Scientists observe a new kind of light emission when electrons in
topological insulators abruptly reverse their direction of motion. The new
findings are reported in the journal “Nature.”
To change the direction of motion of a massive object, such as a car, it has
to be slowed down and brought to a complete standstill first. Even the
tiniest charge carriers in the universe, the electrons, follow this rule.
For future ultrafast electronic components, however, it would be helpful to
circumvent the electron’s inertia. Photons, the quanta of light, show how
this could work. Photons do not carry mass and can thus move at the highest
possible velocity, the speed of light. For a change of direction, they do
not need to slow down; when they are reflected from a mirror, for instance,
they abruptly change their direction without a stopover. Such behavior is
highly desirable for future electronics because the direction of currents
could be switched infinitely swiftly and the clock rate of processors could
be massively increased. Yet, photons do not carry electric charge, which is
a prerequisite for electronic devices.
An international consortium of physicists from the University of Regensburg,
the University of Marburg, and the Russian Academy of Sciences in
Novosibirsk succeeded in flipping the motion of electrons on ultrafast time
scales without slowing them down. In their study, they employed the new
material class of topological insulators. On their surfaces, electrons
behave like massless particles moving almost like light. To switch the
direction of motion of those electrons as rapidly as possible, the
researchers accelerated electrons with the oscillating carrier field of
light – the fastest alternating field in nature controllable by mankind.
When the electrons abruptly reverse their direction of motion, they emit an
ultrashort flash of light containing a broadband spectrum of colors like in
a rainbow. There are strict rules on which colors get emitted: Generally,
when electrons are accelerated by lightwaves only radiation is emitted,
whose oscillation frequency is an integer multiple of the incident light’s
frequency, so-called high-order harmonic radiation. “By carefully adjusting
the accelerating light field, we were able to break this rule. We managed to
control the electrons’ motion such that light of every imaginable color
could be generated,” explains Christoph Schmid, first author of the study.
In a careful analysis of the emitted radiation, the scientists found further
unusual quantum properties of the electrons. It became apparent that the
electrons on the surface of a topological insulator do not move in straight
lines following the electric field of light but rather perform meandering
trajectories through the solid. “Even for a theoretician, it is highly
fascinating to see which phenomena quantum mechanics can produce if you only
look a little closer,” elucidates Dr. Jan Wilhelm, who successfully
explained the experimental findings with a simulation he developed together
with his colleagues in the Institute of Theoretical Physics at the
University of Regensburg.
“These results do not only provide intriguing insights into the microscopic
quantum nature of electrons; they also suggest topological insulators as a
promising material class for future electronics and information processing,”
summarizes Prof. Dr. Rupert Huber, who led the experimental work in
Regensburg. Such expectations perfectly follow the mission statement of the
Collaborative Research Center SFB 1277, funded by the German Science
Foundation. Within this network, experimental and theoretical physicists
explore novel relativistic effects in condensed matter and test
possibilities to implement their findings in future high-tech applications.
Reference:
“Tunable non-integer high-harmonic generation in a topological
insulator” by C. P. Schmid, L. Weigl, P. Grössing, V. Junk, C. Gorini, S.
Schlauderer, S. Ito, M. Meierhofer, N. Hofmann, D. Afanasiev, J. Crewse, K.
A. Kokh, O. E. Tereshchenko, J. Güdde, F. Evers, J. Wilhelm, K. Richter, U.
Höfer and R. Huber, 19 May 2021, Nature. DOI:
10.1038/s41586-021-03466-7