Fundamentals of glass poling: from self-organization to electric-field poling

One decade has passed since the discovery of self-organized (photoinduced quasi-phase-matched) second-harmonic generation (SHG) in optical fibers. This discovery has attracted considerable interest world-wide due to the unusually strong χ(2) (second-order susceptibility) gratings induced purely by optical fields in glass (10 m/V, which were 4-5 orders of magnitude higher than one could explain by known physical processes). In 1991 the value of χ(2) has been increased to a new level of 1pm/V by thermal poling. In this paper we discuss fundamentals of glass poling, trying to answer the question: What is the limit of a second-order nonlinearity in glass