A method of inducing a periodic variation of nonlinearity in a sample of ferroelectric material, comprises applying an electrically insulting mask to surface of the sample, applying an electric field across the sample to produce domain inversion in the sample, and removing the electric field when non-inverted regions of the sample remain only in the vicinity of the surface of the sample beneath parts of the surface covered by the mask. This method can be used to engineer accurate domain periods of submicron dimensions or larger which are confined to a surface region of the ferroelectric material, so that the poled material can be used to fabricate planar waveguide devices for nonlinear optical applications. In particular, the submicron periods can be exploited in the fabrication of one and two-dimensional photonic band gap devices.