Energy Transfer Upconversion or ETU is a physical principle (most commonly encountered in solid-state laser physics) that involves the excitation of a laser-active ion to a level above that which would be achieved by simple absorption of a pump photon, the required additional energy being transferred from another laser-active ion undergoing nonradiative deexcitation.
[1][2] ETU involves two fundamental ideas: energy transfer and upconversion.
Nonradiative emission may be via Auger decay or via energy transfer to another laser-active ion.
Nonradiative decay is itself an inefficiency (in a perfect laser every downward transition would be a stimulated emission event), whilst the excitation of the energy-receiving ion can result in heating of the gain medium.
When ETU occurs due to a clustering of ions within the host medium, it is sometimes termed concentration quenching.