Energy transfer processes and the green fluorescence in heavily doped Er3+: fluoride glasses

The thermalised (4S3/2 + 2H11/2) level plays a significant role in the overall population dynamics of Er3+ in fluoride glasses. A detailed study of the fluorescence waveforms from this level was conducted in Er3+-doped ZBAN glasses over a wide range of doping concentrations (0.2–18 mol%). Direct (520 nm) and up-conversion (800 nm) pumping were used in this study. Rate-equation analysis was used to identify and characterise the energy-transfer processes responsible for the observed fluorescence behaviour. Three-ion processes were found to be very important in the overall population dynamics, even at low doping concentrations. With direct pumping the decay waveforms remained exponential for high Er3+ concentrations (e.g. 18 mol%). Two-ion and three-ion processes, that involve the ground state, were found to decrease the lifetime of the level from about 550 μs (0.2 mol%) to about 3 μs (18 mol%). With 800-nm pumping, the level received its initial population by multiphonon decay from the 2H9/2 level which is populated by excited-state absorption of the pump. The decay of the (4S3/2 + 2H11/2) level was found to be comprised of two components. The initial component was very similar to that observed with direct pumping and was again influenced by the above-mentioned multi-ion processes. The second component was the result of multi-ion processes involving the 4I11/2 level that transfer population from this level to the (4S3/2 + 2H11/2) level.