Excitation spectra of light-induced electric responses in doped ferroelectric crystals

On the basis of studies of light-induced responses of doped lithium niobate crystals, a new approach to the spectroscopy of polar materials is developed. An experimental technique is described and possible applications and advantages of the approach are discussed.     

Electro-optic and dielectric properties of Hafnium-doped congruent lithium niobate crystals

We report the optical and dielectric properties in hafnium (Hf)-doped lithium niobate (LN) crystals. We investigated samples of congruent composition with various doping concentration varying from 0 to 8 mol%. The clamped and unclamped values of the electro-optic coefficient r ₂₂ of Hf-doped LN and the corresponding dielectric permittivity as well, have been experimentally determined and compared with the results obtained in undoped congruent LN crystals. We show that the electro-optic and dielectric properties are only slightly affected by the introduction of hafnium ions, and therefore Hf-doped LN has the advantage of low photorefractive damage compared with the undoped congruent LN.     

Temperature dependence of Raman scattering and anharmonic properties in LiNbO_3

The temperature dependence of the Raman spectrum in LiNbO \(_3\) is investigated from 100 to 700 K. The various sources of asymmetry of Raman bands and artefacts are discussed before analyzing the temperature dependence of A \(_1\) and E first-order phonon lines. The phonon frequency downshift and damping increase on heating are interpreted in terms of normal volume expansion and third- and fourth-order anharmonic potentials. Anharmonic contributions are highly anisotropic and mainly explain the temperature dependences of both frequency and damping of A \(_1\) optical vibrational modes along the ferroelectric axis. Results are consistent with Caciuc et al. (Phys Rev B 61:8806, 2000) predictions.     

Spectral line intensities of Yb3+ ion in LiNbO3 crystals

Optical spectra of absorption and luminescence of LiNbO₃:Yb³⁺ crystals are investigated experimentally and theoretically. Within the framework of the point charge approximation of crystal field, the Stark problem is solved: crystal field parameters and wave functions of Stark states are determined. Line strengths due to indirect electric-dipole and magnetic-dipole inter-Stark transitions are calculated and values of the Judd-Ofelt parameters are determined. Values of the branching ratios of luminescence. Einstein coefficients and, lifetime of the excited sublevel are calculated. A satisfactory agreement of calculated results with experimental data is obtained.     

Crystal LiNbO<Subscript>3</Subscript>-Ho<Superscript>3+</Superscript>: Material for optical cooling

The possibilities of LiNbO₃-Ho³⁺ crystals for optical cooling based on the anti-Stokes luminescence in the wavelength range 2000-2200 nm are investigated. The efficiency and cooling temperature under the continuous wave (CW) excitation at the wavelengths 2035-2071 nm by ~100 W power are estimated. It is shown that under the CW excitation at 2035 nm wavelength the maximum cooling temperature is equal to 2.5 K, and at 2071 nm wavelength is equal to 10.9 K.     

LiNbO<Subscript>3</Subscript>–Yb<Superscript>3+</Superscript> crystal as a material for optical temperature sensor

The method of optical determination of temperature based on cooperative luminescence in the LiNbO₃ crystals doped with rare earth ions is proposed. As a temperature-dependent measurable characteristic, the quantum yield of luminescence is considered taking into account the processes of nonradiative redistribution of the electron excitation energy in the impurity subsystem. It is shown that the LiNbO₃–Yb³⁺ crystals can be used as a material for optical temperature sensors in the temperature range of 200–500 K with the average sensitivity of 0.12%K^–1.     

Low-temperature synthesis and characteristics of fractal graphene layers

Large scale fractal graphene layers are obtained by complex method of liquid phase exfoliation and self-organization. Atomic force microscopy (AFM) is used to study the surface properties of formed layers and to assess their thickness. Surface potential of graphene and potential transition between the graphene and substrate is measured by Kelvin probe method. The influence of the effect of dielectric confinement on the optical properties of graphene is discussed in this work. Raman scattering spectra were used for structural analysis and assessment of the level of defects. Current-voltage characteristics of graphene ribbons were measured and discussed for different number of layers.     

Characterization of stoichiometric LiNbO3 grown from melts containing K2O

The growth of LiNbO₃ single crystals from a melt with the Li/Nb ratio of 0.946, to which 6 wt.% K₂O has been added, leads to stoichiometric specimens, essentially free of potassium, with (50±0.15) mol% Li₂O in the crystal. This is established by studying the composition dependence of the following properties: linewidths of the electron paramagnetic resonance (EPR) of Fe³⁺, energy of the fundamental absorption edge, Raman linewidths of phonon modes, and dispersion of the optical birefringence. Comparison of the results with relevant calibration scales leads to the above composition. In all cases the Li₂O content was found to be closer to 50% than that of a LiNbO₃ crystal vapor-phase equilibrated to 49.9mol% Li₂O. The photorefractive effect at light intensities I10⁷ W/m² is suppressed in this stoichiometric material. The features of the ternary system K₂O-Li₂O-Nb₂O₅, which are possibly responsible for the unexpected growth of stoichiometric LiNbO₃ from the indicated melts, are discussed.     

Estimation of Thermal Expansion Coefficients of Rare-Earth Garnets in a Wide Temperature Range

Simple expressions were obtained for the thermal expansion coefficients of the rare-earth garnets, which allow to estimate their values in a wide temperature range in cases, when either the lattice unit cell parameters or the density of the material are known at 300 K. The expression giving the relationship between the thermal expansion and thermal conductivity coefficients for the rare-earth garnets has also been obtained. It is shown that the calculated values obtained with the use of these expressions are in a good agreement with the available experimental data for the garnets of different composition.     

Мicro-Optical Structures Written by Photothermal Method in a Specially Modified Near-Surface Layer of Lithium Niobate Crystals

Russian Physics Journal -