Charge transfer excitons in Ge-doped silica fibers: Self-organization and second harmonic generation

A Raman and a first hyper-Raman study of pure silica and Ge-doped silica fibers is performed. The Raman spectra of a doped fiber are accompanied by a broad luminescence band corresponding to charge transfer excitons of Ge-centers. Self-organization (orientational ordering) of these excitons under light pumping results in an instability of the system with respect to the weak static phase matched field similar to chi((3))E(2 omega)E(omega)*E(omega)* found by Stolen and Tom. A fiber makes a transition into a polarized state accompanied by the strong phase matched field E(dc) similar to e(i(k2 omega-2k omega)Z) allowing the second harmonic generation (chi((2)=)chi((3))E(dc), E(dc) similar to 10(5) V/cm) to occur.