Thin film micro-lenses stretch to focus
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 10×10 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.
Fig. 3: In different bending configurations, the array of transmissive FZPs enables adjacent lenses to have overlapping fields of view while focus shifts as the substrate bends. All images courtesy of Hongrui Jiang.
Rolled into a cylinder, the array was able to capture a panorama image covering a 170-degree field of view while achieving a depth of focus of about 2.5mm. The overlap between the images allowed them to be stitched with high fidelity (with a measured resolution of 8.8μm), according to the researchers, resulting in a 3.8X increase in the area of the image captured by the lens array as compared to the area captured by a single lens.
Although the actual optical properties of the micro-lenses are themselves set during manufacture, by changing the widths, diameter and number of concentric black rings, they can further be tuned in a single lens by stretching and flexing it.
Such arrays could find their way into miniaturized robotics and surveillance systems, but also in minimally invasive surgery optical systems for endoscopy or in vivo microscopy.
For demonstration purposes the researchers flexed the lens array in one axis for focus control (displacing the centrally located lens), but in order to control the optical focus across all lenses, they would have to control the position of all lenses.
But being stretchable, the film could be wrapped on a cylinder whose radius could be increased or decreased for optical focus control across the complete field of view. Jiang’s future research could be on the integration of MEMS-based actuators behind each lens for very precise radial control. He is confident that such lens arrays could enable 360º field of view micro-camera systems, possibly in combination with a conformable, plastic image sensors such as those co-developed by ISORG and Plastic Logic.
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