BRAGG LIQUID CRYSTAL POLARIZATION GRATINGS

• US 20160033698A1
• Filed: 07/30/2015
• Published: 02/04/2016
• Est. Priority Date: 07/31/2014
• Status: Active Grant

• Current Assignee: META Platforms Technologies LLC (Meta Platforms, Inc. (f/k/a Facebook, Inc.))
• Original Assignee: ImagineOptix Corporation (Meta Platforms, Inc. (f/k/a Facebook, Inc.)), North Carolina State University (University of North Carolina System)
• Inventors: Escuti, Michael J., Komanduri, Ravi K., Kekas, D. Jason
• Granted Patent: US 10,859,740 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: G02B 27/283 used for beam splitting or combining
• CPC Class Codes: G02B 5/1833 comprising birefringent materials birefringent elements per se G02B5/3083
• CPC Class Codes: G02B 5/3016 involving passive liquid crystal elements optical properties of liquid crystals G02F1/0063; polarising elements associated with active liquid crystal devices G02F1/133528
• CPC Class Codes: G02F 1/1334 based on polymer dispersed liquid crystals, e.g. microencapsulated liquid crystals
• CPC Class Codes: G02F 2201/305 diffraction grating

Abstract

An optical element includes a plurality of stacked birefringent sublayers, such as liquid crystal sublayers, configured to alter a direction of propagation of light passing therethrough according to the Bragg condition. The stacked birefringent sublayers respectively include local optical axes that vary along respective interfaces between adjacent ones of the stacked birefringent sublayers to define respective grating periods. The respective thicknesses of the stacked birefringent sublayers may be less than a wavelength of the light. Related apparatus and methods of operation are also discussed.

Claims

1. An optical element, comprising:

a plurality of stacked birefringent sublayers configured to alter a direction of propagation of light passing therethrough according to a Bragg condition,

wherein the stacked birefringent sublayers respectively comprise local optical axes that vary along respective interfaces between adjacent ones of the stacked birefringent sublayers to define respective grating periods.

2. The optical element of claim 1, wherein the stacked birefringent sublayers have respective thicknesses that are less than a wavelength of the light.

3. The optical element of claim 2, wherein the respective thicknesses are less than the respective grating periods defined by the local optical axes thereof, and wherein an aggregate thickness comprising a sum of the respective thicknesses is greater than the respective grating periods.

4. The optical element of claim 3, wherein the wavelength of the light comprises a wavelength λ

, the respective grating periods define a period Λ

of the optical element, the aggregate thickness comprises a thickness d, and an average refractive index of the stacked birefringent sublayers comprises a refractive index n such that a variable Q is greater than 1 for the wavelength λ

, where Q=2 π

λ

d/Λ

² n.

5. The optical element of claim 4, wherein the stacked birefringent sublayers comprise a material having a birefringence of less than about 0.4.

6. The optical element of claim 5, wherein the stacked birefringent sublayers respectively comprise liquid crystal sublayers having liquid crystal molecule orientations that define the local optical axes.

7. The optical element of claim 4, wherein the local optical axes of one or more of the stacked birefringent sublayers are rotated over the respective thicknesses thereof to define respective twist angles.

8. The optical element of claim 7, wherein ones of the respective twist angles and/or respective thicknesses are different among the stacked birefringent sublayers.

9. The optical element of claim 1, wherein the plurality of stacked birefringent sublayers is configured to diffract the light into a zero order beam and a first order beam having a substantially same polarization, which is different than an incident polarization of the light.

10. The optical element of claim 1, wherein the plurality of stacked birefringent sublayers is configured to diffract the light into a first order beam with a diffraction efficiency of about 94% or more responsive to receiving the light at an angle of incidence that is approximately equal to a Bragg angle.

11. The optical element of claim 10, wherein a difference between angles of propagation of a zero order beam and the first order beam is greater than about 45 degrees, responsive to receiving the light at the angle of incidence that is approximately equal to the Bragg angle.

12. The optical element of claim 9, wherein the substantially same polarization of the zero order and first order beams comprises an at least approximately circular polarization of a same handedness.

13. The optical element of claim 2, further comprising:

a reflective element positioned to receive a zero order beam and/or a first order beam output from the stacked birefringent sublayers and reflect the zero order beam and/or the first order beam back theretoward.

14. The optical element of claim 13, wherein the reflective element is configured to output light having a plurality of different polarizations responsive to receiving the zero order beam and/or the first order beam from the stacked birefringent sublayers.

15. The optical element of claim 1, wherein the respective grating periods of the stacked birefringent sublayers are less than a wavelength of the light.

16. The optical element of claim 15, further comprising:

a transparent substrate that is optically coupled to the plurality of stacked birefringent sublayers,

wherein the plurality of stacked birefringent sublayers is configured to diffract the light into a first order beam at an angle that is greater than a critical angle for total internal reflection (TIR) within the transparent substrate.

17. The optical element of claim 1, wherein the plurality of stacked birefringent sublayers comprises first birefringent sublayers having respective first grating periods and are configured to alter the direction of propagation of a first wavelength of the light passing therethrough according to the Bragg condition, and further comprising:

a plurality of stacked second birefringent sublayers on the first birefringent sublayers and respectively configured to alter the direction of propagation of a second wavelength of the light passing therethrough according to the Bragg condition,

the second birefringent sublayers having respective local optical axes that vary along respective interfaces therebetween to define respective second grating periods,

wherein the first and second birefringent sublayers are configured to diffract the first and second wavelengths of the light, respectively, into respective first order beams.

18. The optical element of claim 17, wherein the respective first order beams have a substantially same propagation direction.

19. The optical element of claim 17, wherein the second grating periods, an aggregate thickness of the second birefringent sublayers, and/or an average refractive index of the second birefringent sublayers differ from those of the first birefringent layers.

20. The optical element of claim 2, wherein the wavelength of the light is about 400 nanometers (nm) to about 1700 nm.

21. The optical element of claim 20, wherein the respective grating periods are about 1000 nanometers (nm) or less, and wherein the respective thicknesses are about 100 nm to about 300 nm.

22. A diffractive optical element, comprising:

at least one liquid crystal layer having a thickness d, an average refractive index n, and comprising liquid crystal molecule orientations that vary in a direction along a surface thereof to define a grating period Λ

of the diffractive optical element such that a variable Q is greater than 1 for an operational wavelength λ

of light, where Q=2π

λ

d/Λ

² n.

23. The diffractive optical element of claim 22, wherein the at least one liquid crystal layer comprises a plurality of stacked polymerized nematic liquid crystal sublayers, wherein respective thicknesses of the stacked polymerized liquid crystal sublayers are less than the operational wavelength λ

of the light.

24. The diffractive optical element of claim 23, wherein the respective thicknesses of the stacked polymerized liquid crystal sublayers are less than the grating period Λ

of the diffractive optical element, and wherein the respective thicknesses of the stacked polymerized nematic liquid crystal sublayers collectively define the thickness d, which is greater than the grating period Λ

of the diffractive optical element and the operational wavelength λ

of the light.

25. The diffractive optical element of claim 23, wherein the liquid crystal molecule orientations of one or more of the stacked polymerized nematic liquid crystal sublayers are rotated over the respective thicknesses thereof to define respective twist angles φ

, wherein the respective twist angles φ and

/or the respective thicknesses are different among the stacked polymerized nematic liquid crystal sublayers.

26. The diffractive optical element of claim 22, wherein the grating period Λ

of the diffractive optical element is less than the operational wavelength λ

of the light.