Two-color light controls electrons in graphene's transient
Researchers used two laser fields to create and then steer electrons within a fleeting topological state in graphene, observing valley-polarized currents
A two-color laser pulse can create and then control electrons within a fleeting topological state in graphene. Researchers from Friedrich-Alexander University Erlangen-Nürnberg, Ludwig Maximilian University of Munich, the Technion-Israel Institute of Technology, and the University of Central Florida demonstrated this by illuminating a graphene strip with two precisely timed light fields and measuring the resulting photocurrents.
A circularly polarized 1,550-nanometer laser pulse, lasting 200 femtoseconds, first illuminated a microscopic strip of monolayer graphene. This light field temporarily reshaped the material's electronic band structure, creating a periodic, non-equilibrium condition known as a Floquet topological insulator state. "When a circularly polarized light field interacts with or 'dresses' graphene, it pushes electrons into circular orbits," explained co-senior author Ofer Neufeld. The team then applied a second laser field with double the frequency-775 nanometers-to exert control over the electrons within this transient state.
Steering electrons with light harmonics
The researchers measured the electrical currents generated by the light, known as photocurrents, via gold electrodes connected to the graphene. By adjusting the polarization and timing of the second harmonic field relative to the first, they could dictate the strength and direction of these currents. The experiments revealed photocurrent circular dichroism, where the current's magnitude depended on the rotational direction of the second light field. They also observed an all-optical anomalous Hall effect, where electrons were deflected sideways without any external magnetic field.
Calculations supporting the measurements indicated the generation of valley-polarized currents. In this phenomenon, one of the two distinct electronic valleys in the dressed graphene's band structure contributed more to the current flow than the other. The team's setup was sensitive enough to detect changes occurring within a single cycle of the light oscillation.
A hybrid light-matter platform
"We think the ability to use a second optical field to control electrons in the Floquet state is an important step forward," said co-senior author Peter Hommelhoff. This approach allows manipulation of electrons within a hybrid state that merges the material's properties with light, potentially enabling engineered properties not found in the bare material, such as topologically protected currents. The work connects theoretical predictions about Floquet states with direct experimental observation, a link the researchers said was previously missing.
Toward ultrafast material control
The method moves beyond merely observing light-induced states to actively controlling electrons within them. The team suggests this technique could be applied to realize similar transient topological states in other two-dimensional materials. Hommelhoff noted that prior studies often relied on time-averaged techniques, which can obscure fast non-equilibrium physics due to rapid scattering effects. This new approach could help bridge concepts from cold-atom lattice research to solid-state physics.
Future applications may include advances in ultrafast electronics, sensors, and spectroscopy tools. The ability to create, probe, and control such transient states in two-dimensional materials could provide a new pathway for developing these technologies. The research was led by co-first author Daniel M. B. Lesko alongside Hommelhoff, Tobias Weitz, and Neufeld.
