Breaking of Raman selection rules in photorefractive iron doped Lithium Niobate crystals

International audienceTransmission Raman spectroscopy experiments were performed as function of time on iron doped congruent lithium niobate crystals within two –in principle equivalent symmetric- configurations, namely Y(zx)Y and Y(xz)Y according to Raman selection rules. Whereas only the lines corresponding to E[TO] phonons are detected at any time as expected in the Y(zx)Y configuration, the spectra in the Y(xz)Y configuration shows a large evolution with time. Just at the beginning of illumination, the spectrum in the transmission geometry is very similar to the corresponding spectrum in back-scattering, but rapidly new lines appear and increase in intensity with time. After one minute of illumination, the spectrum completely differs from expected E[TO]. These new lines in fact arise from another mode symmetry and are clearly assigned as due to A1[TO] phonons.This activation with time of forbidden A1[TO] Raman lines for this configuration result from photorefractive internal field which causes the polarization conversion from ordinary to extraordinary polarization and is originated by photorefractivity of the crystal