Optical Nanoantennas Could Improve Data Transfer

Metallic antennas 100 nm in size that efficiently receive optical frequencies in the range of several hundreds of terahertz have been realized by researchers, who say this breakthrough could speed up optical data transfer.

Physicists at the 4th Physics Institute of the University of Stuttgart, in collaboration with researchers at the Max Planck Institute for Solid State Research, modeled this new approach after standard TV antennas, which receive signals carried by electromagnetic waves with frequencies in the megahertz regime and convert them into pulses of electric currents in the connected cables. The antenna connects two very different length scales: the carrier wavelength, ranging from centimeters to meters, and the size of the wiring, typically on the millimeter scale.

Certain antenna geometries are known to receive radiation from designated directions. Such a unidirectional TV antenna is the Yagi-Uda antenna, invented by Hidetsugu Yagi and Shintaro Uda of Tohoku Imperial University in 1926. Consisting of several aligned parallel dipole antennas of different lengths, the Yagi-Uda antenna can be tuned to receive signals from a given direction 5 to 10 times more efficiently than a dipole antenna. The received signal can be enhanced even more by several orders of magnitude when the single antenna is expanded to an array of Yagi-Uda antennas. Such antenna arrays are used to transmit signals over very large distances, for example in satellite communication.

Tilted scanning electron micrograph of the optical Yagi-Uda nanoantenna array. (Image: University of Stuttgart)

The researchers, who teamed up at the Stuttgart Center of Photonics Engineering, scaled down the concept of Yagi-Uda antenna arrays to optical wavelengths. Doctoral student Daniel Dregely fabricated 3-D gold wire arrays of different lengths and stacked them one above another with nanometer precision. A periodic arrangement of the single Yagi-Uda nanoantennas then formed the investigated optical antenna arrays.

Measurements on the 3-D arrays revealed that the amount of absorbed energy strongly depends on the angle of incidence and on the frequency of the incident electromagnetic waves. The scientists showed in particular that maximal absorption of incident radiation occurs at 200 THz only if light impinges from the direction parallel to the antenna axis of the individual Yagi-Uda antenna. For this particular situation, the incoming wave 1500 nm length is confined to a subwavelength region extending only to about 100 nm. In the future, this can be used for very sensitive detection of near-infrared radiation on the nanoscale. One big advantage of their optical antenna arrays is that its three-dimensional character couples to radiation normal to the surface. This is particularly advantageous for light emitters such as LEDs, or for very sensitive photodetectors.

The scientists believe that the experimental realization of a phased array at optical wavelengths should enable optical high-speed data transfer on the micrometer scale, for example on microchips in high-performance computer circuits.

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