Lightning Switch for Electron Waves

Regensburg physicists are researching the fundamentals of future plasma electronics

electron waves
Waves in the plasma of the black phosphorus (below) spread out from the point of their excitation and also vibrate the surrounding silicon dioxide (top). Researchers at the University of Regensburg have thus succeeded for the first time in switching surface vibrations to ultrasound time scales less femtoseconds. Picture credits: Fabian Mooshammer

Researchers from the University of Regensburg and the Scuola Normale Superiore di Pisa have developed a super-fast switch for electron waves, which could allow future electronics to be accelerated many times over.

The characteristic luster of metals is caused by electrons that are free to move in the interior of the material and reflect incident light. Similar to water waves on a pond, waves can also be produced on the surface of this electron lake – so-called surface plasmons. Instead of a stone that is thrown into the water, light is used in the laboratory to produce surface plasmons. When light is bundled to a sharp metal tip of the size of a few nanometers, tiny circular surface waves spread out from this tip. A nanometer is only about ten times the diameter of an atom. These miniature waves could be used in future compact electronic components to rapidly transport digital information. However, there has been no way to turn such surface waves on and off quickly. In conventional electronics, an analogous task of so-called transistors is perceived.

A research team led by Professor Rupert Huber, Chair for Experimental and Applied Physics at the University of Regensburg, has now succeeded, for the first time, in switching electron waves on and off in the electron seas and thus laying an important basis for future plasma electronics.

The physicists, however, did not use metal on which electron waves are always present.Rather, a sophisticated layer structure based on a semiconductor was used.Semiconductors such as silicon are the materials that make up computer chips. The semiconductor used here is a particularly modern material: so-called black phosphorus. By irradiating an intense light flash, free-moving electrons can only be generated therein. Without these, no surface waves are present and the structure is “off”. As soon as, however, the first laser pulse has produced the freely moving electrons, the surface plasmons can be sent from the tip with a subsequent pulse.

To test how fast this switching process can be, the Regensburg team around Prof. Dr. Rupert Huber activated surface plasmons with ultra-bright light flashes with a duration of only a few femtoseconds. A femtosecond is the unimaginably short period of a millionth part of a billionth of a second, that is, 000,000,000,000,000 001 seconds.With a globally unique super-fast and high-resolution microscope, the researchers then followed directly in extreme time how the plasmon wave propagates. It was clear that the switching times were on the femtosecond time scale and were thus many orders of magnitude faster than the fastest existing transistors. A pleasant surprise was that the wavelength of the surface waves is almost independent of the power of the power-on laser.

These results are extremely encouraging for future ultrasound electronics based on surface plasmons. In the next step the Regensburg researchers will test first functional plasmonic devices – from now on the femtosecond time scale.

The novel principle is presented in the current issue of the trade journal “Nature Nanotechnology” (DOI: 10.1038 / NNANO.2016.261)