Thin film micro-lenses stretch to focus

November 26, 2015 // By Julien Happich
The bio-inspired micro-lens array designed by University of Wisconsin-Madison researchers stemmed from the observation of insects' multi-faceted eyes which typically yield a very large field of vision (some dragonflies practically boast 360º surround vision).

Multi-camera domed structures are often used to achieve a large field of view on a fixed macro level. But here the researchers looked at fabricating arrays of micrometre-sized lenses on a thin flexible film whose curvature could determine not only the lenses’ field of view but also their depth of focus.

In order to be able to build their micro-lenses on a flexible polymer film as thin as 100μm, the researchers opted for a flat Fresnel Zone Plate (FZP) configuration, whereby instead of relying on refraction to control the light path, as it would be the case for conventional optical lenses (with layers of varying refraction indexes), they rely on a concentric diffraction barrier that bends the light as it passes its edges.

Lead by Hongrui Jiang, professor of electrical and computer engineering at UW-Madison, the researchers created a 10x10 array of Fresnel zone plate micro-lenses at a 500μm pitch, each lens being about 450μm in diameters. The whole array measured about 5x5mm.

Although they started on a 350μm thick silicon wafer with various lithographic and etching steps to prepare the growth of silicon nanowires (black silicon for the concentric light blocking rings of the Fresnel zone plate), the researchers then coated the wafer with a 100μm thick layer of PDMS before removing the silicon on the backside of the wafer through a dry, bulk etching process, effectively obtaining a fully flexible and transparent polymer film with the Fresnel zone plane embedded into it.



Fig. 1: Silicon nanowires form the dark areas of the FZPs (a,b), they are embedded as concentric rings into the PDMS matrix (c) to form the FZPs (d). The completed arrays can be flexed and stretched (e,f).


The tiny array of lenses can then be bent and stretched into different configurations, either to augment the overall field of view (by stitching the images obtained through the adjacent lenses) or to change the depth of focus as the central lens moves back and forth with the flexing action.

Fig. 2: The cylindrical arrangement allowed researchers to resolve a 170-degree field of view.