July 28, 2015

Smaller, faster, cheaper

(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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