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Flexi Detectors Sharpen Pics

Photonics.com
Jan 2009
MADISON, Wis., Jan. 14, 2008 – The photo distortion created by conventional photo lenses could be eliminated by new curved photodetectors made from extremely thin, flexible sheets of germanium, a light-sensitive material often used in high-end imaging sensors.

University of Wisconsin-Madison electrical and computer engineering associate professor Zhenqiang Ma and colleagues developed the light-sensitive material that could revolutionize photography and other imaging technologies and make distorted cell phone photos and big, clunky telephoto lenses things of the past.
CurvedPhotodetector.jpg
This curved photodetector array was developed by University of Wisconsin-Madison electrical and computer engineering associate professor Zhenqiang Ma and colleagues. Inspired by the human eye,it is made of flexible germanium and could eliminate the photo distortion that occurs in conventional lenses. (Image courtesy of Zhenqiang Ma.)
When a device records an image, light passes through a lens and lands on a photodetector – a light-sensitive material such as the sensor in a digital camera. However, a lens bends the light and curves the focusing plane. In a digital camera, the point where the focusing plane meets the flat sensor will be in focus, but the image becomes more distorted the farther it is from that focus point.

“If I take a picture with a cell phone camera, for example, there is distortion,” Ma said. “The closer the subject is to the lens, the more distortion there is.”

That’s why some photos can turn out looking like images in a funhouse mirror, with an enlarged nose or a hand as big as a head.

High-end digital cameras correct this problem by incorporating multiple panes of glass to refract light and flatten the focusing plane. However, such lens systems – such as the mammoth telephoto lenses that sports photographers use – are large, bulky and expensive. Even high-quality lenses stretch the edges of an image somewhat.

Inspired by the human eye, Ma’s curved photodetector could eliminate that distortion. In the eye, light enters though a single lens, but at the back of the eye, the image falls upon the curved retina, eliminating distortion.

“If you can make a curved imaging plane, you just need one lens,” Ma said. “That’s why this development is extremely important.”

Ma and his group can create curved photodetectors with specially fabricated nanomembranes – extremely thin, flexible sheets of germanium, a very light-sensitive material often used in high-end imaging sensors. Researchers then can apply the nanomembranes to any polymer substrate, such as a thin, flexible piece of plastic. Currently, the group has demonstrated photodetectors curved in one direction, but Ma hopes next to develop hemispherical sensors.

“We can easily realize very high density flexible and sensitive imaging arrays because the photodetector material germanium itself is extremely bendable and extremely efficient in absorbing light,” he said. Ma’s co-authors include UW-Madison materials science and engineering professor Max Lagally and University of Michigan professor Pallab Bhattacharya.

Their technology is featured on the cover of the Jan. 5 issue of Applied Physics Letters.

For more information, visit: www.wisc.edu


GLOSSARY
camera
A light-tight box that receives light from an object or scene and focuses it to form an image on a light-sensitive material or a detector. The camera generally contains a lens of variable aperture and a shutter of variable speed to precisely control the exposure. In an electronic imaging system, the camera does not use chemical means to store the image, but takes advantage of the sensitivity of various detectors to different bands of the electromagnetic spectrum. These sensors are transducers...
germanium
A crystalline semiconductor material that transmits in the infrared.
image
In optics, an image is the reconstruction of light rays from a source or object when light from that source or object is passed through a system of optics and onto an image forming plane. Light rays passing through an optical system tend to either converge (real image) or diverge (virtual image) to a plane (also called the image plane) in which a visual reproduction of the object is formed. This reconstructed pictorial representation of the object is called an image.
lens
A transparent optical component consisting of one or more pieces of optical glass with surfaces so curved (usually spherical) that they serve to converge or diverge the transmitted rays from an object, thus forming a real or virtual image of that object.
photodetector
A device used to sense incident radiation.
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
polymer
A material whose molecular structure consists of long chains made up by the repetition of many (usually thousands) of similar groups of atoms.
sensor
1. A generic term for detector. 2. A complete optical/mechanical/electronic system that contains some form of radiation detector.
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