Sub-wavelength structures for waveguide in-coupling

A method for producing an optical device, the method comprising:

• providing a waveguide comprising 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;
• and
• applying over the first surface of the waveguide a coupling layer comprising a second medium having a second index of refraction at the target wavelength, which is greater than the first index of refraction;
• and
• patterning the coupling layer 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,
• wherein the periodic array comprises a matrix of unit cells, wherein each of the unit cells comprises two or more of the cylinders in a predefined spatial relation, which is uniform over the unit cells.

A method for coupling light, comprising:

• directing a beam of light at a target wavelength toward a first surface of a waveguide comprising a first medium, which is transparent and has a first index of refraction at the 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;
• and
• coupling the light from the beam into the waveguide using a coupling layer, which is disposed over the first surface of the waveguide and comprises a second medium having a second index of refraction at the target wavelength, which is greater than the first index of refraction, and which 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,
• wherein the periodic array comprises a matrix of unit cells, wherein each of the unit cells comprises two or more of the cylinders in a predefined spatial relation, which is uniform over the unit cells.