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140396692Engineers from Stanford and the University of Pennsylvania have used “plasmonic cloaking” in order to create a device that is able to see, without being seen.  The engineers have shown that a coating of reflective metal can make something less visible by creating an invisible, light-detecting device.  The device has silicon nano wires that are covered by a thin cap of gold, and by adjusting the ratio of metal to silicon, the engineers were able to make the reflected light from the two materials cancel each other out. 

Silicon generates electrical current when illuminated, and is commonly found in solar panels and light sensors.  However, the new device uses a relatively new concept known as plasmonic cloaking to make the device invisible.  The engineers created a plasmonic cloak in which the scattered light from the metal and semiconductor cancel each other out, which is known as destructive interference.  The key, according to the researchers, is to create a dipole in the gold that is equal in strength, but opposite in sign to the dipole in the silicon.  Creating equally strong positive and negative dipoles makes them cancel each other out, enabling the system to become invisible.

 “We found that a carefully engineered gold shell dramatically alters the optical response of the silicon nanowire,” Pengyu Fan, the lead author of a paper published online in the journal Nature Photonics, said in a press 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.”  Professor Mark Brongersma, an engineer on the project, said you can cover a semiconductor with metal and still have the light get through to the silicon.

 “As we show, the metal not only allows the light to reach the silicon where we can detect the current generated, but it makes the wire invisible, too,” Brongersma said in a press release.  The researchers say that plasmonic cloaking is effective across most of the visible spectrum of light, and the effect works regardless of the angle of incoming light or the shape and placement of the metal-covered nano wires in the device.

 They said that in order to make a device invisible, what matters the most is the tuning of metal and semiconductor.  “If the dipoles do not align properly, the cloaking effect is lessened, or even lost,” Fan said in the press release. “Having the right amount of materials at the nanoscale, therefore, is key to producing the greatest degree of cloaking.”  The engineers say the device could be used in devices like solar cells, sensors, solid-state lighting, and chip-scale lasers.

 They said in digital cameras, plasmonically cloaked pixels could reduce the disruptive cross-talk between neighboring pixels which could lead to sharper images.  “We can even imagine reengineering existing opto-electronic devices to incorporate valuable new functions and to achieve sensor densities not possible today,” said Brongersma. “There are many emerging opportunities for these photonic building blocks.”  


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