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The electron transition between oxygen vacancies and cationic vacancies shortens the band gap of KDP crystals.

The structure, total system energies, electronic structures and optical absorption of the [010] screw dislocation in potassium dihydrogen phosphate (KH2PO4, KDP) crystals are investigated in the framework of the density functional theory with the HSE06 functional. To describe the complete stress field, six supercells under different shear stress caused by the [010] dislocation are constructed. The results have confirmed that the shear stress will deform the crystal structure and narrow the band gap. One of the more significant findings to emerge from this study is that when the crystal is under a large shear stress, in other words, in the vicinity of the dislocation core the dehydration will occur and it will enhance the linear and nonlinear absorption of the KDP crystal. The H atom and hydroxyl are contributed by different PO4 tetrahedra, respectively. The data reported here appear to support the assumption that the [010] dislocation has a harmful influence on the resistance of KDP to the laser irradiation. In spite of its limitations, these findings have significant implications to understand how to improve LIDT of the KDP crystal and have a number of important implications for future practice.

Intrinsic point defects are regarded as one of the dominant factors that can induce initial structural breakdown under laser irradiation and reduce the laser damage thresholds of potassium dihydrogen phosphate (KDP) and its analog ammonium dihydrogen phosphate (ADP) crystals. In this work, we theoretically investigated the structural stress and extra optical absorption induced by the cation defects (Hi, VP, VK, and VN) in both crystals. VP5− is identified as one of the main defects that can contribute to the reduced laser damage threshold of KDP. It can collapse the PO43− skeletal structure of KDP and introduce extra optical absorption at 310–620 nm. The relatively higher laser damage threshold of ADP is attributed to the difficulties in forming VP defects in the crystal. The dominant cation defects Hi+ and VN3+ in ADP cause less local stress than VP5− in KDP and do not introduce any defect states and extra optical absorption that may reduce the optical damage threshold. These calculation results agree well with the experiments. It is therefore suggested to control the VP concentration during the KDP crystal growth and treatment to effectively enhance its resistance to laser irradiation.

The oxygen vacancy (VO) and interstitial oxygen (Oi) are the easiest formed intrinsic point defects that could decrease the laser damage thresholds of potassium dihydrogen phosphate (KDP) and its analog ammonium dihydrogen phosphate (ADP). In this work, we have grown KDP and ADP crystals by traditional temperature reduction method and “point-seed” rapid growth technique, and proved the existence of the oxygen defects by position annihilation technique. Based on the first-principle calculations, the mechanisms of the structural breakdown and the optical absorption induced by the VO and Oi defects in both crystals have been investigated. It is found that VO2+ and VO0 defects are the dominant point defects that attribute to the collapse of the KDP crystal structure and the weak extra optical absorption at about 155 nm. The higher laser damage threshold of ADP than KDP mainly comes from a mass of hydrogen bonds in ADP crystals. They have found to play an important role in stabilizing the skeleton structure of ADP by forming the strong O-O peroxyl bridge structure and by preventing the electron transitions between and within the NH4 and the PO4 groups. It is therefore suggested to decrease the concentration of VO defects during the growth progress in order to improve the crystal performance under laser irradiation.