Cross-relaxation interactions in ZnO:Mn2+: The ground state optical pumping

A steady-state population inversion in the ground state of Mn2+ in ZnO was detected by application of continuous microwave and circularly polarized optical pumping in the temperature range of 3–6 K. Multiple spin-flip processes occur in view of a simultaneous saturation in the harmonically related transitions of Mn2+ spins. It is found that an additional relaxation channel arises at 2.7 K due to dynamic polarization of the 55Mn nuclei through the saturation of the first order electron-nuclear forbidden transitions. The transient populations are created between 55Mn nuclear sublevels.