A new optical antenna could improve the efficiency of devices that handle just a few photons at a time, such as quantum computers and quantum cryptography circuits. The antenna, which receives and transmits light in one direction, is a few hundred nanometers in size. With five nanoscopic gold bars of diminishing length sitting across one larger bar, it resembles a directional TV antenna.
Niek van Hulst, a professor at the Institute of Photonic Sciences in Barcelona, Spain, led the development of the nano-antenna. He says it could prove useful for quantum computing and quantum cryptography because it can transmit light in a single direction. Currently, the components used to emit and detect photons for these purposes do so in all directions. “It’s very difficult to control where the photons go,” he says.
Van Hulst and colleagues took inspiration for the new device from a type of radio antenna called a Yagi-Uda antenna. “We use exactly the same one used to detect TV signals,” he says. The length of a Yagi-Uda antenna needs to be roughly the same as the wavelength of electromagnetic radiation it’s tuned to. For light, this is the nanometer range.
A Yagi-Uda antenna has five parallel bars, known as elements. Only one of these elements–the second longest, or the feed element–is connected to a circuit. The rest are passive, constructively and destructively interfering with the signal to make it directional.
Van Hulst and colleagues used electron beam lithography to create the elements, depositing very small strips of gold on a glass substrate, each with a specific length and separation. Causing the antenna to resonate at a very high frequency–in the terahertz range–makes it emit light at a wavelength of around 800 nanometers (infrared light).
To test the device, the team used it as a transmitter rather than a receiver. The signal was fed to the antenna in the form of a light signal that caused electrons in the feed element to resonate.
One major challenge was to find a way to stimulate only the feed element. Since these gold elements are smaller than the wavelength of light, it is not possible to focus light precisely enough to do this. The solution was to pepper one end of the feed element with quantum dots–nanoscopic chunks of a semiconductor material (in this case, a colloidal cadmium selenide-based semiconductor). The size of the quantum dots determines the wavelength of light they emit when they are optically stimulated. When the entire structure is illuminated with infrared light, the quantum dots are stimulated, while the element remains unaffected. “The quantum dots lose all their energy to the element,” Van Hulst says, causing electrons within the gold to resonate at a similar frequency and emit infrared. The rest of the Yagi-Uda structure then comes into play, with the remaining gold elements interfering with the emitted light, canceling it out in all directions but one. The work was published last week in the journal Science.
“It’s the best optical antenna I have seen,” says Markus Lippitz, at the Max Planck Institute for Solid State Research, in Stuttgart, Germany. Not only does it have the most complex structure of any optical antenna developed so far, Lippitz says, but it is also the first to be used to send photons.
There are other ways to redirect the light from single-photon devices, notes Harald Giessen, a professor at the University of Stuttgart. One way is to put quantum dots in cavities so that light can only escape in one direction. But using nano-antennas should be more efficient because they ensure that more photons are detected.
Lippitz says further improvements in optical antenna design research can be expected. “Currently people are just copying from radio-frequency schemes,” he says. “The next step would be to make them more optimized for photonics.”