(July 28, 2015) Transmitting large amounts of data, such as those needed to
keep the internet running, requires high-performance modulators that turn
electric signals into light signals. Researchers at ETH Zurich have now
developed a modulator that is a hundred times smaller than conventional models.
In February 1880 in his laboratory in Washington the
American inventor Alexander Graham Bell developed a device which he himself
called his greatest achievement, greater even than the telephone: the
“photophone”. Bell’s idea to transmit spoken words over large distances using
light was the forerunner of a technology without which the modern internet
would be unthinkable. Today, huge amounts of data are sent incredibly fast
through fibre-optic cables as light pulses. For that purpose they first have to
be converted from electrical signals, which are used by computers and
telephones, into optical signals. In Bell’s days it was a simple, very thin
mirror that turned sound waves into modulated light. Today’s electro-optic
modulators are more complicated, but they do have one thing in common with
their distant ancestor: at several centimeters they are still rather large,
especially when compared with electronic devices that can be as small as a few
micrometers.
In a seminal paper in the scientific journal “Nature
Photonics”, Juerg Leuthold, professor of photonics and communications at ETH
Zurich, and his colleagues now present a novel modulator that is a hundred
times smaller and that can, therefore, be easily integrated into electronic
circuits. Moreover, the new modulator is considerably cheaper and faster than
common models, and it uses far less energy.
The plasmon-trick
For this sleight of hand the researchers led by Leuthold and
his doctoral student Christian Haffner, who contributed to the development of
the modulator, use a technical trick. In order to build the smallest possible
modulator they first need to focus a light beam whose intensity they want to
modulate into a very small volume. The laws of optics, however, dictate that
such a volume cannot be smaller than the wavelength of the light itself. Modern
telecommunications use laser light with a wavelength of one and a half
micrometers, which accordingly is the lower limit for the size of a modulator.
In order to beat that limit and to make the device even
smaller, the light is first turned into so-called surface-plasmon-polaritons.
Plasmon-polaritons are a combination of electromagnetic fields and electrons
that propagate along a surface of a metal strip. At the end of the strip they
are converted back to light once again. The advantage of this detour is that
plasmon-polaritons can be confined in a much smaller space than the light they
originated from.
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