Cloak and swagger: Engineers use plasmonics to create an invisible photodetector.

A team of engineers at Stanford and the University of Pennsylvania has for the first time used "plasmonic cloaking" to create a device that can see without being seen – an invisible machine that detects light.

Light detection is well known and relatively simple. Silicon generates electrical current when illuminated and is common in solar panels and light sensors today. The Stanford device, however, is a departure in that for the first time it uses a relatively new concept known as plasmonic cloaking to render the device invisible

The field of plasmonics studies how light interacts with metal nanostructures and induces tiny oscillating electrical currents along the surfaces of the metal and the semiconductor. These currents, in turn, produce scattered light waves.

By carefully designing their device – by tuning the geometries – the engineers have created a plasmonic cloak in which the scattered light from the metal and semiconductor cancel each other perfectly through a phenomenon known as destructive interference.

The rippling light waves in the metal and semiconductor create a separation of positive and negative charges in the materials – a dipole moment, in technical terms. The key is to create a dipole in the gold that is equal in strength but opposite in sign to the dipole in the silicon. When equally strong positive and negative dipoles meet, they cancel each other and the system becomes invisible.

The engineers have shown that plasmonic cloaking is effective across much of the visible spectrum of light and that the effect works regardless of the angle of incoming light or the shape and placement of the metal-covered nanowires in the device. They likewise demonstrate that other metals commonly used in computer chips, like aluminum and copper, work just as well as gold

To produce invisibility, what matters above all is the tuning of metal and semiconductor.

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