Origin and properties of second-order optical non-linearity in ultraviolet-poled GeO₂-SiO₂ glass

Progress in establishing larger second-order optical non-linearity in Ge-doped silica glass (15.7 GeO₂-84.3 SiO₂ in mol%) induced by simultaneous applications of a direct current (DC) electric field (≤ 10⁵ V/cm) and ultraviolet laser irradiation (193 nm), the so-called the ultraviolet (UV)-poling method, is reported. The non-linearity thus induced has been shown to be affected by the GeE' centers created from oxygen deficient vacancies through photo-chemical reactions. In addition, we have found for the first time, to our knowledge, that crystallization in the glass is induced by the UV-poling, which leads to increase of third-order non-linearity, χ⁽³⁾, approximately 15 times larger than that before UV-poling. Based on these results we suggest that origin of the second-order non-linearity in the glass induced by UV-poling is a combined effect of the third-order non-linearity possessed by the crystallites and an internal space-charge field where the charges are assumed to be produced during the formation of GeE′ centers.