Optical piston in glasses

Light-driven electron transitions between different traps in the direction opposite to an electric force eE (e is the electron charge and E is the sta tic electric field) are studied. We show that in Ge-doped silica fibres the electrons are transfered predominantly between different Ge centres at con centrations greater than or equal to 10 mol.% and dominate over the ordinar y electron mobility in the direction of the force eE. Light acts as an opti cal piston, moving electrons against the acting force and therefore resulti ng in positive feedback in response to static electric field. This feedback amplifies the weak initial electric held up to magnitudes of the order of 10(5)-10(6) V cm(-1). This strong field breaks the initial inversion symmet ry and allows effective second harmonic generation.