Optical Antennas

June 13, 2011

One favorite engineering book on my bookshelf is "Antennas" by John Kraus.[1] Kraus invented the axial mode helical antenna, and he was a pioneer in radio astronomy. Antennas (or, antennae, depending on the spelling rules that were drummed into you in high school) are an important part of radio receivers and transmitters, since they couple electromagnetic signals between electronic circuitry and "free space," an archaic term for the vacuum. I wrote about antennas in a previous article (Antennae, June 28, 2010).

One important rule of thumb in antenna design is that the antenna needs to be about the same dimension as the wavelength of the radiation that needs to be transmitted or received. If the antenna is too small, little of the voltage applied to the antenna couples into free space. Unless certain electronic trickery is employed, too large an antenna will be less efficient for other reasons. That's why AM radio towers, which transmit around a frequency of a megahertz, are more than a hundred meters high, and the Wi-Fi antenna on your wireless router (2.45 GHz) is just a few inches long.

A spherical cage antenna invented by John Kraus. Figure 1 of US Patent No. 2,732,551, "Spherical Cage Antenna," January 24, 1956

The advent of nanotechnology has opened an interesting application area for antennae. Since visible light has a wavelength of about 500 nanometers, it's now possible to make a radio antenna that detects light. Not only that, if certain other technologies evolve, we could employ such antennas as photovoltaic energy sources in devices called nantennas.

The nantenna idea goes back to the time of the 1970s energy crisis. The essential nantenna idea was contained in a 1973 patent,[2] but technology at the time did not allow a practical implementation. The nantenna aegis has been picked up again by the Idaho National Laboratory, a laboratory of the US Department of Energy.[3] A research group there has demonstrated creation of alternating electrical currents in antennas at the terahertz frequencies required for solar energy harvesting. In order to harvest light at wavelengths from 0.4 - 1.6 μm, nantennas must be tuned for a 190 - 750 terahertz frequency range.

The major problem in using nantennas to harvest solar energy is conversion of these extremely high frequency electrical signals to useful electrical power. Conventional conversion of AC signals to DC voltages is by semiconductor diode rectifiers, but high frequency Schottky diodes don't perform well at such extreme frequencies. Another possibility is the metal-insulator-metal (MIM) tunnel diode.

MIM diodes demonstrate the utility of quantum mechanics. The device consists of metal electrodes of different work functions separated by a thin dielectric insulator. Electrons will tunnel through the insulator, but the difference in work function makes the tunneling easier in one direction than the other.

Since an MIM diode is a tunneling device, parasitic capacitance, the primary problem of conventional diodes, is not a problem. Efficient MIM diodes have been demonstrated at about 150 terahertz, just outside the range required for a nantenna solar energy collector.[4]

Nantenna solar collectors may be quite a few years into the future, but the use of antennas to process light signals has become common in optics, as demonstrated in a recent paper in Nature Communications.[5-6]

A bowtie nanoantenna structure used to concentrate optical signals in a nanoscale object. The optical signal, including the initial, transient portion, is weak without the antenna (top), but it is amplified by an order of magnitude with the bowtie structure (bottom). Figure 1 of Ref. 5, (Via arXiv).

As shown in the figure, a resonant optical nanoantenna was used to concentrate the optical field on an individual nanoscale object. This antenna structure gave an order of magnitude enhancement in the optical signals used to probe a single gold nanoparticle.

This nanoantenna research was conducted by scientists at the Ultrafast Nanooptics Group of the Max Planck Institute for Solid State Research and the Physics Institute and Research Center of the University of Stuttgart (Stuttgart, Germany).

References:

1. J.D. Kraus, "Antennas," McGraw-Hill (New York, 1988).
2. James C. Fletcher and Robert A. Bailey, "Electromagnetic Wave Energy Converter," US Patent No. 3,760,257, September 18, 1973.
3. Dale K. Kotter, Steven D. Novack, W. Dennis Slafer and Patrick Pinhero, "Solar Nantenna Electromagnetic Collectors," Proceedings of the 2nd International Conference on Energy Sustainability, August 10-14, 2008, Jacksonville, Florida, Report No. ES 2008-54016.
4. B. Berland, "Photovoltaic Technologies Beyond the Horizon: Optical Rectenna Solar Cell," Final Report, NREL/SR-520-33263, National Renewable Energy Laboratory, February, 2003.
5. Thorsten Schumacher, Kai Kratzer, David Molnar, Mario Hentschel, Harald Giessen and Markus Lippitz, "Nanoantenna-enhanced ultrafast nonlinear spectroscopy of a single gold nanoparticle," Nature Communications, vol. 2, no. 5 (May 31, 2011), article 333.
6. Thorsten Schumacher, Kai Kratzer, David Molnar, Mario Hentschel, Harald Giessen and Markus Lippitz, "Nanoantenna-enhanced ultrafast nonlinear spectroscopy of a single gold nanoparticle," arXiv Preprint Server, April 26, 2011.