"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." "We found that a carefully engineered gold shell dramatically alters the optical response of the silicon nanowire," said Fan. When equally strong positive and negative dipoles meet, they cancel each other and the system becomes invisible. 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. 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. These currents, in turn, produce scattered light waves.īy 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 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. 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 Silicon generates electrical current when illuminated and is common in solar panels and light sensors today. Light detection is well known and relatively simple.
Brongersma, a Keck Faculty Scholar in Stanford's School of Engineering, is senior author of the study. He is a doctoral candidate in materials science and engineering working in Associate Professor Mark Brongersma's group. Pengyu Fan is the lead author of a paper demonstrating the new device published online Sunday in the journal Nature Photonics.
By adjusting the ratio of metal to silicon – a technique the engineers refer to as tuning the geometries – they capitalize on favorable nanoscale physics in which the reflected light from the two materials cancel each other to make the device invisible.
They have created an invisible, light-detecting device that can "see without being seen."Īt the heart of the device are silicon nanowires covered by a thin cap of gold. It may not be intuitive, but a coating of reflective metal can actually make something less visible, engineers at Stanford and the University of Pennsylvania have shown. The brighter areas are bare silicon while the dimmer sections are coated with gold demonstrating how plasmonic cloaking reduces light scattering in the gold-coated sections. An image showing light scattering from a silicon nanowire running diagonally from bottom left to top right.
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