The problem of achieving high second-order nonlinearities in glasses: The role of electronic conductivity in poling of high index glasses

Efficient thermal poling of electronically conducting glass is prevented by the inherent difficulty to record a large electrostatic field within such glasses. To overcome this limitation, a waveguide/substrate configuration has been proposed, in which the glass for poling was deposited as a film of appropriate thickness on a substrate chosen for its higher ionic conductivity. Owing to this configuration, the poling voltage drops entirely across the glass film, allowing high electrostatic field to be recorded in spite of the high electronic conductivity of the glass. The proposed method was demonstrated here in the case of bismuth-zinc-borate glasses, which possess high potential for poling because of their high intrinsic χ(3). A four-fold enhancement of χ(2) compared to bulk glass, from ~ 0.5 to ~ 2 pm/V, is demonstrated. It is also shown that the χ(2) values obtained are the highest sustainable by the glass limited by the onset of nonlinear conductivity. The waveguide/substrate configuration intrinsically allows obtaining perfect overlap of the poling induced second-order nonlinearity with the guiding region of the waveguide. An equivalent RC-circuit model describing the poled glass reveals that the value of the poling-induced second-order nonlinearity is strongly dependent on the ratio β between ionic and electronic conductivity. The most promising glass systems for poling are found to be the ones displaying the highest product χ(3)β. This work is performed on bismuth-zinc-borate heavy metal oxide glasses but the waveguide/substrate configuration proposed here is likely to be equally successful in enhancing the second-order nonlinearity in high χ(3) electronic conducting glasses such as for example telluride and chalcogenide glasses.