Study on the effect of defect cluster <i>Cu</i> _<i>K</i> - <i>V</i> _<i>H</i> on laser damage threshold in KDP crystals

Abstract Based on first-principles calculations, the defect formation energy (DFE), lattice distortion, electronic structure, and optical properties of defect clusters <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mspace width="0.25em"/> <mml:mi>C</mml:mi> <mml:msub> <mml:mrow> <mml:mi>u</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>K</mml:mi> </mml:mrow> </mml:msub> <mml:mo>−</mml:mo> <mml:msub> <mml:mrow> <mml:mi>V</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>H</mml:mi> </mml:mrow> </mml:msub> </mml:math> have been studied in the KDP crystal. The DFE results indicate that the defect clusters mainly exist in neutral and -1 charged states. Lattice distortion analysis reveals that the defect clusters produce significant variations in Cu-O bond lengths (up to 37%) for different charge states. Density of states (DOS) calculations reveal that the neutral charged state of the defect cluster generates high-density defect states within the band gap, which are highly likely to participate in multiphoton transition processes. In contrast, the −1 charged state induces an upward shift of the valence band maximum (VBM), resulting in a narrowed effective band gap that facilitates single-photon electronic transitions. By considering electron–phonon coupling, the calculated absorption peaks are located at 271 nm and 378 nm. The 378 nm absorption band closely matches the experimentally observed peak at 390 nm. The transition process is accompanied by significant lattice relaxation energy (nearly half of the absorption energy), which we propose causes severe damage to the crystal and significantly reduces the laser-induced damage threshold (LIDT) of KDP crystals.