Optical device

• US 20040091224A1
• Filed: 10/31/2002
• Published: 05/13/2004
• Est. Priority Date: 04/06/2000
• Status: Active Grant

• Current Assignee: Nanogan Limited (IQE, PLC)
• Original Assignee: BTG International Limited (Boston Scientific Corporation)
• Inventors: Charlton, Martin D.B., Baumberg, Jeremy J., Netti, Maria C., Zoorob, Majd E., Parker, Gregory J.
• Granted Patent: US 6,888,994 B2
• Time in Patent Office: Days
• US Class Current: 385/129
• CPC Class Codes: B82Y 20/00 Nanooptics, e.g. quantum optics or photonic crystals
• CPC Class Codes: C30B 29/60 characterised by shape
• CPC Class Codes: C30B 33/00 After-treatment of single crystals or homogeneous polycrystalline material with defined structure C30B31/00 takes precedence
• CPC Class Codes: G02B 6/1225 comprising photonic band-gap structures or photonic lattices

Abstract

In order to create an optical device with a photonic band gap extending in two dimensions and with very uniform properties in any direction and for any polarisation state, to within 1%, air holes are etched within a substrate of low refractive index material such silicon oxynitride or silica glass. The ratio of air hole area to the remainder of the substrate is low, being less than 35%. The air holes define a quasicrystal structure, having twelve fold symmetry, being based on a square-triangle system. In another development, an etched substrate with a regular crystal structure or quasicrystal structure exhibits a non-linear refractive index. Two adjacent areas in such a substrate have different lattice properties, or have defects in the lattices, to create a unidirectional transmission path (diode action). A further beam of light may be used to modulate the transmission path by reason of the non-linear refractive index.

Claims

1. A method of forming a structure exhibiting a photonic band gap, the method comprising:

providing a material extending in two dimensions, and forming in said material, first regions, having a first refractive index, the first regions being spaced apart by a second region or regions of a second refractive index, the regions defining a quasicrystal exhibiting long range order but short term disorder, and exhibiting n-fold symmetry wherein n is greater than or equal to two, whereby to provide a photonic band gap extending in at least said two dimensions.

6. A method or structure according to any of claims 1 to 5, wherein the photonic band gap is generally uniform in any direction within said two dimensions, having the same width and/or same mid gap frequency.

7. A method or structure according to claim 6, wherein the photonic band gap is uniform in any direction to within 4%, and preferably within 1%.

8. A method or structure according to any of claims 1 to 7, wherein the photonic band gap is generally the same for all states of polarisation.

12. A method according to claim 8, wherein the air holes are subsequently filled with a desired material.

13. A method according to claim 12, wherein the desired material is liquid crystal, ferromagnetic, a dye, or an optical non-linear material, whose optical properties may be influenced by application of an electric, magnetic, electromagnetic field, or optical field, temperature, acoustic radiation, or chemical reagent.

14. A structure or method according to any preceding claim, wherein the quasicrystal exhibits a 12 fold symmetry.

15. A structure or method according to any preceding claim, wherein the quasicrystal has a unit cell structure.

16. A structure or method according to claim 14 or 15, wherein the approximant tiling is based on an assembly of adjacent rectangles, triangles or rhombi.

17. A structure or method according to any of claims 1 to 13, wherein the quasicrystal exhibits 10-fold symmetry, being based on Penrose tiling, or the quasicrystal is based on Archimedian tiling.

18. A structure or method according to any preceding claim, wherein the material is selected from silicon, germanium, silicon nitride, silicon oxynitride, gallium arsenide or indium phosphide, or compounds or mixtures thereof, or a glass, or a plastic.

19. A structure or method according to claim 18, wherein the material is silicon nitride or silicon oxynitride.

20. A structure or method according to claim 18, wherein the material is a glass, preferably a silica glass.

21. A structure or method according to any preceding claim, wherein said first regions are positioned at apices of the crystal lattice, or at the centre of shape sites.

22. A structure or method according to any preceding claim, wherein said first regions define a quasicrystal geometry in two dimensions, and n>

6.

23. A structure or method according to any preceding claim, wherein said material extends in three dimensions, and said first regions extend in a direction perpendicular to said two dimensions.

24. A structure or method according to claim 23, wherein the material comprises an optical fibre, and said first regions extend along the length of the fibre.

25. A structure or method according to any of claims 1 to 21, wherein the first regions are formed as lengths parallel to one another in one of the two dimensions, the spacing of the lengths in the second dimension being such as to provide said photonic band gap in at least said two dimensions, and wherein n is two or more.

2. A structure exhibiting a photonic band gap,

wherein the structure includes a material, extending in two dimensions, comprising first regions of a first refractive index spaced from one another by a second region or regions of a second refractive index in order to provide a quasicrystal exhibiting long range order but short term disorder, and exhibiting n-fold symmetry, wherein n is greater than or equal to two, whereby to create a photonic band gap extending in at least said two dimensions.

