Experimental and theoretical study of the origin of the second-order nonlinearities induced in thermally poled glasses

To model the poling process, the glass is assimilated to a neutral "solid electrolyte". During the poling process, an electric voltage V0 is applied between the cathode and the anode respectively located at x=0 and x=L. The evolution of the ion concentrations is then described by a system of coupled equations containing the equations of diffusion for the different ionic species and Laplace's equation. Since in fused silica, at about 300°C, it is mainly sodium ions, which are mobile, the anions are assumed to be immobile1. According to these approximations and under usual poling conditions (T≈300°C, V0≈3kV), we computed the expression of the potential across the sample during the poling. On Figure 1, we have plotted the potential V(x), the charge distribution ρ(x) and the associated electric field E(x) for a typical silica sample. Since the screening distances di and d2 at the cathode and the anode are much smaller than the sample thickness, the electric field E(x)~V0/d2~108 V/m at the glass interfaces is very high.