Lighting it up: Fast material manipulation by laser
Researchers from the Fritz Haber Institute in Berlin and the MPSD have found out that ultrafast switches in material properties can be prompted by laser pulses – and why this process occurs. Their work may lead to new transistor concepts.
Creating the fastest electronic technology possible is a central aim of contemporary materials research. The key components of fast computing technologies are transistors – switching devices that turn electrical currents on and off very quickly as basic steps of logic operations. In order to improve our knowledge about ideal transistor materials, physicists are constantly trying to determine new methods to create such extremely fast switches. Researchers from the Fritz Haber Institute (FHI) of the Max Planck Society and the MPSD have now figured out that light can be used to produce a novel type of ultrafast switch.
The physicists involved in the project search for new ways to prompt materials to change their properties – to make magnetic metals non-magnetic, for example, or to change the electric conductivity of a crystal. A material’s electrical properties are strongly related to the arrangement of the electrons in the crystal. Controlling the electrons’ arrangement has been a key research challenge for decades. Most control methods, however, are fairly slow.
“We knew that external influences like temperature or pressure variations work,” says Dr Ralph Ernstorfer, Group Leader at the FHI’s Department of Physical Chemistry, “but that takes time, at least a few seconds.” Anyone who regularly uses a smart phone or computer knows that a few seconds can feel like eternity. So Dr. Ernstorfer’s group explored how to switch material properties much faster by means of light. The team focused on a particular phenomenon known as a Lifshitz transition, in which the conducting properties of the material are abruptly changed by an external perturbation, in this case a laser field.
By firing ultrashort optical laser pulses at their chosen material, a semi-metallic crystal composed of tungsten and tellurium atoms, the researchers managed to massively reduce the switching time to only 100 femtoseconds – a millionth of a billionth of a second. The laser light encourages the crystal to re-organize its internal electronic structure, which also changes its conductivity.
The MPSD’s team involving Nicolas Tancogne-Dejean, Michael Sentef and Ángel Rubio had already predicted the possibility of manipulating the interaction of electrons using a laser field. Their state-of-the-art simulations demonstrated that the predicted changes could indeed be realized in real-life materials.
“Through our simulations we could explain the microscopic origin of the experimental findings.” says Tancogne-Dejean, “In particular, they reveal the role of electron-electron interaction. This opens up very exciting perspectives for the laser control of quantum materials and could lead to the next generation of ultrafast electronic devices.“
In addition to achieving and explaining the significant reduction in the switching time, the scientists also managed to observe exactly how the crystal’s electronic structure changed. “We used a new instrument to take pictures of the transition every step of the way,” explains Dr Samuel Beaulieu, a postdoctoral fellow at the FHI at the time of the study and now a permanent researcher at the Centre Lasers Intenses et Applications (CELIA) at CNRS-Bordeaux University. “This is amazing progress – we used to only know what the electronic structure of the material looked like after, but never during the transition.”
The method deployed and described in this work generates new knowledge about possible future transistor materials. The fact that light can drive ultrafast electronic transitions is a first step towards even quicker and more efficient future technologies.
Text by Agatha Frischmuth, FHI / Jenny Witt, MPSD