Researchers have fabricated a silicon optical antenna that is somewhat like an extremely small, special kind of prism. This is because when a red light shines on the optical antenna, the light turns right, but when the light is another colour such as orange, it turns left. This unusual property, which is called "bidirectional colour scattering," enables the optical antenna to function effectively as a passive wavelength router for visible light.
The device could have applications for innovative light sensors, light-matter manipulation, and optical communication.
The new optical antenna was developed by a team of researchers, Jiaqi Li et al., at imec (Interuniversity MicroElectronics Center) and the University of Leuven (KU Leuven), both in Leuven, Belgium. Their work is published in a recent issue of Nano Letters.
Although optical antennas are a relatively new area of research, they are simply the optical version of the radio and microwave antennas that most people are familiar with, which are commonly used for receiving and transmitting signals in radios, cell phones, and Wi-Fi.
In general, an antenna's size corresponds to the wavelengths that it was designed for. Since radio and microwave waves are on the scale of millimeters to kilometers, these antennas can be quite large. Since the wavelength of visible light is on the scale of a few hundred nanometers, tuning in to these signals requires nanosized antennas, which are much more difficult to fabricate.
Over the past few years, the imec and KU Leuven team has been exploring the possibilities of directional light manipulation at these length scales using an antenna consisting of just a single element. In 2013, using gold nanoantennas, they were able to demonstrate the world's smallest unidirectional optical antenna, in the shape of the letter V. These metallic antennas support so-called "plasmonic modes," which are fundamentally different form the optical modes supported by a dielectric antenna.
Now, by switching to a dielectric V-shaped antenna made from silicon, the researchers could achieve bidirectional scattering, in contrast to unidirectional scattering in the case of using gold. In bidirectional scattering, the scattering direction depends on the wavelength of the incoming (incident) light.
The shift in direction is gradual. For example, as the wavelength decreases from 755 nm to 660 nm, the scattering direction gradually changes from the leftward to the rightward direction. The specific wavelengths can be tuned by engineering slight adjustments to the size and shape of the antenna.
"With our work, we demonstrate that by carefully engineering the geometry of a single piece of silicon with dimensions smaller than the light wavelength, it is possible to efficiently direct visible and near-infrared light of different colours into different directions," said coauthor Niels Verellen, a physicist at imec and KU Leuven.
"This, for instance, was not possible with just symmetric particles or similarly shaped metallic (plasmonic) antennas."
Using silicon offers several advantages compared to using gold. For instance, silicon circumvents Ohmic absorption losses, which is one of the main drawbacks of plasmonic nanoantennas. In addition, the silicon antennas have a large scattering cross-section, which means they can interact with light very efficiently. Silicon is also a fully CMOS-compatible material, allowing straightforward integration in large-scale opto-electronic device fabrication.