Metasurfaces with asymmetric 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. An optical system comprising:
an optically transmissive substrate comprising a metasurface, the metasurface comprising:
a grating comprising a plurality of unit cells, 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; and
a reflector, wherein the grating is between the reflector and the substrate.
2. The optical system of claim 1 , wherein the reflector is spaced apart from the grating.
3. The optical system of claim 2 , wherein the grating is embedded in an optically transmissive material.
4. The optical system of claim 1 , wherein the optically transmissive material spaces the reflector apart from the grating.
5. The optical system of claim 1 , wherein the substrate comprises:
a second metasurface on a side of the substrate opposite the metasurface, the second metasurface comprising:
a second grating comprising a plurality of second unit cells, each second unit cell comprising, as seen in a top-down view:
a laterally-elongated third nanobeam; and
a laterally-elongated fourth nanobeam spaced apart from the third nanobeam by a gap, wherein the fourth nanobeam is wider than the third nanobeam.
6. The optical system of claim 1 , wherein the unit cells are laterally-elongated and are parallel to each other.
7. The optical system of claim 1 , wherein the metasurface is configured to diffract incident light of a visible wavelength into a first diffraction order.
8. The optical system of claim 1 , wherein the second width is 10 nm to 1 μm.
9. The optical system of claim 8 , wherein the second width is 10 nm to 300 nm.
10. The optical system of claim 1 , wherein a pitch of the unit cells is 10 nm to 1 μm.
11. The optical system of claim 10 , wherein the pitch of the unit cells is 10 nm to 500 nm.
12. The optical system of claim 10 , wherein the optically transmissive substrate is a waveguide plate.
13. The optical system of claim 12 , further comprising a stack of the optically transmissive substrates, wherein dimensions of features of the unit cells varies between the substrates.
14. The optical system of claim 1 , wherein the first nanobeam and the second nanobeam are separated by a gap of 10 nm to 1 μm.
15. The optical system of claim 14 , wherein the gap is 10 nm to 300 nm wide.
16. The optical system of claim 1 , wherein the optically transmissive substrate comprises a glass.
17. The optical system of claim 1 , wherein the first and second nanobeam comprises silicon.
18. The optical system of claim 17 , wherein the first and second nanobeam comprises silicon nitride.
19. The optical system of claim 1 , wherein the optically transmissive substrate and the metasurface form a polarizing beam splitter.
20. The optical system of claim 1 , wherein the metasurface is an incoupling optical element, further comprising an image injection device configured to project light to the incoupling optical element, wherein the metasurface is configured to redirect the light to propagate the light through the substrate by total internal reflection.
21. The optical system of claim 1 , wherein the metasurface is an outcoupling optical element, wherein the metasurface is configured to extract light out of the substrate.
22. A method for forming a metasurface, the method comprising:
providing an optically transmissive substrate;
forming a grating on the substrate, the grating comprising a plurality of unit cells, 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; and
providing a layer of reflective material on the grating, the layer of reflective material extending into the gap and between the unit cells, wherein the grating is between the reflector and the substrate.
23. The method of claim 22 , wherein providing the layer of reflective material comprises depositing reflective material between and over the first and second nanobeams.
24. The method of claim 23 , wherein the reflective material comprises aluminum.
25. The method of claim 22 , wherein forming the grating comprises:
depositing an optically transmissive layer over the substrate; and
patterning the optically transmissive layer to define the grating.
26. The method of claim 25 , 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.