Devices that convert optical radiation to spatially concentrated energy at the nanoscale are finding applications in data storage, spectroscopy, sensing and nanoscale optical manipulation. They can even serve as a plasmonic “film” for recording images or encoding sound.
For those of a certain generation, “antenna” conjures up early childhood memories of “rabbit ears”—telescopic, upward-pointing metal rods—perched on top of a television. Childhood observations allowed us to infer a few things about antenna functionality with no knowledge of antenna theory or electromagnetics. We observed that antennas were metal and typically long, and that the antenna’s relative orientation was crucial to signal reception. We might even have made an educated guess that antennas somehow converted the “television waves” of our favorite program, beamed through the air from the broadcasting station, into a signal that our television set could translate.
With the advances in personal electronics and communication devices over the last several decades, antennas have become ubiquitous. We generally are oblivious to their presence, however, because of their ubiquity and their increased miniaturization. Cellphone technology, for example, has moved from using carrier frequencies on the order of hundreds of MHz to a few GHz; this, in turn, has reduced the size of cellphone antennas from externally mounted projections to devices small enough to be concealed within the body of the cellphone itself. That’s because the characteristic dimensions of the antenna are proportional to the order of the radiation wavelength—and, thus, higher-frequency, shorter-wavelength devices will require smaller antennas.
By the same physical logic, an antenna for light—an electromagnetic wave oscillating at THz frequencies—would require nanoscale dimensions. And developments in nanotechnology in recent years have made the fabrication of optical antennas, or nanoantennas, quite viable. The ability of these devices to convert optical radiation to spatially concentrated energy at the nanoscale has found application in areas as diverse as data storage, spectroscopy, and sensing, as well as in nanoscale optical tweezers. And our group has recently shown that optical nanoantennas can even be modified to serve as photographic film for recording images or encoding sound in the near field—a finding that opens some fascinating potential new applications for the devices.