d–d Transitions of Fe3+ ions in Fe-doped K2Al2B2O7 crystal

Single crystal of K2Al2B2O7 (KABO) with 6.3% aluminum substituted by iron was grown by top-seeded solution method using KF-B2O3 as flux. Optical transmittance, reflection and photoluminescence spectra of the Fe-doped KABO crystal were investigated by comparing with those of the Fe-free crystal. The absorption and fluorescence bands in the visible and near infrared regions were assigned to d–d transitions of tetrahedrally coordinated Fe3+ ions. The results of theoretical calculation based on the crystal-field approximation show a good agreement with the observed optical absorption peak positions. These evidences reconfirmed that the Fe3+ impurity causes the UV absorption in KABO crystal which prevent it from being used as efficient nonlinear optical materials for the fourth harmonic generation of Nd-based lasers.