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.

The UV absorptions for KABO crystals were assigned to an O2− → Fe3+ charge-transfer process in which Fe3+ substituting Al3+ sites. A new growth method to reduce the iron content in the grown crystal was adopted and a new KABO crystal shows negligible Fe3+ UV absorption was obtained for the first time. Figure optionsDownload as PowerPoint slide