Mechanism of the linear and nonlinear optical effects of SrAlF5 and BaMgF4 crystals

The electronic properties of SrAlF5 and BaMgF4 have been studied using the ab initio plane-wave pseudopotential method. Based on the electronic band structures, the linear and nonlinear optical properties for these crystals are theoretically determined. Both the calculated refractive indices and nonlinear optical effects are in reasonable agreement with the available experimental values. The origin of the nonlinear optical effects is elucidated through a real-space atom-cutting analysis. It is found that in SrAlF5 crystals the contribution of the (AlF6)3− anionic group to both the anisotropy of the linear optical and second harmonic generation effect is dominant while that of the A-site cations Sr2+ can be negligible. With the size increase of the A-site cations the contribution of Ba2+ for the optical effects in BaMgF4 become more pronounced and is comparable to that of the (MgF6)4− anionic group.