Observation of efficient population of the red-emitting state from the green state by non-multiphonon relaxation in the Er31–Yb31 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.