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

The structural origin of the nonlinear optical susceptibility (/Ξ(3)/) of lead-niobium-germanate film glasses with large Nb 2O5 contents has been investigated. /Ξ(3)/ shows a strong enhancement with the Nb content in the films with /Ξ(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 /Ξ(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 Nb 4+ 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 /Ξ(3)/in the film glasses. © 2011 American Institute of Physics.The authors acknowledge the financial support of Spanish Ministry of Science and Innovation (Grants Nos. TEC2008- 01183 and MAT2009-14282-C02-01).Peer Reviewe