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Photosensor sees colour like human eyes

04 Sep 2014  | Jade Boyd

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Zheng's colour photodetector uses a combination of band engineering and plasmonic gratings, comb-like aluminium structures with rows of parallel slits. Using electron-beam evaporation, which is a common technique in CMOS processing, Zheng deposited a thin layer of aluminium onto a silicon photodetector topped with an ultrathin oxide coating.

Biomimetic colour photodetector

Rice University's new biomimetic colour photodetector uses aluminium gratings like the one in this image from a scanning electron microscope. The light-filtering slits in the grating are about 100nm wide. (Source: B. Zheng/Rice University)

Colour selection is performed by utilising interference effects between the plasmonic grating and the photodetector's surface. By carefully tuning the oxide thickness and the width and spacing of the slits, Zheng was able to preferentially direct different colours into the silicon photodetector or reflect it back into free space.

The metallic nanostructures use surface plasmons—waves of electrons that flow like a fluid across metal surfaces. Light of a specific wavelength can excite a plasmon, and LANP researchers often create devices where plasmons interact, sometimes with dramatic effects.

"With plasmonic gratings, not only do you get colour tunability, you can also enhance near fields," Zheng said. "The near-field interaction increases the absorption cross section, which means that the grating sort of acts as its own lens. You get this funnelling of light into a concentrated area.

"Not only are we using the photodetector as an amplifier, we're also using the plasmonic colour filter as a way to increase the amount of light that goes into the detector," he said.

Co-authors include Rice graduate student Yumin Wang and Peter Nordlander, professor of physics and astronomy at Rice. The research was supported by the Office of Naval Research, the Department of Defense's National Security Science and Engineering Faculty Fellowship Program and the Robert A. Welch Foundation.

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