Second-harmonic generation in Ge-doped silica fibres: experiment and theory

Raman and hyper-Raman studies of pure silica and Ge-doped silica fibres reveals charge transfer from the Ge centre to silica in the fibres under pumping by the second harmonic. Self-organization of these charge-transfer excitons results in a strong phase-matched static electric field breaking the inversion symmetry and allowing frequency doubling. Owing to the long-range electron transfer the response of the medium is very slow and a considerable retardation takes place during the preparation of the fibre by light. This retardation makes the second-harmonic amplitude grow at the stage of preparation. The retardation vanishes in the stationary state and the second-harmonic amplitude tends to its initial magnitude in the fully prepared fibre. Despite the fact that the strong phase-matched electric field is still present, the frequency-doubling mechanism is now switched off.