Investigation of thermal annealing by gamma irradiation at room temperature in LiNbO_3 crystals

摘要

An interesting phenomenon of thermal annealing in gamma irradiated undoped, and photorefractive Cu- and Fe-doped, Z-oriented LiNbO_3 crystals has been obrior 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 10~5 Gy, while increasing the doses to 10~6 Gy and 10~7 Gy can profit in a considerable reduction of the optical inhomogeneity. A certain influence of Cu-doping on this effect has also been observed.