Oxygen vacancies in KTiOPO4 : Optical absorption from hybrid DFT

Density functional theory is used to calculate the optical absorption of oxygen vacancies in potassium titanyl phosphate ($\mathrm{K}\mathrm{Ti}\mathrm{O}\mathrm{P}{\mathrm{O}}_{4}$, KTP) crystals. A modified hybrid functional is used for the description of the midgap defect states and the optical excitation energies. Oxygen vacancies in the $+2$ charge state lead to rather minor modification of the bulk KTP optical response, while the $+1$ and neutral charge states give rise to characteristic midgap optical absorption covering the whole near-infrared and visible spectrum. Its intensity is strongly polarization dependent and strongest for light polarized parallel to the $z$ axis. The modification of the KTP optical absorption by oxygen vacancies predicted here corroborates the picture that the gray-track formation in KTP, i.e., its photochromic damage, is related to a successive charging of oxygen vacancies.