Study on the optical mechanism of photorefractive resistance in Zr-doped lithium niobate crystals from first-principle calculations.

Lithium niobate on insulator (LNOI) has attracted widespread interest due to the excellent optical performance of lithium niobate crystals and the integration characteristics of thin film devices. With the improvement of the integration level of photonic integrated chips and the increase in light intensity inside the chip, the impact of optical damage in LNOI on-chip performance has attracted attention. One effective way to suppress the optical damage of lithium niobate is to dope it with Zr to form LiNbO (LiNbO:Zr), which is famous for its high resistance to optical damage from ultraviolet to visible spectrum. However, the mechanism behind the outstanding resistance to optical damage in LiNbO:Zr is still unclear. Here, the density of states, photorefractive center, refractive index, and birefringence of LiNbO:Zr are analyzed as functions of dopant concentrations by first-principle calculations. Electronic property analysis shows that the electrons in the Zr 4d state are all transferred to the Nb 4d state, occupying the shallow impurity level. The refractive index and birefringence change when the doping concentration reaches a threshold concentration of about 2.0 mol%. As the most stable full shell state, Zr-Nb can effectively suppress the photorefractive effect and result in the lowest refractive index and birefringence. These calculations not only establish the defect model corresponding to the different doping concentrations to investigate the optical properties of LN:Zr but also explain the mechanism underlying the high resistance to optical damage in LiNbO:Zr.