*2.6.2 Cooperative emission*

Cooperative emission is a manifestation of electronic interactions, which happens when two nearby centers are electronically excited within nanoscopic distances, resulting in a higher-energy photon being released as a result of a simulated state of the pair centers. This mechanism is seen in rare-earth ions, resulting in an up-converted emission of a higher energy photon than the excitation energy of actual ions [6]. The interaction reappears as two neighboring ions are separated by just a few nanometers and based on the form of electronic excitation of the individual ions, the interaction between these two ions can be multipole–multipole or electron exchange. It's worth noting that the emission comes from a virtual level rather than the ion pair's physical level.
