Multilayer liquid crystal diffractive gratings for redirecting light of wide incident angle ranges
An optical device includes a stack of multiple grating structures, each of which includes a plurality of sublayers of liquid crystal material. Each sublayer of liquid crystal material includes laterally extending repeating units, each formed of a plurality of liquid crystal molecules. The repeating units of the liquid crystal layers are lateral offset from one another, and defined a tilt angle. The grating structures forming the stack of grating structure have tilt angles of different magnitudes. The grating structures may be configured to redirect light of visible or infrared wavelengths. Advantageously, the different tilt angles of the stack of grating structures allows for highly efficient diffraction of light incident on the grating structures at a wide range of incident angles.
1. An optical device comprising: a first polarization grating structure comprising:
a first plurality of liquid crystal sublayers comprising liquid crystal molecules, wherein the liquid crystal molecules form repeating units having a first periodicity along a lateral axis, and
wherein the repeating units are laterally shifted by a first shift distance in comparison to similar repeating units of an immediately adjacent one of the first plurality of sublayers, wherein the repeating units shifted by the first shift distance define a first tilt angle with respect to a normal to the lateral axis, the normal extending along a thickness axis of the first polarization grating structure; and a second polarization grating structure over the first polarization grating structure, the second polarization grating structure comprising:
a second plurality of liquid crystal sublayers comprising liquid crystal molecules, wherein the liquid crystal molecules form repeating units having a second periodicity along the lateral axis, and
wherein the repeating units are laterally shifted by a second shift distance in comparison to similar repeating units of an immediately adjacent one of the plurality of sublayers, wherein the repeating units shifted by the second shift distance define a second tilt angle with respect to the normal to the lateral axis, wherein magnitudes of the first and second tilt angles are different,
wherein the first shift distance is larger than the second shift distance, and
wherein the first plurality of liquid crystal sublayers are laterally shifted by a first aggregate shift larger than a second aggregate shift of the second plurality of liquid crystal sublayers.
2. The device of claim 1 , wherein liquid crystal molecules in each of the plurality of sublayers of the first polarization grating structure are twisted by a first twist angle,
wherein orientations of the liquid crystal molecules, as seen in a side or top-down view, vary in a repeating pattern laterally across each of the sublayers, and
wherein each of the repeating units in the plurality of liquid crystal sublayers of the first polarization grating structure are formed by a plurality of liquid crystal molecules having a similar progression of orientations.
3. The device of claim 1 , wherein the repeating units of successively higher levels of the liquid crystal sublayers of the first and second polarization grating structures are shifted in a same direction.
4. The device of claim 1 , wherein liquid crystal molecules in each of the plurality of sublayers of the second polarization grating structure are twisted by a second twist angle,
wherein orientations of the liquid crystal molecules, as seen in a side or top-down view, vary in a repeating pattern laterally across each of the sublayers,
wherein each of the repeating units in the plurality of liquid crystal sublayers of the second polarization grating structure are formed by a plurality of liquid crystal molecules having a similar progression of orientations.
5. The device of claim 1 , wherein the liquid crystal molecules in the plurality of sublayers are part of a polymerized liquid crystal material.
6. The device of claim 1 , wherein the liquid crystal molecules in the plurality of sublayers comprise liquid crystalline di-acrylate.
7. The device of claim 1 , wherein the plurality of sublayers are doped with chiral dopants.
8. The device of claim 1 , wherein the liquid crystal molecules have a tilt angle (θ 1 ) between about 85 degrees and 85 degrees.
9. The device of claim 8 , wherein the liquid crystal molecules are configured to diffract at least 40% of a beam of light incident on the polarization grating structure at an incident angle (θ inc ) between about −20 degrees and 20 degrees relative to the tilt angle.
10. The device of claim 1 , further comprising a third polarization grating structure over the second polarization grating structure, the third polarization grating structure including a plurality of liquid crystal sublayers comprising liquid crystal molecules,
wherein the liquid crystal molecules form repeating units having a third periodicity,
wherein the repeating units are laterally shifted by a third shift distance in comparison to similar repeating units of an immediately adjacent one of the plurality of sublayers, wherein the repeating units shifted by the third shift distance define a third tilt angle with respect to the normal to the lateral axis,
wherein magnitudes of the first, second, and third tilt angles are different.
11. The device of claim 10 , wherein the liquid crystal sublayers of each of the first, second, and third polarization grating structures have different compositions than others of the first, second, and third polarization grating structures.
12. The device of claim 1 , wherein the first and second polarization grating structures are over a waveguide and further comprising:
a template layer over the waveguide and under the first and second polarization grating structures.
13. The device of claim 12 , wherein the first and second polarization grating structures are in-coupling optical elements configured to in-couple an incident beam of light into the waveguide such that the incident beam of light propagates through the waveguide by total internal reflection.
14. The device of claim 13 , further comprising a light modulation device configured to direct light into the waveguide via the first and second polarization grating structures.
15. The device of claim 13 , wherein the first and second polarization grating structures are out-coupling optical elements configured to out-couple an incident beam of light propagating through the waveguide by total internal reflection.
16. The device of claim 13 , wherein the first and second polarization grating structures are orthogonal pupil expanders configured to redirect light propagating through the waveguide by total internal reflection, wherein the redirected light continues to propagate through the waveguide by total internal reflection.
17. The device of claim 13 , further comprising a stack of waveguides, each waveguide comprising first and second polarization grating structures comprising pluralities of liquid crystal sublayers having tilt angles of different magnitudes.
18. The device of claim 1 , wherein a spacing along the thickness axis between adjacent sublayers of the first plurality of liquid crystal sublayers is larger than a spacing along the thickness axis between adjacent sublayers of the second plurality of liquid crystal sublayers.