A new laser line due to energy transfer from colour centres to erbium ions in CaF2

Two new laser lines at 1.5308 μm and 1.5298 μm have been observed in single crystals of CaF 2 grown from a melt containing Er 2 O 3 . The stimulated emission from such crystals is different from those doped with ErF 3 , presumably reflecting a difference in the crystalline environment at the site of the impurity ions. The emission only occurs in crystals which have been γ-irradiated; this treatment produces strong absorption bands centred on 3620 Å and 5520 Å, which arise from a colour centre associated with the presence of oxygen in the lattice. Absorption in these bands produces excitation which is transferred to the erbium impurity ions, resulting in an enhancement of the fluorescence power. The lines have lower thresholds than previously reported emissions from erbium, but only occur at temperatures in the liquid helium region. The output power shows a saturation effect which is attributed to a rise in the temperature of the lattice during the pumping pulse.