An international team of researchers led by a group at the Australian National University (ANU) is the first to demonstrate ultra-fast transmission of information through an optical nanoantenna that has been imprinted onto an optical waveguide. These results could have significant implications for telecommunication applications, enabling high-speed data transmission through these devices.
Prior to this work, which is described in the journal Science Advances, there were very few examples in which an optical nanoantenna had been imprinted onto an optical waveguide. Additionally, those earlier examples had very limited functionalities, such as coupling light to a waveguide mode.
“What we showed is that such an antenna of sub-micron size can sort and route different streams of information (encoded into the different polarizations of light) into different directions of the waveguide,” said Dragomir Neshev, a professor at ANU, who led the research, in an e-mail interview with IEEE Spectrum. “This is a very important operation used in coherent receivers for any communication link.”
But what may be even more exciting is that Neshev and his colleagues were able to shrink the size of the optical component that performs the polarization sorting to an antenna of sub-micrometer size. This could potentially enable high-density integration of photonics components on a silicon chip.
The basis for the device is similar to the operation of a Yagi-Uda aerial, which emits and collects radio waves only in one direction. “The design of our antenna effectively integrates two such antennas for horizontal and for the vertical polarizations,” adds Neshev.
Of course, the term optical “nanoantennas” often gets used quite broadly. As a result, sometimes even a simple nanoparticle is referred to as an optical nanoantenna. To get an idea of what the ANU researchers have done it is helpful to apply a stricter definition of what an optical nanoantenna is by drawing an analogy to a radio-wave antenna.
In the radio-wave antenna analogy, the aerial has a feed element, which is connected to a cable so that the antenna detects the electromagnetic radiation from air and channels it to the cable. So in this analogy, the gold nanorods of the nanoantenna are the feed elements and the optical waveguide is the cable.
The optical nanoantenna operates through plasmonics. In plasmonics, incoming light excites electrons on the surface of a metal so that they begin to move across the metal’s surface in plasmon waves. These plasmon waves have a much smaller length than even the smallest wavelength of light. As a result, it is possible to make devices on a much smaller scale than those that would depend on light by itself. Plasmonics has led to the very real possibility of creating so-called photonic integrated circuits (ICs) in which photons could replace electrons.
Neshev explains that this latest research is so unusual because it involves expertise in plasmonics, silicon photonics for the fabrication of the waveguides, and telecommunication networks for the high-bit rate transmission.
However, Neshev concedes that more engineering needs to be undertaken before this device could be commercially developed.
“The entire structure needs to be made CMOS compatible,” said Neshev. “The currently used gold bars will have to be replaced with another metal, possibly aluminum, to be compatible with the standard CMOS fabrication.”
In the immediate future, Neshev and his colleagues will be trying to improve the device’s transmission efficiency and also fabricating circuits from several such devices.