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. Cross-relaxation energy back transfer between Yb3+ and Er3+ ions has been shown to be significant in red upconversion luminescence. The mixed rare-earth dopant system of erbium (Er3+) and ytterbium (Yb3+) ions is commonly used to achieve infrared-to-visible upconversion luminescence. Under infrared excitation, the Yb3+ ions absorb the incident light and consequent energy transfer to the Er3+ ions results in red and green emission. Now, Jiahua Zhang and co-workers from Changchun Institute of Optics, Fine Mechanics and Physics in China have conducted luminescence spectroscopy of Y2O3 doped with Er3+ and Yb3+ and discovered that the new energy pathway only takes place between nearest Er3+–Yb3+ pairs. In addition, they also found that this process is faster and more efficient than multiphoton excitation and makes a major contribution to red upconversion.