Molecular dynamics simulations of some amorphous and crystalline photonic materials

Abstract

Constant pressure molecular dynamics (MD) simulations have been used to simulate amorphous and crystalline forms of BaTiO₃ and TiO₂. Simulation results for pure RaTiO₃ and TiO₂ glasses show fourfold titanium coordination with a Ti–O bond distance of 1.8 Å, consistent with experimental evidence for the structure of titania doped glasses for all optical switching and ultra low expansivity (ULE) TiO₂–SiO₂ glasses. Radial distribution function data for crystalline, liquid, and amorphous forms of BaTiO₃ were also obtained. The displacement polarization and its contribution to susceptibilities have been calculated by application of an electric field to the simulation cell. The calculated ionic dielectric constants (Ks_i) for simulated NaCI (crystal), TiO₂ glass, and TiO₂ (crystal) were 3.34, 5.96, and 19.4 as compared to the experimental values of ≍3.34, 3–10, and 78, respectively. The calculated cubic nonlinear susceptibility (ξ₃) values for NaCI (crystal), TiO₂ (glass), and TiO₂ (crystal) were, respectively, 0.194, 2.175, and 4.68 (x 10⁻¹⁸m²V⁻²). The increase in ξ₃ values is consistent with experimentally observed trends of the linear refractive indices. Improved agreement between experimental and calculated values of susceptibilities and dielectric constant was obtained for materials with higher ionicity.