Metasurfaces with asymetric gratings for redirecting light and methods for fabricating

An optical system comprising:

• a stack of optically transmissive substrates, wherein each optically transmissive substrate of the stack is a waveguide plate, wherein each of the optically transmissive substrates comprises a multilevel metasurface, the multilevel metasurface comprising:
• a grating comprising a plurality of multilevel unit cells, each unit cell comprising, as seen in a top-down view:
• on a lowermost level of the unit cell:
• a laterally-elongated, first lowermost level nanobeam having a first width;
• and
• a laterally-elongated, second lowermost level nanobeam having a second width, wherein the second width is larger than the first width;
• and
• on an uppermost level of the unit cell:
• a laterally-elongated, first uppermost level nanobeam above the first lowermost level nanobeam;
• and
• a laterally-elongated, second uppermost level nanobeam above the second lowermost level nanobeam,
• wherein dimensions of features of the unit cells varies between the substrates, 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.