Metasurfaces with asymetric gratings for redirecting light and methods for fabricating
An optical system comprises an optically transmissive substrate comprising a metasurface which comprises a grating comprising a plurality of unit cells. Each unit cell comprises a laterally-elongated first nanobeam having a first width; and a laterally-elongated second nanobeam spaced apart from the first nanobeam by a gap, the second nanobeam having a second width larger than the first width. A pitch of the unit cells is 10 nm to 1 μm. The heights of the first and the second nanobeams are: 10 nm to 450 nm where a refractive index of the substrate is more than 3.3; and 10 nm to 1 μm where the refractive index is 3.3 or less.
1. A method for forming a metasurface, the method comprising:
providing an optically transmissive substrate;
forming a grating comprising a plurality of unit cells across a major surface of the substrate, each unit cell comprising, as seen in a top-down view:
a laterally-elongated first nanobeam having a first width; and
a laterally-elongated second nanobeam spaced apart from the first nanobeam by a gap, the second nanobeam having a second width larger than the first width,
depositing a layer of optically transmissive spacer material in the gap and between the unit cells; and
depositing a reflective layer on the layer of spacer material, wherein the spacer material separates the grating from the reflective layer, wherein the reflective layer comprises a reflective metal material that extends homogeneously and continuously over and across the plurality of unit cells.
2. The method of claim 1 , wherein the spacer material has a refractive index of 1 to 2.
3. The method of claim 1 , wherein the reflective layer comprises aluminum.
4. The method of claim 1 , wherein forming the grating comprises:
depositing an optically transmissive layer over the substrate; and
patterning the optically transmissive layer to define the grating.
5. The method of claim 4 , wherein patterning the optically transmissive layer comprises:
providing a resist layer over the optically transmissive layer;
defining a pattern in the resist layer; and
transferring the pattern from the resist layer to the optically transmissive layer.
6. The method of claim 1 , wherein the first nanobeam extends parallel to the second nanobeam.
7. The method of claim 1 , wherein the optically transmissive substrate comprises a glass.
8. A method for forming a metasurface, the method comprising:
providing an optically transmissive substrate;
providing an optically transmissive layer over the substrate; and
patterning the optically transmissive layer to define a plurality of repeating units, each repeating unit comprising, as seen in a top-down view:
a laterally-elongated first nanobeam having a first width; and
a laterally-elongated second nanobeam spaced-apart from the first nanobeam by a gap, the second nanobeam having a second width larger than the first width; and
depositing an optically transmissive material on the first and second nanobeams and into the gaps between the nanobeams to form spaced apart masses of the optically transmissive material above the nanobeams.
9. The method of claim 8 , wherein the optically transmissive material has a higher refractive index than either the patterned resist or the substrate.
10. The method of claim 8 , wherein patterning the optically transmissive layer comprises patterning resist.
11. The method of claim 10 , wherein patterning the resist comprises imprinting the pattern into the resist.
12. The method of claim 10 , wherein depositing the optically transmissive material comprises spin coating the optically transmissive material on the patterned resist.
13. The method of claim 10 , wherein depositing the optically transmissive material comprises performing a conformal deposition or a directional deposition of the optically transmissive material.
14. The method of claim 13 , wherein the conformal deposition comprises chemical vapor deposition or atomic layer deposition of the optically transmissive material.
15. The method of claim 13 , wherein the directional deposition comprises evaporation or sputtering the optically transmissive material.
16. The method of claim 8 , wherein the first width is 10 nm to 250 nm.
17. The method of claim 16 , wherein the second width is 10 nm to 300 nm.
18. The method of claim 8 , wherein a pitch of the unit cells is 300 nm to 500 nm.
19. The method of claim 8 , wherein the first nanobeam and the second nanobeam are separated by a gap of 10 nm to 300 nm.
20. The method of claim 8 , wherein the optically transmissive substrate is a waveguide.