Lithium thioindate (LiInS2) is a new nonlinear chalcogenide biaxial material transparent from 0.4 to 12 µm, that has been successfully grown in large sizes and good optical quality. We report on new physical properties that are relevant for laser and nonlinear optics applications. With respect to AgGaS(e)2 ternary chalcopyrite materials, LiInS2 displays a nearly-isotropic thermal expansion behavior, a 5-times larger thermal conductivity associated with high optical damage thresholds, and an extremely low intensity-dependent absorption allowing direct high-power downconversion from the near-IR to the deep mid-IR. Continuous-wave difference-frequency generation (5-11 µm) of Ti:sapphire laser sources is reported for the first time.
IR and Raman active modes have been measured in several fluoride scheelites, namely the laser matrix (studied as a reference) and four compounds (Ln = Ho, Er, Tm and Yb). The observed phonons have been assigned in accordance with the various irreducible representations of the point group of the crystal. In three modes never observed before have been measured. The polarized experimental IR reflection spectra have been analysed through a `four-parameters' model to give the mode frequencies and dampings. For the whole set of the compounds, the frequency dependence of the dielectric function, optical indices and absorption coefficient have been deduced from the IR study. The electronic Raman effect has been shown in the low-temperature Raman spectra for Ln= Tm, Ho and Yb. The corresponding measured wavenumbers are compared with the calculated energies between crystal-field levels of L SJ states, each being associated to an irreducible representation of the ion-site group .
A general view of the fluoride scheelite lattice dynamical properties is proposed. The symmetry properties of the vibrations and the symmetry-adapted eigenvectors of the scheelite structure have been determined for the most important directions of the Brillouin zone, namely , and . The low-frequency phonon spectrum along these three directions has been measured by inelastic neutron scattering. The corresponding data, together with the measured sound velocities and the Raman- and infrared-active phonon frequencies, are described in the framework of a rigid ion model. The average error between experimental results and calculated frequencies is less than 5%. The most important normal modes of vibration and the one-phonon density of states are deduced.
A simple, unified form of the fluorine-fluorine interionic potential is deduced from the modelization of many fluoroperovskites through a rigid-ion model. The Born-Mayer potential appears to be the most 0 0 convenient description ( VF F =i,e "~)' with p=0. 582 A and A, =481 nm ' A ). The short-range force constants deduced from this potential enabled the prediction of phonon-dispersion curves and density of states in perovskite-type compounds for which very few experimental data are available.
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