Measurement of the electric screening field in Bi12TiO20

In Bi12TiO20 with an applied ac voltage the coupling for two-wave mixing can reach an extreme value at the transient moment tp when the internal electric 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 from charge redistribution. Therefore, at the moment tp the amount of the screening field is equal to the value of the applied field and thus can be measured. Also tracking of the temporal course of the electric screening field is possible. The buildup of screening fields leads to an intensity dependence of tp for a given shape of the ac voltage.