Why rare earth ions are generally doped as active ions in laser materials?
In most cases, the rare-earth ions replace other ions of similar size and same valence (charge state) in the host medium; for example, a Nd3+ ion in Nd:YAG (yttrium aluminum garnet) substitutes an yttrium (Y3+) ion. The concentration of laser-active rare-earth dopants in the host medium is in most cases only a small molar percentage. A characteristic property of the trivalent rare-earth ions is that their electronic transitions usually occur within the 4f shell, which is somewhat shielded from the host lattice by the optically passive outer electronic shells. This reduces the influence of the host lattice on the wavelengths, bandwidths and cross sections of the relevant optical transitions.
