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Light detector on a chip shines path leading to new apps

Posted: 28 Sep 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Vanderbilt University? sensor? CPL? integrated circularly polarised light? light detector?

The metamaterial that the researchers developed to detect polarised light consists of silver nanowires laid down in a sub-microscopic zig-zag pattern on an extremely thin sheet of acrylic fixed to an optically thick silver plate. This metamaterial is attached to the bottom of a silicon wafer with the nanowire side up.

How CPL passes through the silicon chip

Illustration of how CPL passes through the silicon chip and is absorbed by the metamaterial. (Valentine Lab/Vanderbilt)

The nanowires generate a cloud of free-flowing electrons that produce "plasmon" density waves that efficiently absorb energy from photons that pass through the silicon wafer. The absorption process creates "hot" or energetic electrons that shoot up into the wafer where they generate a detectable electrical current.

The zig-zag pattern can be made either right-handed or left-handed. When it is right-handed, the surface absorbs right CPL and reflects left CPL. When it is left-handed it absorbs left CPL and reflects right CPL. By including both right-handed and left-handed surface patterns, the sensor can differentiate between right and left CPL.

There have been two previous efforts to make solid-state polarized light detectors. According to Li, one used chiral organic materials that are unstable in air, worked only in a narrow range of wavelengths and had a limited power range. Another was based on a more complicated multilayer design that only worked at low temperatures.

Light detector's capability

Three images of the same surface demonstrate the detector's capability. The researchers coated the surface with right- and -left-handed metamaterial in the form of the Vanderbilt logo. The image on the left was taken in plain polarised light. The one in the centre was taken with left-handed CPL. And the image on the right was taken with right-handed CPL. (Valentine Lab/Vanderbilt)

"That is the beauty of metamaterials: You can design them to work in the fashion you desire," said Li.

The efficiency of their prototype is 0.2 per cent, too low to be commercially viable. Now that they have proven the viability of their approach, however, they have a number of ideas for how they can boost the efficiency to a level comparable to conventional photodetectors.

The research was supported by National Science Foundation grant CBET-1336455, Office of Naval Research grant N00014-14-1-0475, U.S. Army Research Office grant W911NF-12-1-0407 and the Volkswagen Foundation.

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