Thin film deposition: an alternative technique for the fabrication of binary optics with high efficiency

Binary optics were fabricated by an ion-beam sputter deposition technique on different substrate materials for the wavelengths of 1.52 μm and 0.63 μm respectively. The blazed profile of thin film dielectric gratings was approximated by a 2 and 4 phase level profile and the kinoform profile in each zone of the Fresnel zone lenses was approximated by a 2,4,8,16 and a 32 level profile. Such stepped profiles were realized with several masks, written with electron-beam and with photolithographic technology. The microlenses were coated with an anti-reflection coating. The reflection of these lenses was optimized on the basis of an angular spectrum approach. A minimum reflectivity as low as 1×10^-4 was realized using in situ controlled multilayers of TiO₂ and SiO₂. The measurements reveal, that the spot-sizes of the fabricated microlenses are close to the diffraction limited values. The highest measured diffraction efficiencies for the 32 level structures are 97%. The effects of fabrication errors, such as level heights, alignment and linewidths errors, on the diffraction efficiency of Fresnel zone lenses were discussed