Evacuated gratings and methods of manufacturing
Improvements to gratings for use in waveguides and methods of producing them are described herein. Deep surface relief gratings (SRGs) may offer many advantages over conventional SRGs and Bragg gratings, an important one being a higher S-diffraction efficiency. In one embodiment, deep SRGs can be implemented as polymer surface relief gratings or evacuated Bragg gratings (EBGs). EBGs can be formed by first recording a holographic polymer dispersed liquid crystal (HPDLC) grating. Removing the liquid crystal from the cured grating provides a polymer surface relief grating. Polymer surface relief gratings have many applications including for use in waveguide-based displays.
1. A method for fabricating a deep surface relief grating (SRG), the method comprising:
providing a mixture of monomer and liquid crystal;
providing a substrate;
coating a layer of the mixture on a surface of the substrate;
applying holographic recording beams to the layer to form a holographic polymer dispersed liquid crystal grating comprising alternating polymer rich regions and liquid crystal rich regions; and
removing at least a portion of the liquid crystal in the liquid crystal rich regions to form a polymer surface relief grating,
wherein the liquid crystal rich regions comprise air gaps after removing at least a portion of the liquid crystal in the liquid crystal rich regions.
2. The method of claim 1 , wherein the monomer comprises acrylates, methacrylates, vinyls, isocynates, thiols, isocyanate-acrylate, and/or thioline.
3. The method of claim 2 , wherein the mixture further comprises at least one of a photoinitiator, a coinitiator, or additional additives.
4. The method of claim 2 , wherein the thiols comprise thiol-vinyl-acrylate.
5. The method of claim 3 , wherein the photoinitiator comprises photosensitive components.
6. The method of claim 1 , wherein providing a mixture of monomer and liquid crystal comprises:
mixing the monomer, liquid crystal, and at least one of a photoinitiator, a coinitiator, multifunctional thiol, or additional additives;
storing the mixture in a location absent of light at a temperature of 22° C. or less;
adding additional monomer to the mixture;
filtering the mixture through a filter of 0.6 μm or less; and
storing the filtered mixture in a location absent of light.
7. The method of claim 1 , further comprising refilling the liquid crystal rich regions with a liquid crystal material.
8. The method of claim 1 , wherein removing at least a portion of the liquid crystal comprises removing substantially all of the liquid crystal in the liquid crystal rich regions.
9. The method of claim 1 , wherein removing at least a portion of the liquid crystal further comprises leaving at least a portion of the liquid crystal in the polymer rich regions.
10. The method of claim 1 , wherein removing at least a portion of liquid crystal comprises washing the holographic polymer dispersed liquid crystal grating with a solvent.
11. The method of claim 5 , wherein the photosensitive components comprise dyes and/or radical generators.
12. The method of claim 7 , wherein the liquid crystal material has a different molecular structure than the previously removed liquid crystal.
13. The method of claim 1 , wherein the substrate comprises a glass substrate or plastic substrate.
14. The method of claim 1 , wherein the substrate comprises a transparent substrate.
15. The method of claim 1 , further comprising sandwiching the mixture between the substrate and another substrate with one or more spacers for maintaining internal dimensions.
16. The method of claim 15 , further comprising applying a non-stick release layer on one surface of the other substrate.
17. The method of claim 16 , wherein the non-stick release layer comprises a fluoropolymer.
18. The method of claim 1 , further comprising applying a protective layer over the deep SRG.
19. The method of claim 18 , wherein the protective layer comprises an anti-reflective layer.
20. The method of claim 18 , wherein the protective layer comprises silicate or silicon nitride.
21. The method of claim 18 , wherein the protective layer comprises a parylene coating.
22. The method of claim 1 , further comprising creating a vacuum in the air gaps or filling the air gaps with an inert gas.
23. The method of claim 1 , wherein removing at least a portion of liquid crystal comprises washing the holographic polymer dispersed liquid crystal grating with a solvent.
24. The method of claim 23 , wherein washing the holographic polymer dispersed liquid crystal grating comprises immersing the holographic polymer dispersed liquid crystal grating in the solvent.
25. The method of claim 1 , further comprising curing the holographic polymer dispersed liquid crystal grating.
26. The method of claim 1 , wherein curing the holographic polymer dispersed liquid crystal grating comprises exposing the holographic polymer dispersed liquid crystal grating to a low intensity white light for a period of about an hour.
27. The method of claim 1 , wherein the polymer surface relief grating extends all the way to contact the substrate.
28. The method of claim 27 , wherein the polymer surface relief grating directly contacts the substrate.
29. The method of claim 28 , wherein there is no bias layer between the polymer surface relief grating and the substrate.
30. A method for fabricating a deep surface relief grating (SRG), the method comprising:
mixing a mixture comprising a monomer, a liquid crystal, and at least one of a photoinitiator, a coinitiator, multifunctional thiol, or additional additives;
storing the mixture in a location absent of light at a temperature of 22° C. or less;
adding additional monomer to the mixture;
filtering the mixture through a filter of 0.6 μm or less;
storing the filtered mixture in a location absent of light;
providing a substrate;
coating a layer of the mixture on a surface of the substrate;
applying holographic recording beams to the layer to form a holographic polymer dispersed liquid crystal grating comprising alternating polymer rich regions and liquid crystal rich regions; and
removing at least a portion of the liquid crystal in the liquid crystal rich regions to form a polymer surface relief grating.