Sub-Wavelength Structures for Waveguide In-Coupling

An optical device includes a waveguide including a first medium, which is transparent and has a first index of refraction at a target wavelength and which has mutually-parallel first and second surfaces arranged so that light at the target wavelength propagates within the waveguide by internal reflection between the first and second surfaces. A coupling layer is disposed over the first surface of the waveguide and includes a second medium having a second index of refraction at the target wavelength, which is greater than the first index of refraction, and is patterned to define a periodic array of cylinders, which have respective cylinder axes perpendicular to the first surface and have respective heights and diameters that are smaller than the target wavelength, and which are spaced apart such that a distance between each of the cylinders and a neighboring cylinder in the array is less than the target wavelength.