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 (QPM) devices.