a method of creating a varying second order non - linearity profile along a waveguide , comprising : providing a waveguide structure with a waveguiding core and a surface adjacent to the waveguiding core ; structuring the surface to produce a structured surface defining a varying distance between the structured surface and the waveguiding core along the waveguide ; and thermally poling the waveguide structure to generate a varying second order non - linearity profile along the waveguide - derived from the varying distance between the structured surface and the waveguiding core . by the surface structuring the modulation of the second order non - linearly induced by the thermal poling can be enhanced . the waveguide structures can be used for making a variety of quasi - phase - matched devices .