DIFFRACTION GRATINGS FOR BEAM REDIRECTION

• US 20190339449A1
• Filed: 09/18/2018
• Published: 11/07/2019
• Est. Priority Date: 05/04/2018
• Status: Active Grant

• Current Assignee: META Platforms Technologies LLC (Meta Platforms, Inc. (f/k/a Facebook, Inc.))
• Original Assignee: Facebook Technologies, LLC (Meta Platforms, Inc. (f/k/a Facebook, Inc.))
• Inventors: Shipton, Erik, Rajeeva, Bharath Bangalore
• Granted Patent: US 10,845,538 B2
• Time in Patent Office: Days
• CPC Class Codes: B82Y 20/00 Nanooptics, e.g. quantum optics or photonic crystals
• CPC Class Codes: G02B 2027/0174 holographic
• CPC Class Codes: G02B 27/0081 with means for altering, e.g. enlarging, the entrance or exit pupil
• CPC Class Codes: G02B 27/0172 characterised by optical features
• CPC Class Codes: G02B 27/0944 Diffractive optical elements, e.g. gratings, holograms gratings per se G02B5/18; holograms used as optical elements per se G02B5/32
• CPC Class Codes: G02B 27/1006 for splitting or combining different wavelengths G02B27/1086, G02B27/141 take precedence
• CPC Class Codes: G02B 27/4272 having plural diffractive elements positioned sequentially along the optical path
• CPC Class Codes: G02B 6/122 Basic optical elements, e.g. light-guiding paths

Abstract

A diffraction grating with independently controlled diffraction angles for optical beams at different wavelengths may be used to redirect and couple light to a waveguide in an efficient, space-saving manner. The diffraction grating can include a layer with optical permittivity and associated index contrast of the grating grooves at different grating periods dependent on wavelength.

Claims

1. A diffraction grating comprising:

a first grating structure having a first refractive index contrast, wherein the first refractive index contrast has a first grating pitch and a wavelength-dependent first amplitude, such that at a first wavelength, the first amplitude is above a first threshold and at a second wavelength, the first amplitude is below a second threshold, wherein the second threshold is lower than the first threshold; and

a second grating structure having a second refractive index contrast, wherein the second refractive index contrast has a second grating pitch and a wavelength-dependent second amplitude, such that at the first wavelength, the second amplitude is below the second threshold and at the second wavelength, the second amplitude is above the first threshold.

2. The diffraction grating of claim 1, wherein the second threshold is below 10% of the first threshold.

3. The diffraction grating of claim 2, wherein the second threshold is below 1% of the first threshold.

4. The diffraction grating of claim 3, wherein at the second wavelength, the first refractive index contrast is substantially zero, and wherein at the first wavelength, the second refractive index contrast is substantially zero.

5. The diffraction grating of claim 1, further comprising a third grating structure having a third refractive index contrast, wherein the third refractive index contrast has a third grating pitch and a wavelength-dependent third amplitude, such that at the first and second wavelengths, the third amplitude is below the second threshold and at a third wavelength, the third amplitude is above the first threshold, wherein at the third wavelength, the first amplitude and the second amplitude are both below the second threshold, and wherein each one of the first, second, and third wavelengths corresponds to a color channel of an electronic display.

6. The diffraction grating of claim 5, wherein ratios of the first grating pitch to the first wavelength;

the second grating pitch to the second wavelength; and

the third grating pitch to the third wavelength are equal to each other, such that in operation, components of an optical beam at the first, second, and third wavelengths emitted by the electronic display and impinging on the diffraction grating are diffracted at a substantially same diffraction angle.

7. The diffraction grating of claim 1, wherein the first and second grating structures are disposed adjacent one another.

8. The diffraction grating of claim 7, wherein the second grating structure is disposed on top of the first grating structure.

9. The diffraction grating of claim 4, wherein the first grating structure comprises a first hyperbolic metamaterial comprising an array of first stripes at the first grating pitch and having a zero refractive index contrast at the second wavelength, and wherein the second grating structure comprises a second hyperbolic metamaterial comprising an array of second stripes at the second grating pitch and having a zero refractive index contrast at the first wavelength.

10. The diffraction grating of claim 9, wherein each first stripe comprises a stack of alternating metal and semiconductor layers having a first set of thicknesses to provide the zero refractive index contrast at the second wavelength, and wherein each second stripe comprises a stack of alternating metal and semiconductor layers having a second set of thicknesses to provide the zero refractive index contrast at the first wavelength.

11. The diffraction grating of claim 10, wherein each first stripe comprises a stack of alternating metal and dielectric layers having a first set of thicknesses to provide the zero refractive index contrast at the second wavelength, and wherein each second stripe comprises a stack of alternating metal and dielectric layers having a second set of thicknesses to provide the zero refractive index contrast at the first wavelength.

12. The diffraction grating of claim 10, wherein a ratio of the first to second grating pitches is equal to a ratio of the first to second wavelengths, such that in operation, components of an optical beam at the first and second wavelengths impinging on the diffraction grating are diffracted at a substantially same diffraction angle.

13. A diffraction grating comprising a layer of metamaterial with a spatially varying optical permittivity dependent on wavelength, such that:

at a first wavelength, the spatially varying optical permittivity comprises a plurality of peaks and valleys at a first pitch; and

at a second, different wavelength, the spatially varying optical permittivity comprises a plurality of peaks and valleys at a second, different pitch.

14. The diffraction grating of claim 13, wherein a ratio of the first to second pitches is equal to a ratio of the first to second wavelengths, such that in operation, components of an optical beam at the first and second wavelengths impinging on the diffraction grating are diffracted at a substantially same angle.

15. The diffraction grating of claim 13, wherein the spatially varying optical permittivity comprises a spatially varying refractive index.

16. The diffraction grating of claim 15, wherein the metamaterial comprises a hyperbolic metamaterial.

17. The diffraction grating of claim 13, wherein the metamaterial comprises an array of first stripes at the first pitch having a zero refractive index contrast at the second wavelength, and an array of second stripes at the second pitch having a zero refractive index contrast at the first wavelength.

18. The diffraction grating of claim 17, wherein each first stripe comprises a stack of alternating metal and semiconductor layers having a first set of thicknesses to provide the zero refractive index contrast at the second wavelength, and wherein each second stripe comprises a stack of alternating metal and semiconductor layers having a second set of thicknesses to provide the zero refractive index contrast at the first wavelength.

19. The diffraction grating of claim 17, wherein each first stripe comprises a stack of alternating metal and dielectric layers having a first set of thicknesses to provide a zero refractive index contrast at the second wavelength, and wherein each second stripe comprises a stack of alternating metal and dielectric layers having a second set of thicknesses to provide a zero refractive index contrast at the first wavelength.

20. The diffraction grating of claim 17, wherein a ratio of the first to second pitches is equal to a ratio of the first to second wavelengths, such that in operation, components of an optical beam at the first and second wavelengths impinging on the diffraction grating are diffracted at a substantially same diffraction angle.