A. A. Sobol

Spontaneous Raman spectroscopy of tungstate and molybdate crystals for Raman lasers

Abstract Spontaneous Raman spectra of tungstate (MeWO4) and molybdate (MeMoO4) crystals with sheelite structure were investigated (Me=Ca, Sr, Ba, Pb). The energy shift and line broadening of internal vibrational modes were measured in the temperature range from 77 K to the melting point by means of a high temperature Raman spectroscopic technique. The phase transition from sheelite to cubic structure was discovered at a temperature of 1600 K in a BaMoO4 crystal. Vibrational level diagrams for “free” [WO4]2− and [MoO4]2− tetrahedrons were built on the basis of spontaneous Raman spectra of tungstate and molybdate crystals in the molten state. The effect of mass and electronegativity of Me2+ cations on the position and linewidth of internal vibrational modes was investigated. It was shown that the variation of vibration level diagrams in sheelite series could change the probability of the dephasing and splitting relaxation mechanisms. As a result, the consequent line narrowing of totally symmetric Raman vibrations of [WO4] and [MoO4] tetrahedron complexes was registered in the Raman spectra in a series of sheelite crystals moving from calcium to strontium and barium. Due to the narrowest linewidth (Δν R =1.6 cm −1 ) and the highest peak cross-section of the Raman line in barium tungstate and molybdate crystals, they were proposed as the most efficient sheelite crystals for Raman laser development.

Calcium niobium gallium and calcium lithium niobium gallium garnets doped with rare earth ions––effective laser media

Abstract The results of spectroscopy and laser action of disordered calcium niobium gallium (CNGG) and calcium lithium niobium gallium (CLNGG) garnets doped with Nd3+, Eu3+, Er3+, Ho3+ and Tm3+ were displayed. The structure and some optical and physical properties of CNGG and CLNGG-crystals were studied. Activator centers construction and the mechanism of inhomogeneous broadening and splitting of spectral lines of rare earth ions (TR3+) in CNGG and CLNGG-crystals were analyzed. Possibilities of laser media based on CNGG and CLNGG with TR3+-ions for pulse free running, Q-switch, CW and ultrashort pulse operation regimes were considered.

Picosecond stimulated Raman scattering in crystals

The comparative values of the peak and integral cross sections of spontaneous Raman scattering and the optical dephasing time of molecular vibrations were determined for several oxide crystals by spontaneous Raman spectroscopy. The spectral, time, and energy parameters of stimulated Raman scattering (SRS) were measured for ten crystals using picosecond YLF: Nd laser pumping with a radiation wavelength of 1047 nm. An analysis of the experimental dependence of the threshold energy of pumping SRS on the integral and peak cross sections of spontaneous Raman scattering showed that the SRS gain increment explicitly depended on the integral cross section and was independent of the peak cross section of spontaneous Raman scattering as the ratio between the pumping pulse width (11 ps) and the time of optical dephasing of molecular vibrations changed from 0.42 to 9.3. The gain coefficients of steady-state stimulated Raman scattering under threshold stimulated Raman scattering conditions were determined for all the crystals studied on the basis of the measured threshold SRS pumping energies, the duration and width of the spectrum of pulses, the nonlinear interaction length, the intensity of pumping, and the theoretical dependences that relate the steady-state and transient SRS gain increments. The steady-state SRS gain coefficients obtained in this work fitted well a linear dependence on the peak cross sections of spontaneous Raman scattering, which substantiated the correctness of our analysis and measurements.

Raman spectra and phase transformations of the MLn(WO4)2 (M = Na, K; Ln = La, Gd, Y, Yb) tungstates

The 300-K crystal structure, phase transformations, and melting of the MLn(WO4)2 (M = Na, K; Ln = La, Gd, Y, Yb) tungstates and the structure of their melts were studied by Raman spectroscopy.

Study of the boron-oxygen units in crystalline and molten barium metaborate by high temperature Raman spectroscopy

Abstract The vibrational spectra of the boron-oxygen structural elements in single crystal β-BaB2O4 and BaB2O4 melts were studied by high temperature Raman spectral analysis. The experimental Raman peaks were assigned to the calculated normal vibrational modes of the free [B3O6] molecular unit. The observed vibrational mode splittings were found to be consistent with C63v symmetry for β-BaB2O4. Experimental vibrational spectra were compared with those of LiBO2 and CsBO2 in order to determine the temperature-dependent populations of chain, ring, and tetrahedral structural elements in the respective melts, and these in turn were shown to correlate with the degree of melt supercooling required for spontaneous crystallization to occur.

