D. A. Lis

Effect of cerium codoping in Er3+,Ce3+:NaLa(MoO4)2 crystals

The detailed polarized spectroscopic investigation of I11∕24 and I13∕24 states of Er3+ ions in Czochralski-grown NaLa(MoO4)2 single crystals doped with Er3+ and codoped with Ce3+ as a function of cerium concentration is presented. The main results of room-temperature lifetime investigation show that the I11∕24 lifetime reduces by about 50 times from the solely Er-doped sample to the crystal codoped with 50at.% of Ce, while in the same experimental conditions, the lifetime of the I13∕24 level reduces by less than two times in the most heavily Ce codoped sample. This result looks very promising from the point of view of obtaining low-threshold 1.5-μm laser oscillation.

High-frequency tunable EPR spectroscopy of Tm3+ ions in synthetic forsterite

The electron paramagnetic resonance spectra of isolated and dimer impurity centers of trivalent chromium ions in the octahedral Ml sites in synthetic forsterite are studied in the frequency range of 65–90 GHz. The measurements are performed at 4.2 K in magnetic field from −0.04 to 0.3 T. The zero-field splitting between spin doublets of the isolated Cr3+ ion Δs= 66.7 GHz and between spin sublevels of the Cr3+-Cr3+ dimer Δd1 = 71.5 GHz and Δd2 = 73.0 GHz is measured directly at zero field. The analysis of the spin Hamiltonian parameters shows that the dimer center consists of a pair of Cr3+ ions with an Mg2+ vacancy between them replacing three Mg2+ ions situated in a quasi-one-dimensional chain aligned parallel to the crystal c-axis. It is found that the exchange interaction in the dimer is ferromagnetic with parameters Jz= 0.47 GHz and Jt = 0.79 GHz.

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.

Growth and spectroscopic studies of NaLa(MoO4)2:Tm3+ crystals: A new promising laser material

Single crystals of scheelite-like disordered double molybdate NaLa(MoO4)2 doped with thulium ions with dopant concentrations of 3.0 × 1019 and 1.2 × 1020 cm−3 are grown by the Czochralski method. Spectral and luminescent studies of crystals grown are performed in wavelength ranges of 800 nm (the transitions 3H6↔3H4) and 2 μm (the transitions 3H6↔3F4), including the measurements of the polarized absorption and luminescence spectra of these crystals and the decay kinetics of their excited states. A model that satisfactorily describes the decay kinetics of the luminescence from the 3H4 level taking into account cross-relaxation interactions is proposed. The prospects for the use of these crystals as active media in two-micrometer solid-state lasers are discussed.

Colors of mixed-substituted double molybdate single crystals having scheelite structure

X-ray diffraction, neutron diffraction, optical and absorption spectroscopy, electron probe microanalysis, and temperature-dependent electrical conductivity measurements were used to study structural features and defect generation to be responsible for coloring of single crystals of nominal compositions (Na0.5LaxGd0.5 − x)MoO4:Tm having scheelite structure which were Czochralski grown under a weakly oxidizing atmosphere followed by air annealing. In nonannealed crystals, the color was found to be due to oxygen vacancies and their compensator electrons (black color) and, most likely, electrons located on Mo6+ ions (lilac color); the concentration of these defects decreased upon air annealing, thereby discoloring the samples. The yellow color of crystals was interpreted as arising from possible formation of a color center (an association of a molybdenum vacancy and a hole).

Multifrequency EPR spectroscopy of Ho3+ ions in synthetic forsterite

Electron paramagnetic resonance (EPR) of Ho3+ single ions and Ho3+−Mg2+-vacancy-Ho3+ associates in holmium-doped forsterite single crystals are studied at 9.4, 37.3 and 65–250 GHz. Crystals were grown from melt by the Czochralski technique in slightly oxidizing atmosphere. For both centers, directions of the principal magnetic axes and parameters of the effective spin Hamiltonians describing dependences of electron-nuclear levels on applied magnetic field are obtained. For Ho3+ substituting Mg2+ in the M2 site as the single ion and for Ho3+ ions in dimer centers, values of crystal field parameters related to a real crystal lattice structure are estimated in the framework of the exchange charge model. The calculated crystal field energies, values of theg-factors of the ground Ho3+ quasi-doublet and the directions of the corresponding magnetic moments agree satisfactorily with the data obtained from measurements of EPR and optical absorption and site-selective luminescence spectra.

