Measurement of the electric screening field in Bi12TiO20

In Bi12TiO20 with an applied ac voltage the coupling for two- wave mixing c an reach an extreme value at the transient moment t(p) when the internal el ectric field is zero. This is because of the changing polarization state of the pump wave during field reversal. The internal field is found to be the sum of the applied field and a time developing screening field arising fro m charge redistribution. Therefore, at the moment t(p) the amount of the sc reening field is equal to the value of the applied field and thus can be me asured. Also tracking of the temporal course of the electric screening fiel d is possible. The buildup of screening fields leads to an intensity depend ence of t(p) for a given shape of the ac voltage. (C) 1999 American Institu te of Physics. [S0021-8979(99)00603-9].