Optical structure and method of fabricating an optical structure

1. A method of fabricating an optical structure, the method comprising:

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;

crystallising the silicon of the further layer so that it forms a single crystal structure with the silicon in the layer, in which the crystallisation uses the silicon in the layer as a seed; 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.

2. A method according to claim 1 , in which the etching is anisotropic wet etching.

3. A method according to claim 1 , in which the layer of single crystal crystalline silicon is supported on the insulating surface of the silicon substrate so that the <111> crystal plane of the silicon is non-parallel to the insulating surface, and in which the etching defines a side wall lying in the <111> crystal plane.

4. A method according to claim 1 , in which the layer of single crystal crystalline silicon is arranged so that its <111> crystal plane is at an angle to the insulating surface in the range of 42 degrees to 48 degrees, or 40 degrees to 50 degrees, or 35 degrees to 55 degrees, or 30 degrees to 60 degrees.

5. A method according to claim 1 , in which the layer of single crystal crystalline silicon is arranged so that its <111> crystal plane is substantially orthogonal to the insulating surface.

6. A method according to claim 1 , in which the etching defines the side wall to be at an angle to the insulating surface of the substrate in the range of 42 degrees to 48 degrees, or 40 degrees to 50 degrees, or 35 degrees to 55 degrees, or 30 degrees to 60 degrees, or at an angle of 54.7 degrees to the insulating surface, or at an angle of substantially 45 degrees to the insulating surface, or substantially orthogonal to the insulating surface.

7. An optical structure comprising:

a silicon substrate with a surface insulating layer;

a first portion of silicon supported on the insulating surface of the substrate and having a first side wall non-parallel and non-perpendicular to the said insulating surface, the silicon being single crystal crystalline silicon;

a second portion of silicon supported on the insulating surface of the substrate and having a second side wall non-parallel and non-perpendicular to the said insulating surface and facing the first side wall so as to define a slot between the first and second side walls; and

an insulating material disposed in the slot.

8. An optical structure according to claim 7 , in which the first side wall lies in the <111> crystal plane of the first portion of silicon.

9. An optical structure according to claim 7 , in which the first and second portions of silicon and the insulating material define a waveguiding plane and the slot extends in an optical propagation direction through the waveguiding plane.

10. An optical structure according to claim 7 , in which the second portion of silicon is polycrystalline silicon or amorphous silicon.

11. An optical structure according to claim 7 , in which the second portion of silicon is single crystal crystalline silicon.

12. An optical structure according to claim 11 , in which the second side wall lies in the <111> crystal plane of silicon of the second portion of silicon.

13. An optical structure according to claim 7 , in which the insulating material is a dielectric material.

14. An optical structure according to claim 7 , in which the first side wall is at an angle to the insulating surface of the substrate in the range of 42 degrees to 48 degrees, or 40 degrees to 50 degrees, or 35 degrees to 55 degrees, or 30 degrees to 60 degrees.

15. An optical structure according to claim 7 , in which the first side wall and the second side wall are parallel to each other, so as to define a slot of a constant width.

16. An optical structure according to claim 7 , in which the first side wall and the second side wall are non-parallel to each other.

17. An optical or electro-optical device comprising an optical structure according to claim 7 .

18. An optical waveguiding structure comprising:

a silicon substrate with a surface insulating layer;

a first portion of silicon supported on the insulating surface of the substrate and having a first side wall non-parallel to the said insulating surface, the silicon being single crystal crystalline silicon;

a second portion of silicon supported on the insulating surface of the substrate and having a second side wall non-parallel to the said insulating surface and facing the first side wall so as to define a slot between the first and second side walls; and

an insulating material disposed in the slot;

in which the first and second portions of silicon and the insulating material define a waveguiding plane and the slot extends in an optical propagation direction through the waveguiding plane; and

the structure further comprises a plurality of supporting fins extending from side walls of the first and second portions of silicon opposite to the first and second side walls in a direction non-parallel to the optical propagation direction.