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 is the first example of what the researchers describe as 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 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 capitalize on favorable nanoscale physics in which the reflected light from the two materials cancel each other to make the device invisible.
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. Photo: Stanford Nanocharacterization Lab.
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.
“We found that a carefully engineered gold shell dramatically alters the optical response of the silicon nanowire,” said Fan. “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.”
“It seems counterintuitive,” said Brongersma, “but you can cover a semiconductor with metal – even one as reflective as gold – and still have the light get through to the silicon. As we show, the metal not only allows the light to reach the silicon where we can detect the current generated, but it makes the wire invisible, too.”
In the future, the engineers foresee application for such tunable, metal-semiconductor devices in many relevant areas, including solar cells, sensors, solid-state lighting, chip-scale lasers, and more.
In digital cameras and advanced imaging systems, for instance, plasmonically cloaked pixels might reduce the disruptive cross-talk between neighboring pixels that produces blur. It could therefore lead to sharper, more accurate photos and medical images.
“We can even imagine reengineering existing opto-electronic devices to incorporate valuable new functions and to achieve sensor densities not possible today,” concluded Brongersma. “There are many emerging opportunities for these photonic building blocks.”
Design and operation of a hybrid gold/silicon nanowire photodetector
Nanotechnology has enabled the realization of hybrid devices and circuits in which nanoscale metal and semiconductor building blocks are woven together in a highly integrated fashion. In electronics, it is well known how the distinct material-dependent properties of metals and semiconductors can be combined to realize important functionalities, including transistors, memory and logic. We describe an optoelectronic device for which the geometrical properties of the constituent semiconductor and metallic nanostructures are tuned in conjunction with the materials properties to realize multiple functions in the same physical space. In particular, we demonstrate a photodetector in which the nanoscale electrical contacts have been designed to render the device ‘invisible’ over a broad frequency range. The structure belongs to a new class of devices that capitalize on the notion that nanostructures have a limited number of resonant, geometrically tunable optical modes whose hybridization and intermodal interference can be tailored in a myriad of useful ways.
The middle segment of a bare Si nanowire (labeled “bare”) appears yellow while the two Au covered segments (labeled “covered”) appear green in color; b. This image shows a Si nanowire of which the diameter gradually decreases from the bottom left to the top right). The bare segments of the wire exhibit a color that changes from orange to green. The neighboring covered segments show a color change from yellow/green to blue/green. These observed color changes are consistent with the scattering spectra acquired by measurements and simulations presented in the main text. From the images above, it is also clear that for both wires, the covered segments appear thinner and exhibit a significantly reduced scattering intensity as compared to the neighboring bare segments. This is consistent with the predicted reduction in the scattering cross section of a Si nanowire due to plasmonic cloaking with by a properly designed Au cover.