Observation of efficient population of the red-emitting state from the green state by non-multiphonon relaxation in the Er3+-Yb3+ system

The rare earth Er3+ and Yb3+ codoped system is the most attractive for showcasing energy transfer upconversion. This system can generate green and red emissions from Er3+ under infrared excitation of the sensitizer Yb3+. It is well known that the red-emitting state can be populated from the upper green-emitting state. The contribution of multiphonon relaxation to this population is generally considered important at low excitation densities. Here, we demonstrate for the first time the importance of a previously proposed but neglected mechanism described as a cross relaxation energy transfer from Er3+ to Yb3+, followed by an energy back transfer within the same Er3+-Yb3+ pair. A luminescence spectroscopy study of cubic Y2O3:Er3+, Yb3+ indicates that this mechanism can be more efficient than multiphonon relaxation, and it can even make a major contribution to the red upconversion. The study also revealed that the energy transfers involved in this mechanism take place only in the nearest Er3+-Yb3+ pairs, and thus, it is fast and efficient at low excitation densities. Our results enable a better understanding of upconversion processes and properties in the Er3+-Yb3+ system.