Local order and non-linear optical properties in bulk nanostructured niobiosilicate glasses

This communication presents and discusses an experimental proof of the correlation among local structure and second harmonic generation (SHG) in bulk nanostructured potassium niobiosilicate (KNS) glasses. In particular, SHG shows a maximum in correspondence of the early stages of nanostructuring, that are characterized by the segregation within the amorphous matrix of nanosized inhomogeneities. EXAFS experiments indicate that these inhomogeneities are determined by the maximum size reached by the niobium second coordination shell combined with the sudden inclusion of potassium atoms in it. Such rearrangement at the local level determines the maximum fluctuation of the bulk glass refractive index and in turn its maximum SHG activity.