K. Polgár

Growth of stoichiometric LiNbO3 single crystals by top seeded solution growth method

Abstract Stoichiometric LiNbO 3 single crystals have been grown using the top-seeded solution growth method from potassium containing flux. Spectroscopic properties (e.g. the UV absorption edge, OH-vibrational band) have been used for the characterization of the crystal composition. Optical homogeneity and the formation of lattice defects were compared with those of undoped off-congruent composition crystals prepared by the Czochralski method.

Temperature dependence of the absorption and refraction of Mg-doped congruent and stoichiometric LiNbO3 in the THz range

The absorption coefficient and the index of refraction of Mg-doped LiNbO/sub 3/ crystals with different compositions are determined in the 30 - 200 cm-1 frequency range. We find that stoichiometric LiNbO/sub 3/ has smaller absorption and index of refraction than congruent samples.

Composition dependence of the ultraviolet absorption edge in lithium niobate

A convenient and fast method to determine the composition of LiNbO3 single crystals consisting of a measurement of the position of the fundamental absorption edge in the near UV region is refined and extended to the stoichiometric range. Using unsophisticated apparatus and simple reflection correction, the proposed two-parameter calibration equation provides an absolute accuracy of 0.1 mol % with the relative accuracy reaching an unusual value of better than 0.01 mol % for near stoichiometric compositions.

Infrared absorption study of the OH vibrational band in LiNbO3 crystals

Abstract Stoichiometry and temperature dependence of the OH vibrational band was studied in LiNbO 3 crystals. The decomposition of the bands using the product function of a Lorentzian and a Gaussian showed that in Li deficient crystals the absorption bands consist of at least four components in contrast to more or less perfect stoichiometric crystals which reveal only one to three components. The two new peaks are interpreted by the appearance of two energetically different proton sites in the LiNbO 3 structure having ilmenite-like segments. Above 300°C only one broad band dominates at about 3465 cm −1 indicating the increased mobility of the protons and the interaction with the lattice.

Spectroscopic and electrical conductivity investigation of Mg doped LiNbO3 single crystals

Abstract Series of Mg doped congruent LiNbO3 crystals were investigated by UV a nd IR spectrophotometry and AC conductivity. The position of the UV absorption edge, the shape of the absorption band of the OH- vibration and the value of the electrical conductivity at 400°C changed monotonously up to a critical concentration of Mg ions. Above the concentration of about 4.5×10-2 mol/mol Mg in the crystal anomalous behaviour was observed. We conclude that the mechanism of the incorporation of Mg ions changes at this concentration.

On the lattice site of trivalent dopants and the structure of Mg2+-OH--M3+ defects in LiNbO3:Mg crystals

The lattice sites of a number of trivalent co-dopants (In, Nd, Gd, Er, Tm, and Lu) have been determined in LiNbO3 crystals also with Mg, using the Rutherford back-scattering and proton-induced X-ray emission channelling techniques. In, Er and Lu have been found mainly on Li sites, but a small fraction of In and Lu may substitute for Nb ions. Gd and Tm ions also replace mainly Li, but some of these ions seem to be present in other positions, possibly related to some kind of aggregate, e.g. self-compensating pairs. A fraction of Nd ions probably occupies interstitial sites in structurally empty oxygen octahedra, while other Nd ions may substitute for host cations. In addition to the already known effect of Cr, Fe and Nd ions, In, Lu and also Sc give rise to an OH- absorption band between 3499 and 3523 cm-1, which is attributed to Mg2+ (Li site)-OH-(O site)-M3+ (Nb site) complex defects. No such band is observed for Y, Gd, Er and Tm co-doped crystals.

Nonlinear refraction and absorption of Mg doped stoichiometric and congruent LiNbO3

The light induced change of refraction is studied in pure and Mg doped LiNbO3 with congruent and stoichiometric compositions by the Z-scan method using all-lines visible argonion laser, up to MW/cm2 intensity level. In Mg-doped congruent and stoichiometric crystals with Mg concentrations above threshold a positive change in the refractive index was found, in contrast to all other cases where beam fanning and negative change of the refractive index were observed. The beam distortion in the samples doped above threshold was related to thermal lensing, while below it to the photorefractive effects. It was also shown that for thermal lensing nonlinear absorption plays a dominant role. The Z-scan method was found to be an alternative technique to decide whether the Mg dopant level is above or below the photorefractive threshold. The damage resistance of the Mg doped samples above threshold was higher for the stoichiometric crystal than for the congruent one and increased with the amount of the built-in Mg conc...

Measurement of laser-induced refractive index changes of Mg-doped congruent and stoichiometric LiNbO3

Light-induced refractive index changes and nonlinear absorption of undoped and Mg-doped LiNbO3 with congruent and stoichiometric composition were investigated by the Z-scan method using an all-visible argon-ion laser with intensities up to the MW/cm2 level. Charge coupled device recordings demonstrate that the Z-scan method is suitable for quantitative study of Mg-doped samples, while it can be used only for qualitative measurements of undoped samples because of beam fanning in the latter case. Stoichiometric LiNbO3 doped with 5 mol % Mg shows positive refractive index changes opposite to those in cases of undoped congruent, undoped stoichiometric and 5 mol % Mg-doped congruent LiNbO3. The refractive index change of Δne=2.5×10−5 measured at 0.37 MW/cm2 light intensity for stoichiometric LiNbO3:Mg is six times smaller than that of congruent LiNbO3:Mg at 0.1 MW/cm2.

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.

Investigation of Cu-doped Li2B4O7 single crystals by electron paramagnetic resonance and time-resolved optical spectroscopy

A low-temperature study of the thermoluminescent dosimeter material, lithium tetraborate (Li2B4O7) doped by Cu, has been carried out by the methods of electron paramagnetic resonance (EPR) and time-resolved polarization spectroscopy using 4–20 eV synchrotron radiation and 1 µs Xe flash lamp pulses in the region 3–6 eV. The observed EPR spectra of an unpaired hole with strong d-character and characteristic hyperfine splittings can be ascribed to Cu2+ substituted at a Li lattice site and displaced due to relaxation. The results on the Cu+-related luminescence strongly support the conclusion about a low-symmetry position of copper impurity ions in the lithium tetraborate lattice. The temperature dependence of the decay kinetics of the Cu+-related 3.35 eV emission indicates a triplet nature for the relaxed excited state of the Cu+ centres. An off-centre position of the Cu+ ion in the relaxed excited state is suggested.