Optical Yagi-Uda Nanoantennas

April 25, 2012

Light is an electromagnetic wave, just like radio, so it should be possible to scale radio antenna designs to function for light. The critical dimensions in antennas need to be about the same as the wavelength of the electromagnetic radiation. That's why AM radio towers, which transmit frequencies of about 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.

Since visible light has a wavelength of about 500 nanometers, optical antennas have only become possible with the advent of routine fabrication of nanoscale structures. I wrote about optical antennas in the context of solar energy conversion in a previous article (Optical Antennas, June 13, 2011).

The Yagi-Uda antenna has been a popular antenna type for more than eighty years. Its main advantage is that it's directional and offers gain over the isotropic; that is, it concentrates its electromagnetic energy in one direction for a transmitter. For a receiver, it enhances sensitivity in one direction, while at the same time rejecting noise away from that direction. This antenna concept was published in Japanese by Shintaro Uda and Hidetsugu Yagi, but it was called a Yagi antenna in the West, since Yagi published descriptions in English.

This Yagi-Uda antenna sits atop the roof of my house. It was part of a my daughter's 1998 science fair project. The antenna is a Winegard HD6065P High Definition FM Antenna. (FM Radio Detection of Meteors Web Site).

As shown in the photograph, above, the construction of such antennas is very simple. The principle of operation is related to the concept of phase interference. The reflector element (see schematic) is spaced a quarter of a wavelength from the driven element, which leads to constructive interference for waves in the plane of the elements and a lesser signal at other angles. The director elements likewise re-radiate energy they receive in a way that enhances the signal of the driven element in the forward direction and diminishes the radiation in other directions.

Schematic diagram of a Yagi-Uda antenna. (Illustration by author, rendered using Inkscape).

Yagi patented the concept in 1932,[1] although his patent may seem strange to radio engineers of today. Yagi showed his directional antenna concept in an array of vertical elements on the ground. This Yagi architecture might be worthwhile resurrecting in an optical analog.

A team of scientists from the Nonlinear Physics Centre and the Centre for Ultrahigh Bandwidth Devices for Optical Systems (CUDOS) of the Research School of Physics and Engineering, the Australian National University (Canberra, Australia) has just published a preprint of a very thorough review article of the current and potential uses of optical Yagi-Uda antennas.[2] A representative optical nanoantenna is shown in the following figure.

An optical Yagi-Uda antenna based on gold nanorods. At a wavelength of 1550 nm, the area occupied by the nanoantenna is about 400 x 400 nm. (Fig. 2 of ref. 2, via the arXiv Preprint Server)[2].

The Australian authors discuss and compare the physics behind electromagnetic wave propagation for radio and optical wavelengths; and they analyze the limits for application of this antenna type in the optical domain. Their paper describes some novel broadband and wavelength tunable Yagi-Uda optical nanoantennas, including arrays of such antennas. Most interesting from a practical viewpoint are the several methods of exciting the active element of such nanoantennas.

One application I found most interesting was the use of paired optical Yagi-Uda antennas to transmit an optical signal across a chip without the need for an optical waveguide, as shown in the figure. In a multichip module, this may even be a means of communicating between chips.

Two optical Yagi-Uda antennas can be used as a mean of transmitting optical signals across a chip, or between chips, without use of an optical waveguide. (Fig. 11 of ref. 2, via the arXiv Preprint Server).[2]

My first experience with Yagi-Uda antennas was as a high school senior. I built a radio telescope as a science fair project. Shown below is the Yagi-Uda antenna I built from copper pipes. This photograph was taken on the roof of my high school, which is now a housing complex for senior citizens. Perhaps I can get a room there, some day, in the same location as my high school homeroom. That's what they call full circle.

My senior high school science fair project included this Yagi-Uda antenna I built from copper piping for a radio telescope. This photograph shows its placement on the school roof (43.100903° latitude, -75.232664° longitude). (click for larger image).

References:

1. Hidetsugu Yagi, "Variable Directional Electric Wave Generating Device," US Patent No. 1,860,123, May 24, 1932.
2. Ivan S. Maksymov, Isabelle Staude, Andrey E. Miroshnichenko and Yuri S. Kivshar, "Optical Yagi-Uda nanoantennas," arXiv Preprint Server, April 2, 2012.
3. J.D. Kraus, "Antennas," McGraw-Hill (New York, 1988).