The electronic and nuclear contributions to nonlinear optical properties of niobium oxide-based glasses have been measured using third-harmonic generation and pump-probe techniques. The results have been found to be in agreement with Hellwarth's model for niobium oxide (Nb_2O_5) concentrations below 30 mol. Above 30 mol Nb_2O_5, discrepancies with the theoretical model have been observed. It has been found that high niobium ion content induces a change of the local structure within the glass and favors the formation of three-dimensional blocks of corner-sharing NbO_6 octahedra. Since similarities with crystalline structures are present, we propose that some specific collective vibrational modes could be at the origin of the extra nuclear contributions to the optical Kerr effect.
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