3. A method of forming a structure exhibiting a photonic band gap, the method comprising:

providing a material, extending in two dimensions, and having a relatively low index of refraction, less than or equal to three, and forming in said layer first regions, having a first refractive index, the first regions being spaced apart by a second region or regions of a second refractive index, the regions defining a quasicrystal, exhibiting long range order but short term disorder and exhibiting an n-fold symmetry where n is greater than or equal to two, whereby to create a photonic band gap extending in at least said two dimensions.

4. A method of forming a structure exhibiting a photonic band gap, the method comprising:

providing a material extending in two dimensions and forming in said material first regions, having a first refractive index, the first regions being spaced apart by a second region or regions of a second refractive index, the regions defining a quasicrystal exhibiting long range order but short term disorder and exhibiting n-fold symmetry, wherein n≧

12, whereby to create a photonic band gap extending in at least said two dimensions.

5. A method of forming a structure exhibiting a photonic band gap, the method comprising:

providing a material, extending in two dimensions, and etching the material to remove material in predetermined areas, the etching extending in a direction perpendicular to said two dimensions, whereby to define first regions having a first refractive index spaced by a second region or regions having a second refractive index, whereby to define a quasicrystal exhibiting long range order but short term disorder and exhibiting n-fold symmetry wherein n is greater than or equal to two, whereby to create a photonic band gap extending in at least said two dimensions, and

wherein the ratio of the area of the first regions to the area of the second regions is relatively low, being less than 35%.

9. A method according to claim 5, wherein the ratio is 28%, or less.

10. A method according to claim 5 or 9, wherein said predetermined areas comprise said first regions.

11. A method according to claim 10, wherein said first regions comprise air holes.

26. A structure exhibiting a photonic band gap,

wherein the structure includes a material comprising first regions of a first refractive index spaced from one another by a second region or regions of a second refractive index in order to provide a crystal or quasicrystal exhibiting a photonic band gap, such as to modify the properties of the material to induce a significant non-linear effect wherein the refractive index of the structure is dependent upon the power of light incident on the structure.

36. An optical switch incorporating a structure according to claim 26, including means for directing a power beam of light onto the structure, and including means for directing a signal beam of light through the structure and whereby the power beam changes the refractive index of material and creates a path for the signal beam of light.

27. An optical device comprising a body having at least one path there through for the transmission of optical radiation wherein the transmission characteristics of said radiation when traversing said path are constrained by a first and a second region disposed within said body and comprising a material or materials exhibiting a photonic band gap wherein said first and second regions are positioned relative to one another so that an evanescent field associated with the photonic band gap of said first region interacts with an evanescent field associated with the photonic band gap of said second region to cause said transmission characteristic to be essentially non-linear, asymmetric or controllable.

37. An optical diode incorporating a device according to any of claims 27 to 35, including means for directing a signal beam of light through a transmission path.

38. An optical switch or transistor incorporating a device according to any of claims 27 to 35, including means for directing a signal beam of light through the transmission path, and means for directing a modulating beam of light onto the first and/or second area to after the refractive index thereof, thereby to alter the transmission properties of the transmission path.

28. An optical device having an optical transmission path through first and second adjacent areas, each area comprising first regions of a first refractive index spaced apart by a second region or regions of a second refractive index, said regions defining a crystal or quasi crystal structure having a photonic band gap, and each such structure having defects therein to create photonic states within the band gap and associated evanescent fields, the evanescent fields of the first and second areas being coupled so as to permit transmission within the photonic band gap, the transmission characteristic being non-linear or asymmetric.

29. A device according to claim 28, wherein for each area, the defect structure is formed by predetermined first regions being of a different size or shape to the remainder of the first regions, or being of a different material, or by omitting predetermined first regions, to create a set of defects coupled to one another by evanescent fields.

30. A device according to claim 29, wherein the defect structure comprises a periodic array of defects.

31. A device according to claim 29 or 30, wherein the defects in the first area are different, and/or have a different spacing, from defects in the second area.

32. An optical device having an optical transmission path through first and second adjacent areas, each area being formed from a layer of material comprising, first regions of a first refractive index spaced apart by a second region or regions of a second refractive index, the regions defining a crystal or quasi crystal structure having a photonic band gap, the photonic band gaps in the two areas having different values, whereby light incident on the one area adjusts the band gap value relative to the band gap of the other area.

33. A optical device according to claim 32, wherein at least one of the first and second areas exhibiting a non-linear refractive index.

34. An optical device according to claim 32 or 33, wherein the band gaps are adjustable so as to be equal in value.

35. An optical device according to claim 32, 33, or 34, wherein the crystal or quasi crystal structures in the respective first and second areas are different, being of a different symmetry, and/or the respective first regions have a different size and/or spacing.