Structural aspects of nonlinear optics: optical properties of K2H(IO3)2Cl and related compounds

Abstract

Linear and nonlinear optical properties of dipotassium hydrogen di-iodate chloride K₂H(IO₃)₂Cl have been measured and are compared with other crystalline systems containing I—O bonds. The nonlinear coefficients d₃₃₃, d₃₁₁, d₁₁₃ and d₃₂₂ relative to d₁₂₃ of KH₂PO₄ are 11.0, 2.1, 2.2 and 0.12, respectively. Microscopic modelling in terms of bonds and lone pairs shows that the major source of the observed nonlinearities (d values) is due to the iodine lone pair and the short iodine-oxygen bond of ∼0.18 nm, rather than to the longer range interactions (∼0.3 nm) between the ions themselves. It is further shown that the 12 nonlinear coefficients from five different crystal systems containing IO^–₃ and IO₂ F^–₂ ions may be geometrically related to only one microscopic polarizability (β_ijk) with components β^∥ and β^⊥ which are (15.0 ± 1.5)× 10^–40 m⁴ V^–1 and (4.2 ± 0.4)× 10^–40 m⁴ V^–1 respectively. Nonlinear coefficients are also predicted for crystalline NH₄IO₃ and XeO₃. The fact that we find a non-negligible β^⊥ term is in disagreement with current theoretical models which maintain that β^∥≫β^⊥.