Photon upconversion (UC) is a process in which the sequential absorption of two or more photons leads to the emission of light at shorter wavelength than the excitation wavelength.[1] It is an anti-Stokes type emission. An example is the conversion of infrared light to visible light.[2][3][4][5][6] Materials by which upconversion can take place often contain ions of d-block and f-block elements. Examples of these ions are Ln3+, Ti2+, Ni2+, Mo3+, Re4+, Os4+, and so on.
Three basic mechanisms are energy transfer upconversion, excited-state absorption (ESA) and photon avalanche (PA). Upconversion should be distinguished from two-photon absorption and second-harmonic generation. An early proposal (a solid-state IR quantum counter) was made by N. Bloembergen in 1959[7] and the process was first observed by F. Auzel in 1966.[8][9] In particular, recent big progress in synthesis of high-quality nano-structured crystals has enabled some new pathways for photon upconversion, such as Tb-mediated interfacial energy transfer.[10]