2.6.2 Rare-Earth Ions

 Rare-earth ions, also referred to as the Lanthanides, play an important role in much

of modern optical technology as the active constituents of materials. There are an

amazing number of applications for these rare-earth-activated optical materials and much

of today's cutting-edge optical technology and future innovations rely on their unique

properties.

 The lanthanide series have an electron configuration of: Is2 2s2 2p6 3s2 3p6 4s2 3d10

4p6 5s2 4d10 5p6 6s2 4f". First, it can easily be shown that f shells of rare-earth ions are

only partially filled. These partially filled shells of f electrons give rise to spectrally

narrow localized electronic transitions that occur at wavelengths ranging from the far-

infrared to the vacuum-ultraviolet and provide the basis for optical technology in which

light may dynamically interact with a material that contains these ions. The qualitative

energy level schemes and emission lines for various rare earth elements, such as Nd, Er

and Tm (Tm and Er are used in this research) are shown in Fig 2.10 [50].

 Secondly, it can also be shown from the electron configuration that the 4f levels of

rare-earth ions are shielded by the 5s and 5p electrons so that the 4f electrons remain

highly localized to the ion and their optical transitions maintain much of an atomic-like

character. This characteristic of the rare-earth ions arises from the unique situation in

which the lowest-energy electrons are not spatially the outermost electrons of the ion, and

thus have a limited direct interaction with the ion's environment. Fig 2.11 shows the

radial positions of the outer orbital of the Ce3+ ion. The "shielding" of the 4f electrons

from the environment by the outer filled shells of 5s and 5p electrons prevents the 4f

electrons from directly participating in bonding and allows them to maintain much of the