Plasmonics Creates Invisible Photodetector

For the first time, plasmonic cloaking has been used to create an invisible light-detecting device that can sense light without being seen. It could lead to a new class of devices that controls the flow of light at the nanoscale to produce both optical and electronic functions.

At the heart of the Stanford and University of Pennsylvania device are gold-covered silicon nanowires. By adjusting the ratio of metal to silicon — a technique called tuning the geometries — the engineers capitalized on favorable nanoscale physics to get the light reflected from the two materials to cancel each other out, making the device invisible. This phenomenon is known as destructive interference.

An image showing light scattering from a silicon nanowire running diagonally from bottom left to top right. 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. (Photos: Stanford Nanocharacterization Lab)

The rippling lightwaves in the metal and semiconductor created a separation of positive and negative charges in the materials, known as a dipole moment. The key was 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.

“We found that a carefully engineered gold shell dramatically alters the optical response of the silicon nanowire,” said Pengyu Fan, a materials science and engineering doctoral candidate at Stanford. “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.”

This scanning electron microscope image shows the structure of the invisible photodetector created by engineers at Stanford. The dark-gray area at the center is a silicon nanowire. The bright cap is made of gold.

The engineers demonstrated that the plasmonic cloak is effective across much of the visible spectrum 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 also showed that common computer chip metals such as aluminum and copper work just as well as gold.

What matters most for producing invisibility is tuning the metal and semiconductor.

Schematic of the Stanford invisible photodetector.

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

The engineers are hopeful that the tunable metal-semiconductor devices could be used for solar cell, solid-state lighting, sensor, chip-scale laser applications and more. In digital camera and advanced imaging systems, for example, plasmonically cloaked pixels might reduce the disruptive crosstalk between neighboring pixels that produces blur. It could lead to sharper, more accurate photos and medical images.

Pengyu Fan

“We can even imagine re-engineering existing optoelectronic devices to incorporate valuable new functions and to achieve sensor densities not possible today,” said Mark Brongersma, a professor at Stanford and senior author of the study. “There are many emerging opportunities for these plasmonic building blocks.”

The study appeared online May 20 in Nature Photonics.

For more information, visit: engineering.stanford.edu