Abstract

The problem of efficient second-harmonic generation in germanosilicate optical fibers and bulk MOCVD glass is explained in terms of the existence of optically driven carrier delocalization of midgap states. At intensities above ~10 GW/cm², most of the localized states within the gap become delocalized, resulting in directional charge transport, a large frozen-in dc field, and a discontinuous jump in the effective (2).¹ The experimentally observed threshold is associated with a delocalization transition that occurs when the photon-renormalized disorder energy between sites is lowered to a value near the tunneling-energy bandwidth, resulting in a type of insulatormetal transition. At lower energies, weak delocalization of states occurs between sites with smaller energy separations, leading to shorter-range charge transport, weaker dc fields and a lower effective (2). The model is consistent with many of the observed experimental results, such as seeding-mode dependence, the measured time evolution, and recent magnetic field experiments.

© 1990 Optical Society of America