Determination of the Raman Polarisabilities of Optical Phonons in Lithium Niobate Uniaxial Single Crystal

Abstract

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.

Accurate determination of the anisotropy factors and the phase differences of Raman polarizabilities in some uniaxial crystals: the case of lithium niobate

“…The experimental set-up and the studied crystal have been described in [5]. The stoichiometric lithium niobate crystal belongs to the C 3v point group whose associated Raman tensors with active modes are [6]…”

“…S and S max are dimensionless quantities. Equation (5) shows that the Raman line intensity can be considered proportional to the integrated area. The integrated areas proportional to the Raman intensities of the transverse modes of A 1 and E y modes are evaluated over an interval 2 as shown in figure 2.…”

“…This is to avoid errors due to the base line asymmetry and to take into account the approximation ω ∼ ω 0 . The use of equation ( 5) then improves the calculation of peak areas rather than the approximation of the isosceles triangle used previously [5].…”

“…Moreover, they used arbitrary data to adjust these Raman intensities. More recently, Djiedeu et al [5], by rotating the crystal in steps of 10 • , have demonstrated a general method for the determination of the Raman efficiencies in the backscattering configuration for parallel and perpendicular polarizers. The method of crystal rotation allows us to express the Raman efficiencies of C 3v crystals as a function of crystal rotation angle.…”

“…The method of crystal rotation allows us to express the Raman efficiencies of C 3v crystals as a function of crystal rotation angle. In previous studies [4,5] such expressions of the Raman efficiencies were directly used to fit experimental curves. Therefore it was very difficult to determine some parameters like phase differences, anisotropy factors and extrema positions of the Raman efficiency.…”