Kinetics of excitation transfer from Cr²⁺ to Fe²⁺ ions in co-doped ZnSe

The transfer of electronic excitations from Cr²⁺ to Fe²⁺ ions in co-doped epitaxially grown ZnSe is studied by time-resolved photoluminescence (PL) spectroscopy with unprecedented sub-10 ns time resolution. Upon excitation of Cr²⁺ ions by a picosecond pulse at 2.05 µm wavelength, PL from Fe²⁺ ions displays a delayed onset and a retarded decay in comparison to Fe²⁺ PL directly excited at 3.24 µm. We measure an extremely rapid 60 ns buildup of the Fe²⁺ luminescence, which is followed by a slower relaxation on the few micrometer scale. The experimental results are analyzed in the framework of Förster radiationless resonant energy transfer. Directly connecting to the work of Fedorov et al. [Opt. Mater. Express9, 2340 (2019)10.1364/OME.9.002340], the 60-ns buildup time of energy transfer is found to correspond to a Cr²⁺-Fe²⁺ distance of 0.95 nm, close to the length of the space diagonal of the ZnSe unit cell. This result demonstrates a significant density of spatially correlated Cr²⁺-Fe²⁺ ion pairs at short distance, in parallel to ions with a random distribution at a larger mutual separation.