CMOS-compatible photodetector 'sees' in colour

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Researchers at Rice University have created an image sensor that integrates light amplifiers and colour filters directly into pixels, enabling colour detection similar to the human eye.

The biomimetic device is expected to pave the way toward smaller, less complex and more organic designs for CMOS sensors and other photodetectors used in cameras. While conventional photodetectors convert light into electrical signals, they have no inherent colour sensitivity. To capture colour images, manufacturers have to add colour filters that can separate a scene into red, green and blue colour components. However, this approach adds bulk to the sensor, and the filters gradually degrade under exposure to sunlight. "Today's colour filtering mechanisms often involve materials that are not CMOS compatible, but this new approach has advantages beyond on chip integration," said lead researcher Naomi Halas. "It's also more compact and simple, and more closely mimics the way living organisms 'see' colours." The Rice team's colour photodetector relies on a combination of band engineering and plasmonic gratings – comb-like aluminum structures with rows of parallel slits. Using electron-beam evaporation, which is a common technique in CMOS processing, the researchers deposited a thin layer of aluminum onto a silicon photodetector topped with an ultra thin oxide coating. Colour selection was 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, the team was able to direct different colours into the silicon photodetector or reflect it back into free space. "With plasmonic gratings, not only do you get colour tunability, you can also enhance near fields," said researcher Bob Zheng. "The near field interaction increases the absorption cross section, which means that the grating sort of acts as its own lens. You get this funneling 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."