Study of the electronic structure and second-order non-linear optical polarizabilities at surfaces of ionic crystals

Abstract The electronic structure of (111) surfaces of BaF 2 and CaF 2 crystals have been studied by the use of molecular cluster calculations, within the local density approximation (LDA). The surface is modelled by a 10–13-ion cluster embedded in an external potential from the surrounding crystal ions. In centrosymmetric crystals, such as BaF 2 and CaF 2 , any optical second harmonic generation (SHG) has to be due to electric quadrupole (EQ) interactions. However, in the surface region, electric dipole (ED) interactions will give SHG contributions. Using the wavefunctions obtained from the molecular cluster calculations, we have evaluated the ED part of the second-order microscopic polarizability for (111) surfaces of BaF 2 and CaF 2 . The results show strong frequency dependencies which may explain experimentally observed SHG signals from these materials. The influence of local field corrections for these systems is briefly discussed.