Atomic scale study of stress-induced misaligned subsurface layers in KDP crystals.

We carried out ab initio calculations to study the atomic configuration, band structure and optical absorption of the lattice misalignment structure (LMS) in a subsurface layer of a machined KHPO (KDP) crystal. By varying the different degrees of misalignment, the changes in the corresponding atomic position and bond and energy are obtained, and their correlations are analysed in detail. The results indicate that in the LMS evolution, the variation in the proton distribution around the oxygen atoms plays an important role, and many local stable LMSs appear. Interestingly, at a certain misalignment value, the total system energy of the local stable LMS is near that of a perfect KDP crystal. For some local stable LMSs, the electronic and optical properties related to the laser damage threshold (LDT) of KDP are further studied. The results show that in comparison with a perfect KDP crystal, the band gaps of local stable LMSs at some certain misalignment values become narrow, and their optical absorption curves produce an obvious redshift. These facts demonstrate that the emergence of the LMS could have a significant impact on the optical absorption of the KDP material and thus affect the LDT of KDP under certain working conditions.