The high oscillation frequencies of light waves make them ideally suited to fast data transmission. They can be sent through optical fibres and easily carry hundreds of billions of bits (Gigabits) per second. The “last mile” from a central fibre optic cable to the internet socket at home, however, is the most difficult and expensive. Some alternatives, for instance 4/5G mobile telephony, are cheaper, but they cannot provide all users simultaneously with the extremely high transmission rates required by today’s data-hungry applications such as streaming TV.



Jürg Leuthold, professor at the Institute for Electromagnetic Fields at ETH Zurich, and his collaborators have now, with support by colleagues at the University of Washington in Seattle, developed a novel light modulator that will make it possible in the future to cover the last mile efficiently and at a low cost with high-frequency microwaves – so called millimetre waves - and hence high data transmission rates.

Light modulator without electronics

To transfer data encoded in optical fibres through a variation in the light intensity onto millimetre waves, very fast – and hence expensive – electronic components are needed. In the opposite direction, millimetre waves first have to be received by an antenna, then amplified and mixed down to baseband and finally injected into a light modulator, which translates the data contained in the radio waves back into light pulses.



Leuthold and his colleagues have now succeeded in building a light modulator that works entirely without batteries and electronics. “That makes our modulator completely independent of external power supplies and, on top of that, extremely small so that it can, in principle, be mounted on any lamppost. From there, it can then receive data via microwave signals from individual houses and feed them directly into the central optical fibre”, explains Yannick Salamin, a PhD student who made crucial contributions to the development of the new modulator.

Modulation through plasmons

The modulator built by the ETH researchers consists of a chip measuring less than a millimetre that also contains the microwave antenna. That antenna receives the millimetre waves and converts them into an electric voltage. The voltage then acts on a thin slot at the centre of the chip – the actual heart of the modulator. There, a narrow slit, just a few micrometres long and less than a hundred nanometres wide, is filled with a material that is particularly sensitive to electric fields. The light beam from the fibre is fed into that slit. Inside the slit, however, the light propagates – differently from the fibre optic cable or air – no longer as an electromagnetic wave, but as a so-called plasmon. Plasmons are hybrid creatures made of electromagnetic fields and oscillations of electric charge at the surface of a metal. Owing to this property, they can be confined much more tightly than light waves.

