, Phone/Fax: 00237699491389In this work, the analysis of both experimental and theoretical angular dispersion of the Raman frequencies of the optical polar phonons in LiNbO 3 (LN) single crystal has been determined with the emphasis of the determination without ambiguity of a new and complete set of the normal mode assignments that clarify long-standing debates. The coherency of such couple assignments is assessed by the computed oscillator strengths that are revealed to be in satisfactory agreement with others on the one hand and by the comparison with the oblique frequencies measured from the compilation of the Raman spectra acquired under different geometrical configurations in the 908 scheme on the other hand. Finally, the directional behavior of the quasinormal vectors has been established and discussed with regard to the symmetry mixing of modes or the mode with oblique polarization.

High-temperature Raman and infra-red spectra measurements have been performed on KTiOPO, single crystals in a temperature range from room temperature to 1MX)K. Special attention is paid to the low-frequency Raman spectra which exhibit an anomalously large quasi-elastic scattering: thisappearswithan increase intemperature. The Raman spectra were fitted using a response function involving one oscillator and two Debye relaxations. The resultingparametersare analysedand discussed inorder to clarify the phase transition mechanism and, more specifically, the influence of the potassium (K) ion on this transition and on the ionic conductivity. The high-temperature infra-red reflectivity spectra were fittedforthe two polar modesconfigurations A, and B,and have been analysedcritically within the previously-mentioned framework.

Cascaded conical diffraction where optical elements modifying the local polarization state are intercalated between the aligned biaxial crystals is analyzed theoretically in the framework of paraxial diffraction theory. The obtained expressions are verified and confirmed experimentally for the case of a two-crystal cascade intercalated by a polarizer or a wave plate. The present approach can be used to realize a variety of vector beams with complex beam shapes composed of concentric rings with strongly modulated azimuthal intensity distribution. A potentially very fast switching of the overall beam shape is possible if the intercalated elements are electro-optically tunable retarders.

Raman scattering is an excellent tool to probe matter for the research of its dynamical properties particularly in the aim of its integration in non linear optical applications. However, only few efforts have been devoted to the connection of the Raman scattering measurements with the nonlinear optical properties of inorganic materials such as LiNbO 3 (LN) and LiTaO 3 (LT). Such a relation requires at the first glance the determination of the Raman polarisability tensor elements whose absolute or relative measurement is experimentally very difficult. In the present study, we described a simple method based on crystal rotation, which leads us to measurements of the scattering efficiencies and their relative phase. More specifically, we characterized a congruent lithium niobate and we recorded its Raman spectra in the backscattering geometries e.g. for the following configurations (x(yy)x , x(yz)x , x(zy)x , x(zz)x). The measured spectra were fitted by using the Lorentz or Fano functions. The parameters deduced from the fit are presented as functions of rotation angles of crystal. This allows us to validate the selection rules and to determinate the relative phase and as the absolute values of the Raman tensor elements.

The present study highlights self-consistently helpful improvements dedicated to overcoming the difficulty resulting from the fitting procedure of integrated Raman intensities recorded according to the rotation crystal method described earlier. To this end, the anisotropy factors of Raman polarizabilities and the corresponding relative phases are determined within the framework of the exact mathematical derivation of the phase factors. These are the relevant parameters of the Raman efficiency relations which are numerically difficult to obtain from the fitting of the integrated areas. The present theoretical approach is then applied to the modes of the A(1) and Ey symmetry species of the lithium niobate (LN) crystal point group. All the expressions of the Raman absolute intensities of the A(1) and Ey irreducible representations initially imply three parameters to be determined from the fitting computations. However, from the derived analytical expressions of the phase differences, the number of parameters involved in the fitting procedure is reduced from 3 to 2, thus improving the statistics of the numerical treatment.