Trivalent lanthanides, also called rare-earth ions (REIs), show light emissions in almost all the near infrared and visible spectrum. Such luminescent properties are often related to potential applications in a variety of fields since sharp emission lines, meta-stable states, the ability of converting long/short-wavelengths (up/down-conversion process), energy transfer processes (resonant and non-resonant) and high quantum efficiency make them appropriate for the manufacture of many optical devices.