Subwavelength structures open up the possibility to create an artificial index material which enables the realization of high-efficient diffractive structures. This can be used to generate optical elements with nearly arbitrary phase profiles. We demonstrate the realization of computer-generated holograms based on this effective medium approach. High diffraction efficiencies can be realized by multi-phase-level modulation based on two-dimensional binary nanostructures. The fabrication is performed by one lithographical step using a high-speed e-beam writer which allows high-resolution patterning even on large areas. A diffractive element in the visible range is experimentally demonstrated using the presented effective index approach.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.