Optical waveguide and devices

1. An optical waveguide for guiding light in a photonic circuit comprising a layer of phase change material for modulating the phase of the guided light,

wherein the phase change material is switchable between at least a stable crystalline state and a stable amorphous state each with different refractive indexes, and the phase change material exhibits an extinction coefficient of less than 0.1 in both states for wavelengths greater than 1000 nm, wherein the phase change material is formed of or comprises a chalcogenide comprising antimony selected from: antimonide trisulphide (Sb 2 S 3 ) and antimonide triselenium (Sb 2 Se 3 ).

2. The waveguide of claim 1 , wherein the phase change material exhibits an extinction coefficient of less than 0.05 or 0.01, or substantially zero, in both states for wavelengths greater than 1000 nm.

3. The waveguide of claim 1 , wherein the phase change material exhibits an extinction coefficient of substantially zero in both states and a refractive index difference between the two states of at least 0.1 at a wavelength of 1550 nm.

4. The waveguide of claim 1 , comprising a plurality of layers of phase change material distributed along the waveguide.

5. The waveguide of claim 1 , wherein the waveguide comprises a first layer formed of a first material, and the layer of phase change material is provided on the first layer in a pattern to define a lateral width of the waveguide; or

wherein the waveguide comprises a first layer formed of a first material that defines the width of the waveguide, and the layer of phase change material is provided over the first layer and is evanescently coupled thereto.

6. The waveguide of claim 5 , wherein refractive index of the phase change material is lower than the refractive index of the first material in at least one of the two states.

7. The waveguide of claim 5 , wherein the first layer is formed of or comprises a material selected from the group comprising: silicon, silicon nitride, indium phosphide, gallium phosphide, germanium, and lithium niobate.

8. The waveguide of claim 5 , wherein the first layer is formed of or comprises a silicon layer of a silicon-on-insulator substrate.

9. The waveguide of claim 1 , further comprising:

i) a resistive heating element in thermal contact with at least a longitudinal portion of the layer of phase change material for switching the state of the phase change material at least at that longitudinal portion; or

ii) a plurality of resistive heating elements, each in thermal contact with separate longitudinal portion of the layer of phase change material, for switching the state of the phase change material at least at the respective longitudinal portions.

10. The waveguide of claim 1 , wherein the layer of phase change material has a thickness in the range 50 nm to 200 nm.

11. An optical device for a photonic circuit, comprising a first waveguide and a second waveguide each as defined in claim 1 , wherein:

the first and second waveguides are arranged substantially parallel and spaced apart by a lateral gap, over at least a coupling region thereof, such that light can evanescently couple from the first to the second waveguide in the coupling region, and

the degree of optical coupling from the first to the second waveguide is dependent on the relative states of the phase change material of the first and second waveguides in the coupling region.

12. The device of claim 11 , wherein the phase change material of the first and second waveguides in the coupling region is configurable to be in the same state or different states to selectively couple light from the first waveguide to the second waveguide.

13. The device of claim 12 , wherein the length of the coupling region is equal to a critical coupling length for phase-matching to selectively couple substantially all light from the first waveguide to the second waveguide.

14. The device of claim 11 , further comprising a third waveguide configured the same as either the first or the second waveguide, the third waveguide arranged substantially parallel to and spaced apart from the first or second waveguide by a lateral gap, over at least a respective coupling region thereof, such that light can evanescently couple from the first or second waveguide to the third waveguide in the respective coupling region, and wherein the degree of optical coupling from the first or second waveguide to the third waveguide is dependent on the relative states of the phase change material of the third waveguide and the first or second waveguide in the coupling region.

15. The device of claim 14 , wherein the phase change material of the third waveguide and the first or second waveguide in the coupling region is configurable to be in the same state or different states to selectively couple light from the first or second waveguide to the third waveguide.

16. The device of claim 15 , wherein the length of the coupling region of the first or second waveguide and the third waveguide is equal to a critical coupling length for phase-matching to selectively couple substantially all light from the first or second waveguide to the third waveguide.

17. The device of claim 11 , wherein the length of the coupling region is equal to a critical coupling length for phase-matching, and at least a longitudinal portion of the phase change material of each waveguide in the coupling region is in the same state and overlaps by an overlap length that is variable for varying the degree of optical coupling between respective waveguides.

18. A photonic field programmable coupler array comprising an array of waveguides as defined in claim 1 arranged substantially parallel to and spaced apart from each by a lateral gap, over at least a respective coupling region thereof, such that light can evanescently couple from one waveguide to an adjacent waveguide in the respective coupling region,

wherein at least a longitudinal portion of the phase change material of each waveguide in the respective coupling region is configurable to be in the same state and overlap the corresponding longitudinal portion of an adjacent waveguide by an overlap length to selectively transfer light across two or more adjacent waveguides in the array.

19. A Mach-Zehnder interferometer for a photonic circuit comprising first and second parallel optical paths, wherein the first and/or second optical path comprises a waveguide as defined in any of claim 1 .