Effect of Eu2O3 doping on Ta2O5 crystal growth by the laser-heated pedestal technique

High energy band gap hosts doped with lanthanide ions are suitable for optical devices applications. To study the potential of Ta2O5 as a host compound, pure and Eu2O3-doped Ta2O5 crystal fibers were grown by the laser-heated pedestal growth technique in diameters ranging from 250 to 2600 μm and in lengths of up to 50 mm. The axial temperature gradient at the solid/liquid interface of pure Ta2O5 fibers revealed a critical diameter of 2200 μm above which the fiber cracks. X-ray diffraction measurements of the pure Ta2O5 single crystals showed a monoclinic symmetry and a growth direction of [1 1̄ 0]. An analysis of the pulling rate as a function of the fiber diameter for Eu2O3-doped Ta2O5 fibers indicated a well defined region in which constitutional supercooling is absent. Photoluminescence measurements of pure Ta2O5 crystals using excitation above the band gap (3.8 eV) were dominated by a broad unstructured green band that peaked at 500 nm. Three Eu3+-related optical centers were identified in the doped samples with nominal concentrations exceeding 1 mol%. Two of these centers were consistent with the ion in the monoclinic phase with different oxygen coordinations. The third one was visible in the presence of the triclinic phase.