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

Abstract

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.