Phase structure on volume Bragg grating-based waveguide display

1. A waveguide display comprising:

a first substrate transparent to visible light;

a second substrate transparent to the visible light;

a holographic material layer sandwiched by the first substrate and the second substrate and including a reflective volume Bragg grating characterized by a polarization-dependent diffraction efficiency, the reflective volume Bragg grating configured to more preferentially reflectively diffract incident light of a first polarization state over incident light of a second polarization state that is orthogonal to the first polarization state; and

a phase structure on the first substrate or the second substrate and configured to change a polarization state of display light incident on the phase structure before the display light is diffracted by the reflective volume Bragg grating such that the display light incident on the reflective volume Bragg grating is predominantly in the first polarization state than the second polarization state with respect to the reflective volume Bragg grating to improve diffraction efficiency.

2. The waveguide display of claim 1 , wherein the phase structure comprises a waveplate.

3. The waveguide display of claim 2 , wherein the waveplate is characterized by a waveplate thickness between zero and one wavelength.

4. The waveguide display of claim 1 , wherein the phase structure comprises a layer of a birefringent material.

5. The waveguide display of claim 1 , wherein the phase structure comprises a subwavelength structure and an overcoat layer.

6. The waveguide display of claim 5 , wherein the subwavelength structure is etched in the first substrate or the second substrate.

7. The waveguide display of claim 5 , wherein the subwavelength structure is etched in a material layer formed on the first substrate or the second substrate.

8. The waveguide display of claim 5 , wherein a difference between a refractive index of the first substrate or the second substrate and an effective refractive index of the phase structure including the subwavelength structure and the overcoat layer is less than 0.35.

9. The waveguide display of claim 1 , wherein the phase structure is in selected regions of the first substrate or the second substrate.

10. The waveguide display of claim 1 , wherein the phase structure is characterized by a spatially varying phase retardation across different regions of the phase structure.

11. The waveguide display of claim 1 , wherein:

the phase structure is on the second substrate; and

the waveguide display further comprises a second phase structure on the first substrate.

12. The waveguide display of claim 1 , wherein the holographic material layer includes two or more volume Bragg gratings that include an input grating coupler and an output grating coupler.

13. The waveguide display of claim 12 , wherein the phase structure is in a region of the waveguide display where the input grating coupler is located.

14. The waveguide display of claim 12 , wherein the phase structure is in a region of the waveguide display where the input grating coupler and the output grating coupler are located.

15. The waveguide display of claim 12 , wherein:

the input grating coupler includes one or more volume Bragg gratings; and

the output grating coupler includes at least two volume Bragg gratings configured to expand an eyebox of the waveguide display in two directions.

16. A waveguide display comprising:

a first substrate;

a second substrate;

an input grating coupler between the first substrate and the second substrate and configured to couple display light into the first substrate or the second substrate via reflective diffraction that more preferentially reflectively diffracts incident light of a first polarization state over incident light of a second polarization state that is orthogonal to the first polarization state;

an output grating coupler between the first substrate and the second substrate and configured to at least partially couple the display light out of the waveguide display towards an eyebox of the waveguide display via reflective diffraction that more preferentially reflectively diffracts incident light of the first polarization state over incident light of the second polarization state; and

a phase structure on the first substrate or the second substrate and configured to change a polarization state of the display light coupled into the first substrate or the second substrate before the display light coupled into the first substrate or the second substrate reaches the output grating coupler such that the display light incident on the output grating coupler is predominantly in the first polarization state than the second polarization state with respect to the output grating coupler.

17. The waveguide display of claim 16 , wherein the phase structure includes:

a layer of a birefringent material; or

a subwavelength structure formed in an isotropic material or the birefringent material.

18. The waveguide display of claim 16 , wherein:

the phase structure includes a subwavelength structure and an overcoat layer; and

a difference between a refractive index of the first or second substrate and an effective refractive index of the phase structure is less than 0.35.

19. The waveguide display of claim 16 , wherein the phase structure includes a subwavelength structure etched in the first substrate, in the second substrate, or in a material layer formed on the first substrate or the second substrate.

20. The waveguide display of claim 16 , wherein:

the phase structure is on the second substrate; and

the waveguide display further comprises a second phase structure on the first substrate.