Impact of Fe3+ on UV absorption of K2Al2B2O7 crystals

K₂Al₂B₂O₇ (KABO) is a new nonlinear optical crystal capable of laser harmonic generation in the UV range. However, abnormal UV absorption prevents its application in effectively generating UV light with wavelength shorter than 300 nm. The transmittance spectra of the grown crystals show distinct absorption bands at 216 nm and 264 nm. It is observed that the UV absorption is strongly correlated with iron impurity at a parts per million (ppm) level. Furthermore, electron paramagnetic resonance (EPR) spectra of the absorbing crystals show a strong signal at g = 2.00 position corresponding to a Fe³⁺ center. A new crystal growth method which reduces the iron content has been proposed and results show that the new KABO crystal is free from the Fe³⁺ UV absorptions.