Design and fabrication issues in subwavelength diffractive optical elements

Present lithography techniques allow etching of deep binary subwavelength gratings, i.e. periodic structures with a spacing much finer than the wavelength and a depth on the order of the wavelength for operation in the visible or near-infrared region. If nearly periodic rather than strictly periodic elements are considered, this opens the way for a new generation of optical components, where phase is modulated by changing the duty cycle of the grating, while preserving the binary character for easier fabrication. The component can be considered to have a variable local effective index. We report on design issues in relation with the modal structure of binary gratings and on our first experimental results that demonstrate lenses, prisms and beam splitters.