History of upconversion discovery and its evolution

This History” of Upconversion (UC), starts from a critical discussion about the idea of Bloembergen for an Infra Red Quantum Counter (IRQC) with my thesis Director: “it could not be a thesis subject because during the ions lifetimes not enough energy could be stored”. So it has not been my thesis subject which has been “glass lasers at 1,5microns”, but the object of my thoughts at home all the time. It directed me towards energy transfers study and the discovery of accepting ions already in their excited states which are the necessary steps for UC. © 2020 Elsevier B.V.