EVANSTON, Ill. --- Digital cameras have transformed the world of photography. Now new technology inspired by the human eye could push the photographic image forward even more by producing improved images with a wider field of view.
Yonggang Huang, Joseph Cummings Professor of Civil and Environmental Engineering and Mechanical Engineering at Northwestern University's McCormick School of Engineering and Applied Science, has collaborated with John Rogers, the Flory-Founder Chair Professor of Materials Science and Engineering at the University of Illinois at Urbana-Champaign, to create an array of silicon detectors and electronics that can be conformed to a curved surface. Like the human eye, the curved surface can then act as the focal plane array of the camera, which captures an image.
The results of this research will be published as the cover story in the Aug. 7 issue of the journal Nature.
On a normal camera, such electronics must lie on a straight surface, and the camera's complex system of lenses must reflect an image several times before it can reflect on the right spots on the focal plane.
"The advantages of curved detector surface imaging have been understood by optics designers for a long time, and by biologists for an even longer time," Huang says. "That's how the human eye works -- using the curved surface at the back of the eye to capture an image."
But exactly how to place those electronics on a curved surface to yield working cameras has stumped scientists, despite many different attempts over the last 20 years. The electronics lie on silicon wafers, which can only be compressed 1 percent before they break and fail. Rogers and Huang have established experimental methods and theoretical foundations, respectively, for an effective way to transfer the electronics from a flat surface to a curved one.
Their creation builds on the strength and innovation of both professors. Rogers created a hemispherical transfer element made out of a thin elastomeric membrane that can be stretched out into the shape of a flat drumhead. In this form, planar (flat) electronics can be transferred onto the elastomer. Popping the elastomer back into its hemispheric form enables the transfer of the electronics onto a hemispherical device substrate. A major challenge is that such a process applied to conventional electronics leads to catastrophic mechanical fracture in the brittle semiconductor materials.
Rogers and Huang got around this by creating an array of photodetectors and circuit elements that are so small -- approximately 100 micrometers square -- they aren't as affected when the elastomer pops back into its hemispheric shape. Think of them like buildings on the Earth -- though flat buildings are built on the curved Earth, the area they take up is so small that the curve isn't felt.
In addition, each of these devices on the array is connected by thin metal wires on plastic, which form arc-shaped structures that Huang and Rogers call "pop-up bridges." These bridges interconnect the silicon devices, thereby relaxing all of the strain associated with return of the elastomer to its curved shape.
The researchers also designed the array so that the silicon component of each device is sandwiched in the middle of two other layers, the so-called natural mechanical plane. That way, while the top layer is stretched and the bottom layer is compressed, the middle layer experiences very small stress.
When tested, more than 99 percent of the devices still worked after snapping the elastomer back to its hemispherical shape. Researchers found that the silicon in the devices was only compressed .002 percent -- far below the 1 percent compression where silicon fails.
Early images obtained using this curved array in an electronic eye-type camera indicate large-scale pictures that are much clearer than those obtained with similar, but planar, cameras, when simple imaging optics are used.
"In a conventional, planar camera, parts of the images that fall at the edges of the fields of view are typically not imaged well using simple optics," Huang says. "The hemisphere layout of the electronic eye eliminates this and other limitations, thereby providing improved imaging characteristics."
Huang and Rogers will continue to optimize the camera by adding more pixels.
"There is a lot of room for improvement, but early tests show how well this works. We believe that this is scalable, in a straightforward way, to more sophisticated imaging electronics," Huang says. "It has been a very good collaboration between the two groups."
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