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 Er 3+ and Yb 3+ codoped system is the most attractive for showcasing energy transfer upconversion. This system can generate green and red emissions from Er 3+ under infrared excitation of the sensitizer Yb 3+ . 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 Er 3+ to Yb 3+ , followed by an energy back transfer within the same Er 3+ –Yb 3+ pair. A luminescence spectroscopy study of cubic Y 2 O 3 :Er 3+ , Yb 3+ 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 Er 3+ –Yb 3+ 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 Er 3+ –Yb 3+ system. Rare earths: energy back transfer Cross-relaxation energy back transfer between Yb 3+ and Er 3+ ions has been shown to be significant in red upconversion luminescence. The mixed rare-earth dopant system of erbium (Er 3+ ) and ytterbium (Yb 3+ ) ions is commonly used to achieve infrared-to-visible upconversion luminescence. Under infrared excitation, the Yb 3+ ions absorb the incident light and consequent energy transfer to the Er 3+ 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 Y 2 O 3 doped with Er 3+ and Yb 3+ and discovered that the new energy pathway only takes place between nearest Er 3+ –Yb 3+ 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.