Raman spectroscopy of lithium niobate (LiNbO3)- Sample temperature and laser spot size effects

The Raman spectrum of lithium niobate (LiNbO3) was investigated in the 83-823 K temperature range and as a function of different laser spot sizes. The measurements considered a Z-cut (congruent, undoped) LiNbO3 crystal, HeNe laser excitation (632.8 nm photons), and the z(x, xy)z geometry. The main LiNbO3-related phonon modes were identified and analyzed in terms of their peak position (omega) and linewidth (gamma), according to which it was possible to identify the contributions originating from anharmonic phonon-coupling and thermal lattice -expansion processes. The analyses of the temperature-dependent omega(T) and gamma(T) data took into account the 3 -and 4-phonon model and the experimental Gruneisen and linear thermal expansion coefficients -as available from literature. Among all phonon modes of LiNbO3, the A1(TO4) mode presented a peculiar behavior (at room temperature) that depends on the laser spot size during the Raman measurements. The development of this forbidden mode has been associated with the congruent-photorefractive nature of LiNbO3 as well as with small variations in the geometry of polarization (as a result of changes in the laser spot size).