A comprehensive experimental and theoretical investigation of dispersion of the linear and nonlinear optical susceptibilities for amino acidL-alanine single crystals is reported. The state-of-the-art full potential linear augmented plane wave method, within a framework of the density functional theory was applied. The atomic positions from X-ray diffraction have been optimized so that the force on each atom is around 1 mRy au−1. This relaxed geometry has been used for the theoretical calculations. The complex dielectric susceptibility dispersion, its zero-frequency limit and the birefringence of amino acidL-alanine single crystals were studied. The crystal exhibits a large uniaxial dielectric anisotropy resulting in a significant birefringence. The calculated birefringence at static limit is 0.072 and 0.074 at λ = 1064 nm (corresponding to 1.165 eV) in good agreement with the measured value (0.073). We also report calculations of the complex second-order optical susceptibility dispersions for the principal tensor components: χ(2)123(ω), χ(2)231(ω) and χ(2)312(ω). The calculated second order susceptibility tensor components |χ(2)123(ω)|, |χ(2)231(ω)|, and |χ(2)312(ω)| at λ = 1064 nm are compared with those obtained from our measurements performed using the 25 ps Nd:YAG pulsed laser at λ = 1064 nm. Our calculations are in reasonably good agreement with our experimental data. In addition we have calculated the microscopic second order hyperpolarizability, β123, vector component along the principal dipole moment directions for the dominant component χ(2)123(ω) and it is found to be 0.21 × 10−21 pm V−1 in the static limit and 0.27 × 10−21 pm V−1 at 1.165 eV (λ = 1064 nm) in comparison with our measured value (0.31 × 10−21 pm V−1) at λ = 1064 nm. Additional study of the second order susceptibilities versus the external laser treatment is performed.
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