Upconversion

Photoexcitation at a certain wavelength in the NIR followed by luminescence at a shorter wavelength in the VIS is called NIR to VIS photon upconversion.
This is a rather unusual process since low energy photons are "converted" to higher energy photons. At least two NIR photons are required to generate one VIS photon.
Upconversion can only occur in materials in which multiphonon relaxation processes are not predominant, thus allowing more than one metastable excited state. In rare-earth compounds, the 4f or 5f electrons are efficiently shielded and thus not strongly involved in the metal-to-ligand bonding. As a consequence, electron-phonon coupling to f-f transitions is reduced, and multiphonon relaxation processes are less competitive. The phenomenon of upconversion is therefore most common and best studied in materials containing lanthanide ions. But there are also transition-metal systems and rare-earth / transition-metal combinations which show this phenomenon.