7. Conclusion

In this article, we have shown the origin of the optical and spectral properties of trivalent rare-earth ions, as well as the particular aspects that make them so interesting. We have deliberately limited ourselves to new applications involving the coherence of photons emitted by rare earths, or the effects associated with them when placed in a solid environment. In the field of telecommunications, for example, research is focused on controlling clusters of dopant ions, while increasing their concentration so as to move towards shorter, more compact structures, such as planar guiding structures, possibly hybrids combining these structures with semiconductors, and involving glass-ceramics to increase the overall gain.

Although it is often assumed that the properties of rare-earth ions, due to their particular electronic structure, depend little on their environment...