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Influence of impurity ions on nonlinear coloration of alkali silicate glasses

A study was made of the role of iron impurity ions in the formation and decay of color centers in alkali-silicate glasses under conditions of multiphoton absorption of laser radiation. An investigation was carried out on sodium-silicate glasses which are typical representatives of crown glasses. In the experiments the glasses of the Na2O:CaO:SiO2 equals 22:3:75% compositions synthesized from materials of the `specially pure' grade were used. The impurity was introduced in the form of the compound Fe2O3 in amounts of 0.1% by weight into some of the samples. Oxidative conditions during synthesis ensured that the main part of the iron ions in the glasses was in the form of Fe3+. Since the band gap of alkali- silicate glasses is approximately 6 eV, the third and fourth harmonics of a neodymium laser were used for nonlinear coloration of these glasses. It was shown that an increase in the concentration of Fe2O3 enhanced the residual absorption by stable hole color centers and reduced considerably the lifetime of unstable electron color centers. Repeated irradiation of the glasses containing a small amount of the impurity increased the residual absorption, whereas the induced absorption (measured immediately after the action of a radiation pulse) did not change from one pulse to the next and was governed solely by the radiation irradiance. An increase in the Fe2O3 concentration destroyed this effect.