Jacob Scheuer and others at Tel-Aviv University presented a paper at a recent conference of the International Society for Optics and Photonics describing the properties and showing the potential of nano-scale antennae that can transmit optical light.
Common antennae transmit radio waves, electromagnetic radiation with long wavelengths, low frequency and low energy. But now that nano-scale manufacturing has become possible, researchers are looking for ways to transmit high energy, short wavelength electromagnetic radiation in the form of UV, infrared and visible light.
Antennas work by turning radiation into magnetic fields and causing currents. With a very small antenna, it is hard to measure the magnetic and electric fields being generated, so Scheuer constructed an array of many tiny antennae and took far field measurements of their radiative properties. This allowed his team to characterize, for the first time, the essential properties of nano-antennae.
When an antenna is hit by an electromagnetic signal it resonates between high resistance and current. The length and material of the antenna determines its resonant properties and the properties of its electrical output. The bandwidth of an antenna describes the wavelengths of light that produce enough resonance for the antenna to operate efficiently. In the paper, researchers established the bandwidth and resonance of nano-scale gold antennae. This information facilitates further study of nano-antennae as well as potential future applications.
The potential applications of this technology are widespread. Arrays of nano-antennae could be used to measure how light waves interact with matter, in a science called spectroscopy. They could also be used to create high resolution images of small targets.
Another application is in the harvesting of radiated energy. Solar panels use semiconductors like silicon to capture some energy from the sun. However, they are usually very inefficient. The best solar panels only capture about 21 percent of the energy that hits them. But these nano-antennae are small enough to absorb solar energy and, thus, could be used to make a better solar panel. In fact, Scheuer showed that his array of antennae captured 95 percent of the energy radiated onto it.
Another advantage of nano-antenna arrays is that they can absorb a higher variety of light than silicon solar panels. The sun emits a broad range of radiation. Semiconductors can only capture a fraction of the available wavelengths, but nano-antennae of different lengths can be printed on the same array. This means that a nano-antenna based solar array could not only use energy more efficiently, but it could also harvest it more inclusively. However, the nano-antenna solar array has a long way to go. Light was polarized and aimed in this experiment, and silicon is relatively inexpensive compared to the nano-antennae, which are made of gold and difficult to manufacture. Despite these steep hurdles, Scheuer thinks that nano-antenna arrays have the potential to surpass their silicon couterparts in both cost and efficiency. He also wants to investigate how he could improve the electric capabilities of solar arrays, more specifically how they turn the absorbed radiation into electric current.
