Lattice Site of Rare-Earth Ions in Stoichiometric Lithium Niobate Probed by OH− Vibrational Spectroscopy

Rare-earth (RE = Er3+, Nd3+, or Yb3+) ion-doped stoichiometric LiNbO3 crystals were grown by the Czochralski and the high-temperature top-seeded solution growth methods. For the 0.22–0.87 mol% concentration range of the RE oxides in the melt/solution, in addition to the well-known hydroxyl (OH−) vibrational band in undoped stoichiometric LiNbO3, a new infrared absorption band was observed at about 3500 cm−1, similar to the case of the trivalent optical damage resistant (ODR) dopants In3+ and Sc3+. By comparing the frequencies and polarization dependences of the bands to those detected for ODR ion containing crystals, they are attributed to the stretching vibration of OH− ions in RE3+Nb-OH− complexes. Consequently, above a given concentration threshold, some of the rare-earth ions are assumed to occupy niobium sites in the LiNbO3 lattice. The same model is also suggested for RE-doped congruent LiNbO3 crystals containing over-threshold (>5 mol %) amounts of the Mg-co-dopant

Photorefractive damage resistance threshold in stoichiometric LiNbO_3:Zr crystals

Several optical methods including ultraviolet absorption, infrared absorption of the hydroxyl ions, Raman spectroscopy, and the Z-scan method have been used to determine the damage resistance threshold in 0–0.72 mol. % Zr-containing, flux-grown, nearly stoichiometric LiNbO3 single crystals. All spectroscopical methods used indicate that samples containing at least ≈0.085 mol: % Zr in the crystal are above the threshold while Z-scan data locate the photorefractive damage threshold between 0.085 and 0.314 mol. % Zr

Index of Refraction and Absorption Coefficient Spectra of Paratellurite in the Terahertz Region

Index of refraction and absorption coefficient spectra of pure paratellurite (α-TeO2) crystal as a potential material for terahertz (THz) applications were determined in the 0.25–2 THz frequency range at room temperature by THz time domain spectroscopy (THz-TDS). The investigation was performed with beam polarization both parallel (extraordinary polarization) and perpendicular (ordinary polarization) to the optical axis [001] of the crystal. Similarly to the visible spectral range, positive birefringence was observed in the THz range as well. It was shown that the values of the refractive index for extraordinary polarization are higher and show significantly larger dispersion than for the ordinary one. The absorption coefficient values are also larger for extraordinary polarization. The measured values were fitted by theoretical curves derived from the complex dielectric function containing independent terms of Lorentz oscillators due to phonon-polariton resonances. The results are compared with earlier publications, and the observed significant discrepancies are discussed

Measurement of Refractive Index and Absorption Coefficient of Congruent and Stoichiometric Lithium Niobate in the Terahertz Range

Time domain THz spectroscopy measurements were performed on a series of undoped and Mg-doped congruent lithium niobate crystals with 1.2, 6.1, and 8.4 mol% Mg concentrations and on undoped and Mg-doped stoichiometric lithium niobate crystals with 0.7, 1.5, and 4.2 mol% Mg concentrations with polarization parallel (extraordinary) and perpendicular (ordinary) to the z axis of the crystal at 300 K. The absorption coefficient and refractive index spectra were determined in the THz frequency range from 0.25 to ~2.5 THz. In the case of congruent samples for both polarizations, both the refractive index and the absorption coefficient have minimal values for compositions close to the photorefractive threshold. In the case of stoichiometric samples, similar tendencies close to the photorefractive threshold at lower Mg concentration were observed but only for extraordinary polarization, while for ordinary polarization the measured values, especially for the absorption coefficient, were only weakly dependent on the Mg content

Refractive Index and Absorption Coefficient of Undoped and Mg-Doped Lithium Tantalate in the Terahertz Range

Dielectric material parameters of lithium tantalate (LT) in the terahertz region have been investigated using terahertz time-domain spectroscopy (THz-TDS). Undoped congruent, undoped stoichiometric, and Mg-doped stoichiometric LT crystals were measured. The Mg content was 0.5 and 1.0 mol% for the stoichiometric composition. Index of refraction and absorption coefficient spectra were determined in the 0.3 2.0-THz frequency range for beam polarization both parallel (extraordinary polarization) and perpendicular (ordinary polarization)to the optical axis [001] of the crystal at room temperature. For the calculation of the refractive index and absorption coefficient spectra from the measured data, we used TeraMat software (Menlo System) belonging to the spectrometer. The refractive index and the absorption coefficient for stoichiometric crystals were lower than for the congruent one. In the case of stoichiometric crystals, the Mg dopant caused a slight reduction of both ordinary and extraordinary refractive index compared to the undoped crystal. However, the presence of Mg did not reduce the absorption coefficient either for the ordinary or for the extraordinary polarization. In order to fit the measurement data, a Lorentz oscillator model was used. Good agreement was obtained between the measured data and the fitting curves by using the Lorentz oscillator model containing three terms