<title>Sub-wavelength structures for broadband diffractive optics</title>

Abstract

Diffractive optical elements in the form of surface-relief 'blaze' (echelette-type) structures diamond-turned onto the surface of conventional refractive lens elements are well-established and widely used. However, they suffer from limited broadband diffraction efficiency, which prevents the full benefits of hybrid optics from being realised. A family of diffractive optics, the blazed-binary optical element, is investigated to improve the broadband efficiency. Blazed-binary optical elements are diffractive components, composed of subwavelength (ie. with size smaller than the wavelength) ridges, pillars or other simple geometries carefully etched in a dielectric film, that mimic standard blazed-echelette diffractive elements. Their operation exploits effective-medium theory. We show that by exploiting the high dispersion of artificial material, diffractive optical elements which are blazed over a broad spectral range can be synthesized. A blazed-binary grating is designed to validate the broadband behaviour and practical aspects are investigated through the manufacture of sub-wavelength structures in a Gallium Arsenide substrate.