Boris S. Redkin

Properties of Nd 3+ -doped and undoped tetragonal PbWO 4 ,NaY(WO 4 ) 2 , CaWO 4 , and undoped monoclinicZnWO 4 and CdWO 4 as laser-active and stimulated Ramanscattering-active crystals

Spectroscopic, laser, and chi((3)) nonlinear optical properties of tetragonal PbWO(4), NaY(WO(4))(2), CaWO(4), and monoclinic CdWO(4) and ZnWO(4) were investigated. Particular attention was paid to Nd(3+)-doped and undoped PbWO(4) and NaY(WO(4))(2) crystals. Their absorption and luminescence intensity characteristics, including the peak cross sections of induced transitions, were determined. Pulsed and continuous-wave lasing in the two 4F(3/2)-->4I(11/2) and 4F(3/2)-->4I(13/2) channels was excited. For these five tungstates, highly efficient (greater than 50%) multiple Stokes generation and anti-Stokes picosecond generation were achieved. All the observed scattered laser components were identified. These results were analyzed and compared with spectroscopic data from spontaneous Raman scattering. A new crystalline Raman laser based on PbWO(4) was developed for the chi((3)) conversion frequency of 1-microm pump radiation to the first Stokes emission with efficiency up to 40%. We classify all the tungstates as promising media for lasers and neodymium-doped crystals for self-stimulated Raman scattering lasers.

Modifications of Light Emission Spectra and Atomic Structure of Europium Molibdate Bulk Crystals Caused by High Pressure and Heat Treatment

Essential enhancement of light emission ability of europium molybdate - Eu2(MoO4)3 (EMO) crystals has been achieved by combined application of high pressure (9 GPa) and heat treatment (a procedure was repeated from 3 to 25 times for different samples). Multifold increase in light emission of EMO single crystals can be achieved, without deterioration of the optical transparency of the material. Simultaneously the light emission spectra change substantially. Nature of this phenomenon is explained by the modifications of the crystal field caused by structural transformations induced by the treatments. These modifications of the crystal fields bring to increase in probability of light emission optical transitions between 4f sublevels of europium ions.

Change of the stoichiometry and electrocoloration due to the injection of Li and O ions into lithium niobate crystals

Congruently grown LiNbO3 single crystals show both high oxygen and lithium ion conductivity at temperatures above 500 °C. The high oxygen ion conductivity can be understood in terms of a certain amount of oxygen vacancies already present in congruently grown LiNbO3 single crystals. Thermal treatment of LiNbO3 produces additional oxygen vacancies. The absorption bands introduced by this procedure are investigated. It is found that the electrons which are generated during the reduction process are homogeneously distributed among all oxygen vacancies in the LiNbO3 single crystals. The electrocoloration phenomenon in LiNbO3 single crystals is due to the process of injection of lithium ions and electrons into LiNbO3 by a double charge mechanism. Investigations of the optical and electrical properties of electrocolored LiNbO3 crystals are reported. It is shown that the absorption spectra of thermally and electrochemically reduced samples are identical and that the origin of the absorption processes has to be therefore the same in both cases. That means, additional electrons produced by the double charge injection of lithium ions and electrons are also homogeneously distributed among the oxygen vacancies. This supports our hypothesis that a certain amount of oxygen vacancies has to be present already in as-grown LiNbO3 single crystals.

Structural aspects of solid state amorphization in Eu2(MoO4)3 europium molybdate

The samples of Eu2 (MoO4)3 has been investigated by Xray diffraction method. These samples have been subjected high pressure 9 GPa at room temperature. It was established that powder samples undergo at this pressure solid state amorphization. At consequent annealing with temperature increasing unusual sequence of phase states have been observed. Instead of expected “amorphous”→ α→ β sequence the “amorphous”→ β→ α→ β sequence come to pass. The single crystalline samples in contrast to polycrystalline ones do not undergo solid state amorphization. In this case phase transition from tetragonal β-phase into a new tetragonal high-pressure phase take place. We have shown that in the case of incomplete phase transition “βphasehigh-pressure phase” two phase state has been characterized of existence of wide interphase boundaries. The parameters along the boundaries change continuously from one into other phase. In the case of complete phase transition into high-pressure tetragonal phase at subsequent annealing instead of narrow diffraction reflection halo-like reflections were observed. On the base of these results we have supposed that no amorphization in polycrystalline samples take place. The halo-like reflections in this case characterize the transitional state between high-pressure phase and β-phase, namely the diffraction from the set of transitional boundaries.