Bowtie Nanoantennas

August 1, 2014

Antenna design is one of the more creative parts of radio frequency engineering. Although there's the rule of thumb that an antenna's dimension should be of the order of its working wavelength, it's often not intuitive how electric currents will be induced into shaped conductors by electromagnetic fields. Therefore, computer modeling is now used to determine an antenna's characteristic impedance and radiation/reception pattern.

One novel antenna, invented in the days before computers, is the axial mode (end fire) helical antenna. This antenna was invented in 1946 by John Kraus (1910-2004). As Kraus wrote in his classic textbook, "Antennas," he wound a helix of seven turns of copper wire with a 12 centimeter circumference, this being the wavelength of his signal source. He detected a narrow beam with circular polarization.[1] Helical antennas are now ubiquitous in many high frequency applications, such as satellite communication (see figure).

A satellite communications antenna pictured in 1984. This is a typical helical antenna. (US Air Force photograph by SSgt Louis Comeger, via Wikimedia Commons.)

A helix is a simple geometrical object, so the possibility of its being a useful antenna was tested just a little more than a half century after the first generation of radio waves in 1887 by Heinrich Hertz. Thirty years after the helical antenna, Benoit Mandelbrot popularized fractals, so it was just a matter of time before fractal antennas were designed and tested (see figure).[2]

Figure 7E of US Patent No. 6,452,553, "Fractal antennas and fractal resonators," by Nathan Cohen, September 17, 2002. Engineers will notice the resemblance to a shorted twin-lead. (Via Google Patents.)[2]

One simple geometrical shape, the triangle, is used as the basis for the bowtie/batwing antenna. These antennas are useful for broadcasting, especially television broadcasting, since they emit in a nearly omnidirectional pattern. When a stack of these are mounted with the plane of the antennas perpendicular to the ground, the signals are directed along the Earth's surface where they are most useful.

A famous bowtie. Photograph of Samuel Langhorne Clemens, better known as Mark Twain (1835-1910) Twain, who was friends with Nikola Tesla, liked science, but he lost his fortune on investments in technology. Twain's 1889 novel, A Connecticut Yankee in King Arthur's Court, was based on time travel. (Photograph from the 1922 book, "American Portraits," by Bradford, Gamaliel, via Wikimedia Commons.)

Engineers from the University of Illinois at Urbana-Champaign (Urbana, Illinois) Department of Electrical and Computer Engineering, the Micro and Nanotechnology Laboratory, and the Department of Mechanical Science and Engineering have built novel arrays of nanoscale bowtie antennas. The novelty is in the fact that the individual triangular elements of the bowtie are atop movable pillars on a silicon wafer.[3-4]

The elements of their so-called pillar-bowtie nanoantennas (p-BNA) sit atop 500-nanometer tall silicon dioxide pillars. The principal advantage of doing this is that the gap between the triangle sections of the bowtie can be made much smaller that the lithographic process allows. The gap can be tuned to about 5 nm, which is about four times smaller than that obtained from electron-beam lithography (see figure).[4]

Illustration of the University of Illinois bowtie nanoantenna array. The bowties are made of gold. (University of Illinois Illustration.)

A scanning electron microscope (SEM) was used to deform individual p-BNA structures, or groups of p-BNAs within a sub-array. The deformation rate was as large as 60 nanometers per second.[3-4] The mechanical actuation was facilitated by the relatively high 4.2 aspect ratio of the p-BNAs.[4] A video of the actuation is available online.[5]

These gold nanoantennas interact with light via plasmons, so modification of the antenna gap modifies the optical response. Says Kimani Toussaint, an associate professor of mechanical science and engineering at the University of Illinois and leader of the research project,

"For our approach, one can take a nanoarray structure that was already fabricated and further reconfigure the plasmonic, and hence, optical properties of select antennas. Therefore, one can decide after fabrication, rather than before, how they want their nanostructure to modify light."[4]

The research team envisions spatially addressable devices for particle manipulation and sensing.[4]

References:

1. J.D. Kraus, "Antennas," McGraw-Hill (New York, 1988), pp. 265f. (2001 paperback edition, via Amazon).
2. Nathan Cohen, "Fractal antennas and fractal resonators," US Patent No. 6,452,553 , September 17, 2002.
3. Brian J. Roxworthy, Abdul M. Bhuiya, Xin Yu, Edmond K. C. Chow, and Kimani C. Toussaint, Jr., "Reconfigurable nanoantennas using electron-beam manipulation," Nature Communications, vol. 5, Article no. 4427 (July 14, 2014), doi:10.1038/ncomms5427.
4. Researchers demonstrate novel, tunable nanoantennas, University of Illinois Press Release, July 14, 2015.
5. Video: Top view of the bowtie antenna deformation process using 450k magnification (Nature Web Site).