Low-phonon energy glasses are desirable hosts for rare-earth (RE) ions because they enable emission from RE energy levels that would otherwise be quenched in high-phonon energy glasses. Such emissions are of interest for fiber amplifiers operating at telecommunications wavelength band s of 1.31, 1.46, and 1.55 μm, and for up-conversion lasers and three-dimensional displays.

Phonons are optical-frequency molecular vibrations in a material. If the RE energy level of interest lies only a few phonons in energy above the next lower lying level such as the 1G4 level of Pr3+, which is only 3,000 cm −1 above the 3F4, only three Si—O vibrational phonons (1,100 cm−1) are required to bridge the gap as shown in Figure 1. Thus any electrons excited to the 1G4 level via an external pump source will be deexcited to the 3F4 on down to the 3H4 ground state via phonons, and no radiation of usable light will be produced. This is why emission from the 1G4 level of Pr3+ is absent in silicates and why researchers have gone to great lengths to make low-phonon energy glasses.