First principles study on the doping mechanism and optical properties of KDP:Al crystals

Experimental results show that there is a transition from rising to falling for the laser damage threshold of the potassium dihydrogen phosphate (KDP) crystals when Al doping concentration changes from low to high; however, the mechanism is unclear. Therefore, this paper investigates five possible defect models of Al doping by examining the effects of these defect models on the crystal properties and the possible transformation of these defect models. We found, based on the defect formation energy and spectral analysis, that when the concentration of Al impurities is relatively low, Al mainly exists in the form of Al substituting for P (Al_P); when the concentration increases, interstitial Al will be formed. Both types of defects seldom directly affect the optical properties of KDP crystals. Meanwhile, Al_P can slightly inhibit the formation of H vacancies in the crystals but exerts a promotional effect on the formation of self-trapped holes. H defects are considered to be one of the main reasons for the decrease of the laser damage threshold of the crystals. Al_P therefore slightly increases the laser damage threshold of the crystals. Interstitial Al will greatly promote the formation of H defects in the crystals, thus significantly reducing the damage threshold of the crystals. Through the analysis of the lattice distortions caused by these two types of defects, we think that this is related to the amount of charge of Al ions and the coordination state of the atoms around them. This conclusion explains well the reason for the opposite changes in the laser damage threshold of KDP crystals doped with different concentrations of Al impurities.