An aluminium surface grating can add colour selectivity to CMOS chips, without separate colour filters, claims a team of US researchers.

The grating works by surface plasmon – where photons interact with a wave of electrons.

It is deposited over oxide on a silicon photodetector. “By carefully tuning the oxide thickness and the width and spacing of the slits, it preferentially directs different colours into the photodetector or reflects it back into free space,” said Rice University, which hosts part of the team.

Colour selection comes from interference between the plasmonic grating and the photodetector’s surface, which also focusses.

“With plasmonic gratings, not only do you get colour tunability, you can also enhance near fields,” said Rice 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 funnelling of light into a concentrated area.

The light-filtering slits in the grating are about 100nm (see diagram)

The project was inspired by cephalopod (squid, for example) skin which may be colour sensitive – enabling cephalopods to change colour to match their surroundings despite their eyes being colour blind, said Rice.

The above story is based on materials by Electronics Weekly. 
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