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.

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. They have created an invisible, light-detecting device that can "see without being seen."

At the heart of the device are silicon nanowires covered by a thin cap of gold. By adjusting the ratio of metal to silicon - a technique the engineers refer to as tuning the geometries – they capitalise on favorable nanoscale physics in which the reflected light from the two materials cancel each other to make the device invisible.

Pengyu Fan is the lead author of a paper demonstrating the new device published online Sunday in the journal Nature Photonics. He is a doctoral candidate in materials science and engineering working in Associate Professor Mark Brongersma's group. Brongersma, a Keck Faculty Scholar at Stanford's School of Engineering, is senior author of the study.

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.