Raman spectroscopic study of structural disordering in YVO4, GdVO4, and CaWO4 crystals

The Raman spectra of single-crystal YVO4, GdVO4, and ZrSiO4 with a zircon structure, as well as CaWO4 and BaWO4 with a scheelite structure, are studied in detail over a wide temperature range 14–800 K. An inhomogeneous splitting of the A1g(ν2) vibrational lines in the Raman spectra of YVO4 and GdVO4 and the Ag(ν1) vibrational lines in the spectrum of CaWO4 is detected. It is shown that the profiles of these lines can be decomposed into two components, whose integrated intensities are redistributed with temperature and also depend on the matrix kind in which they are detected. The phenomenon observed is associated with the thermally activated processes of disorientation of the tetrahedral anions in the zircon and scheelite structures.

Intensity of the f-f transitions of Nd3+, Er3+, and Tm3+ rare-earth ions in calcium niobium gallium garnet crystals

This paper reports on the results of the investigation into the intensities of the f-f transitions of Nd3+, Er3+, and Tm3+ ions in calcium niobium gallium garnet (CNGG) crystals. The values of the oscillator strengths and line strengths obtained for hypersensitive transitions and the intensity parameters Ωt of the rare-earth ions in the CNGG crystals are compared with the corresponding quantities for crystals of other garnets and some oxide and fluoride crystals. The assumption is made that an increase in the oscillator strengths and line strengths for the hypersensitive transitions and the intensity parameters Ω2 of the Nd, Er, and Tm ions in the CNGG crystals as compared to those for crystals of other garnets is associated with the specific features revealed in the crystal structure of the calcium niobium gallium garnet, in particular, with the lowering of the symmetry of the positions occupied by rare-earth ions in the crystal structure.

Spectroscopic investigations of NaGd(WO4)2 and NaLa(MoO4)2 single crystals doped by Yb3+ ions

New potential ytterbium-doped laser crystals-disordered scheelite-like double sodium-gadolinium tungstate NaGd(WO4)2 and sodium-lanthanum molybdate NaLa(MoO4)2, had been grown by Czochralski technique. Polarized absorption and fluorescence spectra as well as fluorescence decay kinetics had been measured for these crystals, doped with Yb3+ ions. Gain cross-section for different population inversion of 2F5/2 level had been calculated.

Monoclinic-tetragonal phase transition in zirconium and hafnium dioxides: A high-temperature raman scattering investigation

Zirconium dioxide ZrO2 and hafnium dioxide HfO2 are investigated using high-temperature Raman spectroscopy in the temperature range 300–2080 K, including the regions of the monoclinic-tetragonal phase transitions revealed in these materials. An analysis is made of the specific features observed in the evolution of the high-temperature Raman spectra of both the monoclinic (m) and tetragonal (t) modifications of ZrO2 and HfO2 with variations in the temperature. The polarized Raman spectra of the metastable tetragonal phases in solid solutions based on zirconia and hafnia are used to identify the symmetry of vibrations in the spectra of the tetragonal modifications of pure zirconium and hafnium dioxides, which exist at high temperatures.

Raman spectra and structure of silicon-oxygen groups in crystalline, liquid, and glassy Mg2SiO4

We have measured the Raman spectra of magnesium orthosilicate at temperatures from 300 to 2170 K and have shown that liquid magnesium orthosilicate can be supercooled to 1600 K and can be prepared in the form of glass at quenching rates above 400 K/s. Analysis of Raman results indicates that, in contrast to crystalline Mg2SiO4, liquid and glassy magnesium orthosilicate contains not only isolated [SiO4]4− groups but also polymerized anions made up of corner-shared [SiO4] tetrahedra. This accounts for the observed deep (down to 600 K) supercooling and vitrification of liquid magnesium orthosilicate. During cooling, large [SiO4] groups have a tendency to depolymerize, and the concentration of isolated [SiO4]4− tetrahedra in the super-cooled magnesium orthosilicate melt increases, leading to spontaneous crystallization below 1550 K.