M. D. Fontana

The composition dependence of the Raman spectrum and new assignment of the phonons in

The Raman spectrum of has been measured for various scattering configurations in crystals with different compositions. The comparison between spectra recorded for the congruent and the nearly stoichiometric crystals shows significant differences in the shape and the number of Raman peaks. The analysis of results leads to a new and complete assignment of the long-wavelength optical phonons in .

Band structure treatment of the influence of nonstoichiometric defects on optical properties in LiNbO3

We present a band structure approach in connection with a molecular dynamics cluster optimization that accounts for various structural modifications related to the nonstoichiometry of LiNbO3 crystals. Variation of optical properties with the deviation from the stoichiometric composition can be understood within this approach. The particular role of the electron-phonon contributions to the electro-optic coefficient is shown. Model calculations yield a large dependence of the electro-optic coefficient r22 on the crystal composition, in agreement with the experimental data. The observed minimum of the r22 coefficient versus nonstochiometry is explained within a microscopic approach. The r33 tensor component shows substantially different behavior versus the stoichiometry.

Lattice site of Mg ion in LiNbO3 crystal determined by Raman spectroscopy

The defect structure of Mg-doped lithium niobate crystals was studied with varying Mg doping concentration by means of Raman scattering measurements. The Mg content dependences of both the frequency and damping of the two lowest-frequency A1(TO) phonon modes were used to derive the Mg incorporation mechanism in the LiNbO3 lattice. Results indicated that Mg ions are located on the Li site for low doping and then replace both Li and Nb ions for larger concentration.

Influence of the temperature‐dependent spontaneous birefringence in the electro‐optic measurements of LiNbO3

In spite of the large utilization of LiNbO3 in optical devices, its electro‐optic coefficients are not perfectly known. We show in the present paper that not taking into account the temperature dependence of the linear spontaneous birefringence can lead to an erroneous determination of the electro‐optic coefficients. The electro‐optic coefficient rc of LiNbO3 has been determined by an accurate technique issued from the Senarmont method. At room temperature, rc=19.9×10−12 m/V, with an estimated error of ±5%. The measurements are performed for values of the dc applied electric field as a function of the temperature around room temperature. We point out the drastic influence on the electro‐optic response of the spontaneous birefringence variation due to the temperature. Accurate and reproducible values of the electro‐optic coefficients are obtained only if this dependence is taken into account.

Optical damage resistance in undoped LiNbO3 crystals

Abstract Photo-induced birefringence changes are investigated on several undoped LiNbO 3 crystals as a function of laser-irradiation time and intensity. Comparison points out that the changes are the smallest in the crystals grown from high temperature top seeded solution (TSS). It is concluded that the optical damage resistance in undoped LiNbO 3 crystals is increased with a decrease of the amount of intrinsic defects related to the non-stoichiometry. The necessity to carefully control the true crystal composition is stressed.

Underdamped soft phonon in orthorhombic BaTiO3

Abstract Raman scattering measurements are reported in orthorhombic phase and through the orthorhombic-rhombohedral transition of BaTiO3. Spectra reveal the presence of an underdamped low frequency phonon which slightly softens with decreasing temperature and abruptly stiffens at the transition to the rhombohedral phase. Results are discussed in comparison with isomorphous KNbO3 and tetragonal BaTiO3 and are interpreted as due to a dominant displacive mechanism.

Displacive to Order-Disorder Crossover in the Cubic-Tetragonal Phase Transition of PbTiO3

Light scattering measurements extended down to 2 cm−1 of the laser line were performed on a single-domain PbTiO3 crystal. Spectra reveal the appearance of a central peak below the soft-phonon line when the cubic-tetragonal phase transition is approached with decreasing and increasing temperature. The relaxation process associated with this central peak presents a characteristic frequency smaller than 1 GHz and is shown to possess the main contribution to the dielectric permittivity near the transition temperature. This feature constitutes the first direct evidence of some order-disorder mechanism in the ferroelectric transition of PbTiO3 which was up to now considered as a typical example of a displacive mechanism.

Electric-field frequency dependence of Pockels coefficients in 2-amino-5-nitropyridium dihydrogen phosphate organic–inorganic crystals

The electro-optic and dielectric properties of organic–inorganic 2-amino-5-nitropyridium dihydrogen phosphate crystals were measured near room temperature at a wavelength of 632.8 nm and as a function of an electric-field frequency that varied from 1 kHz to 40 MHz. The frequency dispersion of the electro-optic coefficients and their comparison with the second-harmonic generation coefficients showed that the largest part of the electro-optic properties is electronic in origin (whereas the piezo-optic contribution is small). Results reveal a large anisotropy of the ionic and electronic contributions that is related to the contributions of the organic molecule to nonlinear polarizability.

Assignment of the Raman lines in single crystal barium metaborate

A Raman-scattering study performed on (-BBO) at room temperature allows us to assign all the vibrational modes detected in the Raman spectra. The internal and external vibration modes are properly obtained by taking account of the light polarization, mode contamination and isotope effects. A correspondence between the lattice and the free-ring modes is also presented.

Raman spectroscopy as a probe of the rare-earth ions location in LiNbO3 crystals

Abstract The LiNbO 3 crystals doped with Nd 3+ , Dy 3+ and Er 3+ ions were investigated by means of Raman scattering measurements. Data were compared with results recorded in undoped congruent crystal and highly Mg-doped samples. The doping dependence of frequency and damping in the E(TO 1 ) and the two lowest frequency A 1 (TO) phonon modes were employed to deduce the incorporation site of rare earth doping ions in LiNbO 3 lattice.