Polar potassium rare earth nitrates K2[RE(NO3)5(H2O)2] (RE = La, Ce, Pr and Nd). II. Linear and nonlinear optical properties

Linear and nonlinear optical properties of the four isomorphic orthorhombic (point group mm 2) potassium rare earth nitrates K 2 [RE(NO 3 ) 5 (H 2 O) 2 ], with RE = La, Ce, Pr or Nd, have been investigated. Refractive indices in a wavelength range from 0.35 to 2.5 µm and the transmission spectra between 0.25 to 3.0 µm were measured. Using the Maker fringe technique, the d_{ijk}^{\rm\, SHG} coefficients of second-harmonic generation for a fundamental of the Nd–YAP laser (λ = 1079.5 nm) were determined for all four compounds. The d_{ijk}^{\rm\, SHG} coefficients of all four substances are about three times larger than those of KDP, as was first published by Ebbers et al. [ IEEE J. Quantum Electron. (1993), 29 , 497–507] for K 2 [La(NO 3 ) 5 (H 2 O) 2 ]. These nitrates are promising materials for application to nonlinear optics ( e.g. second-harmonic generation of Nd laser radiation). The nonlinear optical properties of these compounds seem to have their origin mainly in the nitrate groups, as indicated by a simple model (summing the molecular hyperpolarizabilities of the NO 3 group). Neglecting local field effects, the hyperpolarizability of the NO 3 group was calculated as β 222 = 2.52 × 10 −40 m 4 V −1 .