Figure 4.

Principles of spectral conversion through photoluminescence. a) In up‐conversion, two low‐energy photons are converted into one high‐energy photon. Down‐conversion and down‐shifting rely on conversion of a high‐energy photon into one or more low‐energy photons. b) A simplistic scheme of the associated electronic transitions in the active center. UC, as a two‐photon process, involves coherence of ground state (GSA) and excited state absorption to generate high‐energy photons by direct relaxation or through energy transfer (ETU). In DC, emitted photons may derive from photon cascade emission (PCE) or from energy transfer reactions (ETD). For clarity, non‐radiative relaxation (NR) is depicted only for the DS reaction.