Excitation Transfer from Cr2+ to Fe2+ Ions in Co-doped ZnSe as a Pumping Scheme for Infrared Solid-State Lasers

We present the results of the photoluminescence behavior reflecting Cr2+ → Fe2+ excitation transfer in co-doped ZnSe:Cr2+Fe2+. This transfer can be seen as a possible promising pump mechanism to create short pulse lasers for the 3- to 6-µm wavelengths that can be excited using inexpensive 2-µm pump light sources. In addition to the kinetics, emphasis was put on comparing the intensities of both emissions, those of Cr2+ and Fe2+. With resonant excitation of Cr2+, the kinetics of the Fe2+ emission shows a very clear picture of the transfer with 60-ns rise time and the peak delayed by 200 ns. However, the evaluation of the PL intensities brought a surprise. With efficiencies above 80%, the observed Cr2+ → Fe2+ transfer is much more efficient than theoretically expected, and even the overall efficiency including the losses at the Fe2+ ions of almost 4% is still an order of magnitude higher than the theoretical values given for the Förster transfer alone. This leads to the suspicion that either the dopant atoms might not be uniformly distributed in the samples or that the transfer mechanism might be more effective than previously thought.