Embodiments of the present invention relate to planar optical devices composed of one or more sub-wavelength diffraction grating layers. In one embodiment, an optical device includes a first substantially planar reflective structure (104,1904), a second substantially planar reflective structure (106,1906), and a substantially planar sub-wavelength grating layer (102,1902) disposed between the first reflective structure and the second reflective structure. The grating layer is configured with lines (208-211,214-217) having line widths, line thicknesses, and line period spacing selected to control phase changes in different portions of a beam of light transmitted through the optical device.