Structural origin of the nonlinear optical properties of lead niobium germanate film glasses

The structural origin of the nonlinear optical susceptibility (/χ⁽³⁾/) of lead-niobium-germanate film glasses with large Nb2O5 contents has been investigated. /χ⁽³⁾/ shows a strong enhancement with the Nb content in the films with /χ⁽³⁾/ values close to 2 × 10^-11 esu at 800 nm for a Nb content as high as 0.71. Boling-Glass-Owyoung and Lines’ semiempirical models predict accurately the values of /χ⁽³⁾/ for transparent bulk glasses but not for film glasses. This discrepancy is related to the remarkable structural differences between them. Raman spectroscopy suggests the formation of a three-dimensional (3D) structure of [NbO6] octahedra in the case of film glasses having large Nb contents, while X-ray photoelectron spectroscopy shows that a significant fraction of these units contain Nb⁴⁺ ions. The combination of a 3D structure of [NbO6] with the presence of Nb⁴⁺ polarons and their migration through electron intervalence transfer is proposed as the origin of the observed enhancement of /χ⁽³⁾/ in the film glasses.