A method of fabricating an optical structure comprises providing a layer of single crystal crystalline silicon supported on an insulating surface of a silicon substrate; using etching to remove part of the silicon layer and define a side wall which is non-parallel to the insulating surface of the substrate; forming a layer of insulating material over the side wall; forming a further layer of silicon over at least the insulating material; and removing the silicon of the further layer to a level of the layer of silicon such that the layer of insulating material occupies a slot between a portion of silicon in the layer and a portion of silicon in the further layer, a thickness of the layer of insulating material defining a width of the slot.