For high frequency electromagnetic light waves in the frequency range of several 100,000 gigahertz (500,000 GHz corresponds to yellow light with 600 Nm wavelength), one needs very small antennas, which are not larger than a half light wave length, thus maximally 350 nanometers (1 nanometer = 1 millionth millimeter). The scientists of the DFG Heisenberg Nanoscale Science group used an electron-beam to make gold nanoantennas of 70-300 nanometers. The results were published in the journal Nanotechnology (Nanotechnology 20 (2009) 425203).
The nano-antennas could be used for future terabit information transfer, but also as tool for the optical microscopy.
The wavelengths the antennas are designed for correlate with frequencies of 500 THz and more. Of course, no semiconductor elements are available to drive these antennas. For this reason, they are excited with white light; each antenna gets into resonance for its specific frequency (= light color), forming a frequency multiplex broadband array for data transmission with data rates 10 thousand times higher than existing wireless broadband technologies, the institute claims. Modulation of the light beams is achieved by application of the superposition principle, Eisler explained.
Since no semiconductors are available to drive the antennas electrically, the research group focuses on unconventional methods to transmit and receive data. “Actually we should develop nano switching elements that make use of quantum technology”, Eisler said. While quantum computers still are in a very early stage of development, a technical use of these optical antennas could be possible within five to ten years, Eisler believes.
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In this paper we present our approach for engineering gold dipole nanoantennas. Using electron-beam lithography we have been able to produce arrays of single gold antennas with dimensions from 70 to 300 nm total length with a highly reproducible nanoengineering protocol. Characterizing these gold nanoantenna architectures by optical means via dark-field microscopy and scattering spectroscopy gives the linear optical response function as a figure-of-merit for the antenna resonances, spectral linewidth and integrated scattering intensity. We observe an enhanced integrated scattering probability for two arm gold dipole nanoantennas with an antenna feed gap compared to antennas of the size of one arm without a gap.
We have presented a nanoengineering e-beam lithography fabrication protocol that enables us to fabricate reliably two arm and single arm gold dipole nanoantennas with feature sizes as small as 20 nm. For the smallest gold antennas, we measured the linear optical scattering response function by systematically varying the antenna dimensions. We have characterized those resonance scattering spectra as damped Lorentzian oscillators. From this model, we have been able to extract the resonance energy peak position, the spectral width and the relative scattering intensity. We found that a nanoantenna feed gap with smaller than 30 nm synergistically combines the near-field coupling of two antenna arms of length Larm each, creating localized electromagnetic hot spots at a well-chosen subwavelength volume, with exceptionally enhanced far-field photon scattering probabilities. The physical volume does not explain the enhanced photon scattering probability completely when classical scattering theory is applied. We thus speculate that the nanoantenna feed gap as an electromagnetic hot spot may act as an additional dipole source that highly perturbs the internal field of the gold nanoantenna and effectively contributes to the enhanced far-field scattering intensity of visible photons. Quantitative near-field experiments at subwavelength volume are needed to characterize the antenna feed gap and its contribution to the far-field scattering response as well as to the near-field localization capability that go hand in hand with detailed topography information in those nanogaps. Moreover, the dipole allowed longitudinal eigenmode of nanoantennas with and without antenna feed gap needs to be considered in detail.