David Chapron

Raman frequency shift induced by photorefractive effect on Fe–doped lithium niobate

We report on the observation of a time dependent frequency shift of the Raman modes in photorefractive Fe:LiNbO3. The origin of this shift is attributed primarily to the space charge field which builds up due to photo-induced charge transport by the excitation beam, and to the subsequent piezo-induced strain. This phenomenon can be regarded as the Raman analogous of the photorefractive effect, in which the space charge field is detected by an electro-optic variation of the refractive index. It should be possible therefore to analyze it using standard models for photorefractivity. In this paper, we verify this claim discussing its validity.

Ca doping in BaTiO3 crystal: Effect on the Raman spectra and vibrational modes

Polarized Raman measurements were carried out in a 3% Ca- doped BaTiO3 (BCT) single crystal and compared with those in undoped BaTiO3 (BT) in order to study the effect of Ca2+ incorporation into the BT lattice. Spectra recorded at room temperature in BCT show peaks unexpected by the Raman selection rules in BT, proving an off-centering of Ca with respect to the Ba site in BT. Main phonon lines A1 polarized along the ferroelectric c axis are notably shifted to lower frequency by Ca doping, while the phonons E in the plane normal to c are nearly unaffected. In addition, the modes A1 are highly anharmonic, even more than in BT, as revealed by the temperature dependences of their frequency and damping which are analyzed in terms of 3rd and 4th order anharmonic potential.

Zr doping on lithium niobate crystals: Raman spectroscopy and chemometrics

Raman measurements were investigated on Zr-doped lithium niobate LiNbO3 crystals with different concentrations. Spectra were treated by fitting procedure and principal component analysis which both provide results consistent with each other. The concentration dependence of the frequency on the main low-frequency optical phonons provides an insight of site incorporation of Zr ions in the host lattice. The threshold concentration of about 2% is evidenced, confirming the interest of Zr doping as an alternative to Mg doping for the reduction of the optical damage in lithium niobate.

Photorefractive Lithium Niobate crystals: light polarisation rotation highlighted by transmission Raman spectroscopy

Transmission Raman spectroscopy experiments as a function on time were carried out on iron doped photorefractive congruent lithium niobate crystals. A big change with time in Raman spectra is observed when the incident light is polarized along ordinary axis. The mentioned breaking of Raman selection rules is originated by photorefractive properties of the crystal.