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Physicists at the University of Pennsylvania and Stanford University have created a nearly-invisible photodetector, by exploiting the different ways that silicon and gold scatter light.

By tuning the nanoscale geometries of silicon nanowires, using a gold doping, the team has demonstrated that the destructive interference of the scattered light can render the device nearly invisible – while it is still able to operate as a photodetector.

The work, published in Nature Photonics (abstract as PDF), is based on the way that light interacts with, and produces currents in, the surfaces of metals and semiconductors (plasmonics).

Bright areas are bare silicon in a nanowire.

Dimmer gold-coated areas are plasmonically "cloaked".

Image: Stanford Nanocharacterization Lab

In the University of Pennsylvania demonstration, the light waves create a dipole (separation of positive and negative charges) in the two materials. The tuning of the surfaces results in a dipole in the gold that is equal in strength to that in the silicon, but opposite in sign. Where the equal-but-opposite dipoles meet, the material becomes invisible.

“We found that a carefully engineered gold shell dramatically alters the optical response of the silicon nanowire,” says Pengyu Fan, doctoral candidate from Standford University and lead author of the paper, in Stanford’s release.

“Light absorption in the wire drops slightly – by a factor of just four – but the scattering of light drops by 100 times due to the cloaking effect, becoming invisible.”

The universities say the cloaking effect works across much of the visible light spectrum, and even more usefully, it is not dependent on having exactly the right angle of incoming light. The effect is most dependent on the tuning of the metal-semiconductor combination – and it has been shown to work with other doping materials like copper and aluminium.

“If the dipoles do not align properly, the cloaking effect is lessened, or even lost,” said Fan. “Having the right amount of materials at the nanoscale, therefore, is key to producing the greatest degree of cloaking.”

One application cited by Stanford for the cloaking effect is in cameras and medical imaging, to reduce the inter-pixel crosstalk that can lead to blurring. ®

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