The dependence of laser-induced refractive index changes in glass on the chemical composition

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Abstract
The properties of permanent, laser induced refractive index gratings in rare-earth doped glasses have been characterized in a variety of materials including lithium borates, lead and magnesium silicates, lead borate-germanates and lead germanates. This work is combined with the previous results obtained on phosphate and silicate glasses to develop an understanding of the effects of glass composition (both network former and modifier ions) on the ability of a material to produce gratings with high scattering efficiencies. The grating efficiencies and the writing/erasing dynamics were studied, and the two-level-system-model developed previously was employed to explain the results. The results show the importance of having flexible glass networks with low polarizing power of the network former ions, and a high content of nonbridging oxygen ions. In addition, it is important to have light, highly polarizable modifier ions with weak chemical bond strengths to the oxygen ions. This type of glass combined with rare-earth ions that like to exist in more than one local coordination environment and that couple strongly to high energy vibrational modes of the host, are found to be very effective in producing local structural changes that lead to room-temperature-stable refractive index gratings.