Effect of dislocations on the optical homogeneity and UV absorption of KABO crystals

The K2Al2B2O7 (KABO) crystal shows great promise for the fourth harmonic generation of an Nd:YAG laser. However, its high absorption in the UV range greatly affects the conversion efficiency at 266 nm, and up to now the mechanism of the absorption in the crystal has not been understood. This paper discusses the influence of the dislocation density on the optical homogeneity and UV absorption in KABO. Our KABO crystals were grown from high-purity materials with NaF as the flux. Samples cut perpendicular to the (0 0 1) face from different KABO bulk crystals were polished and chemically etched. Triangular etched pits were observed with densities varying from 103 to 104 cm−2, corresponding to decreasing optical homogeneities which are estimated to be 10−5-10−3 cm−1. The UV transmittance spectra also indicated that samples with higher etch pit densities had larger absorption coefficients. On the basis of this study the growth conditions were further optimized, and a bulk KABO crystal 45×21×12 mm3 in size was obtained which had an absorption coefficient of α=0.9 cm−1 at 266 nm, the lowest reported so far to our knowledge.