More than 120 years after the discovery of the
electromagnetic character of radio waves by Heinrich Hertz, wireless data
transmission dominates information technology. Higher and higher radio
frequencies are applied to transmit more data within shorter periods of time.
Some years ago, scientists found that light waves might also be used for radio
transmission. So far, however, manufacture of the small antennas has required
an enormous expenditure. KIT scientists have now succeeded for the first time
in specifically and reproducibly manufacturing the smallest optical
nanoantennas from gold.
In 1887, Heinrich Hertz discovered the electromagnetic waves
at the former Technical College of Karlsruhe, the predecessor of Univer-sität
Karlsruhe (TH). Specific and directed generation of electromagnetic radiation
allows for the transmission of information from a place A to a remote location
B. The key component in this transmission is a dipole antenna on the transmission
side and on the reception side. Today, this technology is applied in many areas
of everyday life, for instance, in mobile radio communication or satellite reception
of broadcasting programs. Communication between the transmitter and receiver
reaches highest efficiency, if the total length of the dipole antennas
corresponds to about half of the wavelength of the electromagnetic wave.
Pictured are nano dipole antennas under a microscope. The colors reflect the different transmission frequencies. Credit: LTI
Radio transmission by high-frequency electromagnetic light
waves in the frequency range of several 100,000 gigahertz (500,000 GHz
correspond to yellow light of 600 nm wavelength) requires minute antennas that
are not longer than half the wavelength of light, i.e. 350 nm at the maximum (1
nm = 1 millionth of a millimeter). Controlled manufacture of such optical
transmission antennas on the nanoscale so far has been very challenging
worldwide, because such small structures cannot be produced easily by optical
exposure methods for physical reasons, i.e. due to the wave character of the
light. To reach the precision required for the manufacture of gold antennas
that are smaller than 100 nm, the scientists working in the "Nanoscale
Science" DFG-Heisenberg Group at the KIT Light Tech-nology Institute (LTI)
used an electron beam process, the so-called electron beam lithography. The
results were published recently in the Nanotechnology journal (Nanotechnology
20 (2009) 425203).
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Dr. Hans-Jürgen Eisler and Matthias Wissert from the LTI developed a method to manufacture minute nanoantennas close to the technical limits. Credit: LTI
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These gold antennas act physically like radio antennas.
However, the latter are 10 million times as large, they have a length of about
1 m. Hence, the frequency received by nanoantennas is 1 million times higher
than radio frequency, i.e. several 100,000 GHz rather than 100 MHz.
These nanoantennas shall transmit information at extremely
high data rates, because the high frequency of the waves allows for an
extremely rapid modulation of the signal. For the future of wireless data
transmission, this means acceleration by a factor of 10,000 at reduced energy
consumption. Hence, nanoantennas are considered a major basis of new optical
high-speed data networks. The positive side-effect: Light in the range of 1000
to 400 nm is not hazardous for man, animals, and plants.
In the future, nanoantennas from Karlsruhe may not only be
used for information transmission, but also as tools for optical microscopy:
"With the help of these small nano light emitters, we can study individual
biomolecules, which has not been established so far", says Dr. Hans-Jürgen
Eisler, who heads the DFG Heisenberg group at the Light Technology Institute.
Moreover, the nanoantennas may serve as tools to characterize nanostructures
from semiconductors, sensor structures, and integrated circuits. The reason is
the efficient capture of light by nanoantennas. Thereafter, they are turned
into light emitters and emit light quantums (photons).
The LTI scientists are presently also working on the
specific and efficient capture of visible light by means of these antennas and
on focusing this light on a few 10 nm, the objective being e.g. the optimization
of photovoltaic modules.
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