SURFACE-RELIEF GRATING WITH PATTERNED REFRACTIVE INDEX MODULATION
Techniques disclosed herein relate generally to surface-relief structures. In one embodiment, a surface-relief grating includes a plurality of grating ridges. The plurality of grating ridges includes a first set of grating ridges characterized by a first refractive index, and a second set of grating ridges interleaved with the first set of grating ridges and characterized by a second refractive index different from the first refractive index. The plurality of grating ridges is imprinted in a polymer layer by a nanoimprint lithography process and is exposed to a light pattern to form the first set of grating ridges and the second set of grating ridges that have different refractive indices.
1 . A method comprising:
imprinting, in an organic material layer, a surface-relief structure including a plurality of grating ridges, the organic material layer including a photosensitive base resin that includes monomers;
generating a first light pattern that includes bright regions and dark regions; and
exposing the surface-relief structure to the first light pattern to polymerize the monomers in a set of grating ridges of the plurality of grating ridges in the bright regions and increase a refractive index of the set of grating ridges of the plurality of grating ridges in the bright regions.
2 . The method of claim 1 , wherein an intensity of the first light pattern varies in one dimension, two dimensions, or three dimensions.
3 . The method of claim 1 , wherein generating the first light pattern includes:
illuminating, by a light beam having a uniform intensity in a cross-section of the light beam, a photomask or a diffractive optical device; or
generating an interference pattern using two coherent light beams.
4 . The method of claim 1 , wherein:
the first light pattern includes a converging light pattern; and
the first light pattern has a highest intensity in the set of grating ridges of the plurality of grating ridges in the bright regions.
5 . The method of claim 1 , further comprising:
depositing an overcoat layer on the organic material layer, the overcoat layer filling regions between the plurality of grating ridges and having a refractive index different from the refractive index of the set of grating ridges.
6 . The method of claim 1 , further comprising:
generating a second light pattern; and
exposing the surface-relief structure to the second light pattern.
7 . The method of claim 6 , wherein the first light pattern and the second light pattern expose different regions of the surface-relief structure.
8 . The method of claim 6 , wherein the first light pattern and the second light pattern expose different layers of the surface-relief structure.
9 . The method of claim 6 , wherein the first light pattern and the second light pattern has different maximum intensities.
10 . The method of claim 6 , wherein an exposure time of exposing the surface-relief structure to the second light pattern is different from an exposure time of exposing the surface-relief structure to the first light pattern.
11 . The method of claim 1 , wherein:
the first light pattern includes a plurality of converging light beams generated by an array of micro-lenses; and
the plurality of converging light beams has a highest intensity at a same layer of the surface-relief structure.
12 . The method of claim 1 , wherein the first light pattern includes a binary light pattern.
13 . The method of claim 1 , wherein the first light pattern includes a gray-scale light pattern having more than two different intensity levels.
14 . The method of claim 1 , wherein the surface-relief structure includes a slanted surface relief grating.
15 . The method of claim 1 , wherein the organic material layer includes nanoparticles with refractive indices greater than 1.7.