Unusual structural properties of (Na0.5La0.5)MoO4:Er,Ce crystals

The structure of single crystals grown by the Czochralski technique from (Na0.500La0.495 − xCexEr0.005)MoO4 (x = 0.10, 0.125, 0.15, 0.175, 0.20) melts in a weakly oxidizing atmosphere (99–99.5% N2 + 0.5–1% O2) (NLM-0) was studied by x-ray diffraction. Some of the crystals received short-term annealing at 1000°C (NLM-2) and/or long-term annealing at 700°C (NLM-1). The oxygen in the structure of NLM:Er,Ce-0 and NLM:Er,Ce-1 with x = 0.20 was found to occupy two crystallographic sites at random. In the structure of the NLM:Er,Ce-2 crystal with x = 0.20, the oxygen atoms are fully ordered into one site, as in the scheelite structure. It cannot be ruled out that these crystals contain both Ce3+ and Ce4+ ions.

Down-conversion in ytterbium-doped NaGd(MoO4)2 crystals

A series of NaGd(MoO4)2 single crystals doped with various Yb3+ concentrations (from 0.5 to 30 at %) is grown. The optical absorption spectra of the crystals are measured within the range from 800 to 1200 nm and used to calculate the actual concentrations of Yb in the samples. The luminescence of crystals is studied under UV excitation. It is found that there exist two types of active centers that are not directly related to Yb3+ ions and exhibit broadband luminescence in the visible spectral region. In addition, UV excitation of crystal gives rise to Yb3+ luminescence in the region of 1 μm owing to nonradiative excited-state energy transfer from, probably, donor centers of the host to ytterbium. The character of the dependence of the UV-excited luminescence of Yb on its concentration in the samples, in comparison with the corresponding dependence measured under IR excitation, points to the occurrence of cooperative down-conversion in crystals with high ytterbium concentrations, which means that the excited-state energy of donors is simultaneously transferred to two Yb3+ ions. In other words, excitations are multiplied; i.e., the quantum yield of this down-conversion exceeds unity.

Growth and spectroscopic characterization of Er3+:CaWO4

In this work we present the growth and spectroscopic investigation of Er3+ in CaWO4 single crystals. Er3+ polarized absorption has been measured at room temperature from UV to 1700nm, and the fluorescence from the most important emitting states from the visible to the infrared has been recorded at 10 and 300K in the two possible polarizations. The lifetimes of the lower lying excited states have been measured as a function of the temperature to evaluate the contribution of nonradiative decay processes. The absorption spectra enabled us to determine radiative lifetimes of the emitting states by means of the the Judd-Ofelt approach. The stimulated emission cross section has been estimated for the 1.5μm transition. A comparison of the theoretical and experimental results permitted to obtain an estimate of the actual Er3+ doping level. A calculation of the effective gain cross section for the 1.5μm transition is also presented. These experimental results are discussed in order to evaluate the potentialities o...

Synthesis and spectroscopy of sodium-gadolinium tungstate NaGd(WO4)2 crystals activated by Tm3+ ions

Single crystals of disordered double sodium-gadolinium tungstate NaGd(WO4)2 doped with Tm ions were grown by the Czochralski method. The polarized absorption and emission spectra of these crystals with different activator concentrations and the decay kinetics of the 3H4 and 3F4 excited states of Tm3+ ions at 300 K were investigated. The absorption and emission cross sections of the expected laser transition 3F4 ↔ 3H6 in the Tm3+ ion were determined and the amplification spectra of the active laser medium NaGd(WO4)2:Tm at different levels of inverse population of the upper laser level 3F4 of the Tm3+ ion were constructed. The prospects for using these crystals as active media in 2-μm solid-state lasers are discussed.