Two-color light steers electrons through graphene’s transient topological state
[post_content]
Disclaimer: This article has been automatically aggregated from
The electronic properties of materials are typically determined by their structure under normal, undisturbed conditions, when they are in a state known as equilibrium. Intense light beams, however, can temporarily reshape a material’s electronic band structure (i.e., the range of energy states available to electrons), potentially giving rise to new electronic behaviors.
Researchers at Friedrich-Alexander University Erlangen-Nürnberg, Ludwig Maximilian University of Munich, the Technion—Israel Institute of Technology, and the University of Central Florida recently demonstrated that illuminating graphene with a specific type of light temporarily modifies its electron states, prompting the emergence of a so-called Floquet topological insulator.
This is a transient, out-of-equilibrium state created when a periodically oscillating field reshapes a material’s electronic structure, resulting in topological properties that are absent at equilibrium.
In this study, a fundamental light field created the state, while a second laser field with a doubled frequency was used to control electrons within it. The team’s paper, published in Nature Physics, builds on recent demonstrations of how bicircular two-color light fields can alter the electronic properties of two-dimensional (2D) materials.
“After presenting some early results at a conference, Ofer Neufeld asked several tough questions during the Q&A about the topological properties of the system,” Daniel M. B. Lesko, co-first author of the paper, told Phys.org.
“Over the following months, Peter Hommelhoff, Tobias Weitz, and I worked through numerous simulations and derivations, as well as ab initio simulations with Ofer, and eventually connected several phenomena that were missing from experimental observations of Floquet states to our two-color-driven system. It was a unique position to be in.
“The moment we made that connection, we realized that many of the unusual measurements we’d taken were directly tied to theoretical predictions that hadn’t yet been observed experimentally.”
Steering electrons in graphene dressed by circularly polarized light
In their experiments, Lesko, Hommelhoff, Weitz, Neufeld and their colleagues used a microscopic strip of monolayer graphene that was grown on a silicon carbide substrate and connected to gold electrodes. This graphene strip was placed under high vacuum at room temperature while the team directed laser pulses onto its center.
First, the sample was illuminated with circularly polarized 1,550-nm laser pulses, each about 200 femtoseconds long. This light field temporarily modified the electronic band structure of the graphene strip.
“When a circularly polarized light field interacts with or ‘dresses’ graphene, it pushes electrons into circular orbits,” explained Neufeld, co-senior author of the paper. “Because these orbits repeat periodically, they generate a new time-periodic state called a Floquet state, which has different properties from the material’s equilibrium (non-driven) state. To control electrons within this Floquet state, we use a harmonic of the dressing field, which lets us take full advantage of the system’s periodicity.”
The second-harmonic field applied to the sample had double the frequency of the original light used by the team, resulting in 775-nm pulses with twice the photon energy of the first pulses. To examine how this second field drove electrons through the dressed graphene strip, the researchers measured the resulting photocurrents (i.e., electrical currents generated by light) via the connected gold electrodes.
By changing the polarization and relative timing of the two light fields, they could control the strength and direction of the photocurrents. They observed photocurrent circular dichroism, meaning that the current depended on the rotational direction of the second light field, as well as an all-optical anomalous Hall effect, in which electrons were deflected sideways without an applied magnetic field.
The team’s measurements, supported by calculations, offered evidence of valley-polarized currents, whereby one of dressed graphene’s two electronic valleys (i.e., distinct regions of its band structure associated with different electron momenta) contributed more strongly to the current than the other. They also showed that photocurrents were sensitive to changes occurring within a single cycle of light.
“We think the ability to use a second optical field to control electrons in the Floquet state is an important step forward,” said Hommelhoff, co-senior author of the paper.
“This means that we can now play with electrons in a state that’s a marriage of the material’s state and light, allowing plenty of new ideas about how this hybrid light-matter state might be engineered to offer properties not available in a bare material, for example, topologically protected currents.”
Paving the way for ultrafast topological electronics
Using their experimental methods, the researchers were able to control electrons within a light-induced band structure, instead of merely observing this structure. The same methods could be used to realize similar light-induced topological states in other materials.
“Previous work in this area has largely relied on time-averaged techniques to study nonequilibrium systems,” explained Hommelhoff.
“That’s a real challenge, because scattering and decoherence happen so fast in many materials that much of the interesting physics of the nonequilibrium state gets washed out in the averaging. We’re hopeful this work helps bridge ideas from the cold-atom lattice community to solid-state physics.”
In the future, the recent efforts by Lesko, Hommelhoff, Neufeld and their colleagues could contribute to the advancement of various devices, including electronics, sensors and spectroscopy tools. Most notably, they could help researchers create, probe and control transient states in 2D materials, which could be advantageous for the development of these technologies.
“We’re now interested in exploring how these hybrid matter-light states could be used to encode and process information, as well as studying the underlying dynamics of the electrons within them,” added Lesko.
Written for you by our author Ingrid Fadelli, edited by Sadie Harley, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive.
If this reporting matters to you, please consider a donation (especially monthly). You’ll get an ad-free account as a thank-you.
Publication details
Daniel M. B. Lesko et al, Optical control of electrons in a Floquet topological insulator, Nature Physics (2026). DOI: 10.1038/s41567-026-03429-7.
© 2026 Science X Network
Citation:
Two-color light steers electrons through graphene’s transient topological state (2026, September 19)
retrieved 20 September 2026
from https://phys.org/news/2026-09-electrons-graphene-transient-topological-state.html
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no
part may be reproduced without the written permission. The content is provided for information purposes only.
for informational purposes only. We do not claim ownership, accuracy, or liability for the content provided. All rights belong to the original publisher.