Gradient Index Fresnel Lens
Optical microscope image of an F#1 Fresnel microlens designed for lambda=1.5 mm. The microlens is 400 ´ 400mm, and is a 2-D “blazed binary” phase grating, meaning each phase zone is composed of a “binary mixture” of two materials, in this case silicon (index of refraction 3.4) and silicon dioxide (index of refraction 1.5). Varying the ratio of silicon to silicon dioxide smoothly across each zone creates a “blazed grating” profile in which the index of refraction varies smoothly from one edge of the zone to the other. Theoretical diffraction efficiency of such a structure can approach 100%. The index of refraction variation leads to color separation within each zone in this reflected white light image.
The variation in material composition is accomplished by etching a pattern of holes into a silicon wafer, and then oxidizing the wafer at elevated temperature in a steam atmosphere to convert exposed silicon into silicon dioxide – filling up the holes in the silicon with silicon dioxide “plugs”. If the pattern of holes has a period much smaller than the optical wavelength, no diffraction from these features is possible and the material takes on an average “effective index of refraction” that can be adjusted by changing the pattern of the holes. In this microlens the etched holes are 160 nm in diameter, which produce 214 nm diameter silicon dioxide plugs after oxidation. The hole period is everywhere smaller than 700 nm.
For more information, see: David L. Dickensheets, Hans Peter Herzig, Wataru Nakagawa, Kaspar Suter,Urs Staufer, “Nanostructured effective-index micro-optical devices based on blazed 2-D sub-wavelength gratings with uniform features on a variable-pitch,” 2008 IEEE/LEOS International Conference on Optical MEMS, Freiburg, Germany, August 11-14, 2008.