Self-organization of excitations in Ge-doped silica fibers and its role in second harmonic generation

Based on Raman and hyper-Raman scattering experiments and on direct green light absorption measurements, excitations of a new type — charge transfer excitons — are found to exist in Ge-doped silica fibers, whose self-organization (orientation ordering of dipole moments) leads to positive feedback in response to a static electric field. An electric field of 10⁵ V/cm then results breaking the inversion symmetry of the initially centrally symmetric medium and allowing the second harmonic to be generated. Generation persists only at the medium preparation stage and self-switches off upon a transition to the stationary state.