Optical upconversion is a process in which a material absorbs two or more photons of lower energy and emits a single photon of higher energy. For example, two infrared photons may be absorbed followed by the emission of a single visible photon. Upconversion is fundamentally different, and far more challenging, than the more common process of down-conversion where a single high energy photon is absorbed followed by the emission of lower energy photons.
Upconversion is desirable in many applications because it can make accessible the optical energy or optical information contained in low energy photons that is otherwise not available to the human eye or to certain classes of photoelectric devices.
Known optical upconversion materials include rare earth doped materials. Such materials, however, are sensitive only at specific wavelengths having a very narrow bandwidth. Another challenge of making such materials practically useful is their low absorption probability. Given the limitations in the state of the art, it would be a valuable advance in the field to provide improved optical upconversion materials.