers by measuring distributions of birefringence, the principal azimuth, transmission, and parameters associated with birefringence dispersion, and also by measurements of additional absorption in a few wafers' points. It has been rather unexpectedly observed that the classical thermal annealing can lead to a decrease in optical homogeneity in the majority of cases. It is attributed to generation of an internal electric field by the pyroelectric effect, and to the electrooptic effect involved thereafter. On the other hand, the secondary electrons generated by gamma irradiation are believed to increase the optical homogeneity by increasing the crystal's conductivity and dissipating this field. A uniform temperature heating across the wafer generated by this irradiation is also a helpful factor in this gamma-annealing. It has been found that this effect at room temperature is small for gamma irradiation of 105 Gy, while increasing the doses to 106 Gy and 107 Gy can profit in a considerable reduction of the optical inhomogeneity. A certain influence of Cu-doping on this effect has also been observed.

Automated birefringence dispersion measurements

“…that thermal annealing in air in a resistant-heated chamber led to a noticeable decrease in the optical homogeneity, while this effect has later been cured by gamma irradiation. In [15], it has been suggested that a certain optical inhomogeneity involved in these heat treatments could be associated with temperature gradients in LiNbO 3 wafers, and was always present especially when the samples after the thermal annealing had been cooled down to room temperature. These gradients through the pyroelectric and electrooptic (also secondary electrooptic or piezo-electrooptic) effects could just be the primary source for the induced birefringence.…”