Dispersion of linear and non-linear optical susceptibilities for amino acid 2-aminopropanoic CH3CH(NH2)COOH single crystals: experimental and theoretical investigations

Abstract

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.