Vladimir Volkov

Polarization and local disorder effects on the properties of Er 3+ -doped XBi(YO 4 ) 2 , X=Li or Na and Y=W or Mo, crystalline tunable laser hosts

Pure and Er-doped ([Er]crystal≤2.4 × 1020 cm−3), NaBi(WO4)2 (NBW), NaBi(MoO4)2, and LiBi(MoO4)2 crystals have been grown by the Czochralski method. The three crystal hosts have similar structural and optical properties. The noncentrosymmetric space group I4¯ (No. 82) crystallographic structure has been established through 300 K single-crystal x-ray and neutron (for NBW only) diffraction. Er3+ energy levels were determined experimentally and simulated in the S4 symmetry through a crystal-field analysis. With this background, the large spectroscopic bandwidths observed were ascribed to the presence of two 2b and 2d sites for Er3+ and to different short-range Na+ and Bi3+ distributions around both sites. The radiative properties of Er3+ are described by the Judd-Ofelt theory achieving branching ratios and radiative lifetimes for transitions useful as laser channels. The 4I13/2—>4I15/2 laser channel (λ≈1.5 μm) shows a peak emission cross-section σEMI(λ≈1530 nm)≈0.5×10−20 cm2 and a quantum efficiency η∞0.68 to 0.74. The laser emission is envisaged to be tunable by Δλ≈100 nm.

Measurement and crystal-field analysis of Er3+ energy levels in crystals of NaBi(MoO4)2 and NaBi(WO4)2 with local disorder

Abstract The polarised optical absorption and photoluminescence of Er 3+ -doped tetragonal I 4 NaBi(MoO 4 ) 2 (NBMo) and NaBi(WO 4 ) 2 (NBW) single crystals have been measured at 10 K. In these hosts Na + and Bi 3+ cations are statistically distributed over 2b and 2d crystallographic sites both with S 4 symmetry. Er 3+ is expected to replace Bi 3+ , therefore inhomogeneous broadening of its spectral bands may arise simultaneously from the occupancy of the above two sites and from the different short-range Na and Bi distributions around them. Despite the broad bands observed, well-defined polarisation rules have been determined, indicating that no reduction of host site symmetry occurs. The positions of 52 and 66 observed Kramers doublets in NBMo and NBW crystals have been established and labelled with their corresponding irreducible representations. Detailed single-electron Hamiltonians combining together free-ion and crystal-field interactions have been used to reproduce the energy levels and associated wavefunctions of the 4f 11 configuration of Er 3+ in both hosts. Very satisfactory correlations were obtained between experimental and calculated crystal-field levels, with rms deviations σ =9.5 and 13.3 cm −1 for NBMo and NBW, respectively. In an attempt to account for the linewidths observed, the energy separation due to the two non-equivalent 2b and 2d sites where Er 3+ can be found has been calculated taking into account only the first oxygen shell (i.e., neglecting the Na and Bi disorder). The site-associated calculated energy separation is generally lower but close to the experimental 10 K linewidths.

Optical emission properties of Nd3+ in NaBi(WO4)2 single crystal

Room temperature optical absorption measurements of Nd3+ in NaBi(WO4)2 single crystals grown by the Czochralski method have been used to calculate the following Judd-Ofelt intensity parameters Ω2 = 30.9 × 10−20, cm2, Ω4 = 12.0 × 10-20 cm2 and Ω6 = 9.3 × 10−20 cm2. Using these parameters the emission properties of Nd3+ in this host have been estimated with particular attention to the 4F3/2 energy level responsible for stimulated emission channels. The experimental branching ratios of this level are obtained from photoluminescence and agree with those calculated using the above Ωk parameters. The 4F3/2 emission has neligible non-radiative intrinsic losses and the 10 K lifetime at low Nd concentration, 0.9 × 1018 cm−3, is τexp = 143 μs. Even at 300 K and for a much higher Nd concentration, 4.0 × 1019 cm−3, non-radiative losses are moderate, giving a quantum efficiency n ≊ 0.85. The 4F3/2 emissions show a stronly polarized character. The largest emission cross-section occurs at λ = 1060.9nm for the 4F3/2 → 4I11/7 laser channel with a π-polarized character, σEMI = 16 × 10−20 cm2.

Photoluminescence and up-conversion of Er3+ in tetragonal NaBi(XO4)2, X=Mo or W, scheelites

Abstract Er doped NaBi(XO 4 ) 2 , X=Mo or W, single crystals with tetragonal structure have been grown with optical quality by the Czochralski method. Er concentrations up to 1.4×10 20 cm −3 and 1.3×10 20 cm −3 have been attained in Mo and W compounds respectively. 4 S 3/2 → 4 I 15/2 emission of Er 3+ has been achieved at room temperature by up-conversion exciting the 4 I 11/2 or 4 I 9/2 multiplets. This green emission is strongly polarised parallel to the crystallographic c -axis. The efficiency of the up-conversion process in tetragonal NaBi(MoO 4 ) 2 and NaBi(WO 4 ) 2 crystals are similar and also similar to that found in monoclinic KGd(WO 4 ) 2 crystal. The slope of the logarithmic dependence between up-converted and pump intensities suggests a two-photon excitation process. A schematic model of the up-conversion process is outlined.

Growth and 10 K spectroscopy of Nd3+ in NaBi(WO4)2 single crystal

Abstract The experimental procedures to grow Nd doped NaBi(WO 4 ) 2 single crystals by the Czochralski method and the difficulties or limitations encountered for doping are described. Nd concentrations in the crystal up to 0.3×10 20 cm −3 have been produced with optical quality. Polarised optical absorption and photoluminescence of Nd 3+ ion in this host have been studied at 10 K and the relative energies of 90 levels have been determined. An attempt to explain the large observed spectroscopic line-widths has been conducted through crystal field calculations of these levels supposing either a single Nd position with C 2 symmetry or two centres both with S 4 point symmetry. The latter assumption produces energy differences between both optical centres that can justify the enlargement of lines, but it is not consistent with the OA intensity changes observed in polarised spectra. The coexistence of several sites with statistically modified ordering of the short range Na and Bi cation distribution is suggested to account for this disagreement.