A team of Stanford and University of Pennsylvania engineers carried out the first experimental demonstration of a plasmonic cloaking sensor, a device that manipulates scattered light to render itself invisible across much of the visible light spectrum.
The device, a photodetector, is made up of semiconductors covered by a thin layer of metal. The experiment used silicon nanowires coated in gold, but researchers state that aluminum and copper could be substituted for gold to the same effect.
Adjusting the ratio of silicon to metal creates the cloaking effect. When light interacts with the metallic nanostructure, tiny electrical currents produce scattered light waves that separate charges in both materials. Carefully engineering the metal coat to create an equal but opposite electric charge to the silicon’s charge allows the charges to cancel each other out, creating invisibility.
The charges must align perfectly for cloaking to occur, which requires meticulous balancing of the amount of materials in the device. The cloaking effect, however, works regardless of shape and placement of the semiconductor and metal, as well as regardless of the angle of light.
“These structures can find application in broadband, chip-scale nanodevices that naturally interface with the outside world and as building blocks for transmissive metamaterials,” the paper reads. For example, the device can be used in digital cameras for sharper images.
The researchers described the device as part of a “new class” of chip-scale devices that combine the geometrical properties of a device with its materials selection to achieve both electric and optical functions.
The experiment’s results were published on May 20 in the journal Nature Photonics. Materials science and engineering doctoral candidate Pengyu Fan served as lead author, and Mark Brongersma, Keck Faculty Scholar in the School of Engineering was the senior author. Linyou Cao Ph.D. ‘09 and materials science and engineering doctoral candidate Farzaneh Afshinmanesh contributed to the research.
-Marwa Farag