Researchers from Friedrich-Alexander University Erlangen-Nürnberg, Ludwig Maximilian University of Munich, the Technion—Israel Institute of Technology, and the University of Central Florida have shown that shining a specific type of light on graphene can temporarily change its electronic properties. This creates a unique, non-equilibrium state known as a Floquet topological insulator. This state is formed when a periodically oscillating field, such as light, alters a material's electronic structure. The study, published in Nature Physics, expands on recent research showing how special light fields can change the properties of two-dimensional materials like graphene. In their experiments, the team used a thin layer of graphene grown on a silicon carbide substrate and connected it to gold electrodes. The graphene was placed in a high vacuum at room temperature, and laser pulses were directed at its center. The researchers used circularly polarized laser pulses with a wavelength of 1,550 nm, each lasting about 200 femtoseconds. This light temporarily altered the graphene’s electronic band structure. The interaction between the circularly polarized light and the graphene caused electrons to move in circular orbits, creating a new time-periodic state called a Floquet state. This state has different electronic properties than the material's normal, non-driven state. To control the electrons within the Floquet state, the researchers used a second laser field with double the frequency of the first. This second field, called a harmonic, produced 775-nm pulses with twice the energy of the original light. By adjusting the polarization and timing of both light fields, the researchers could influence the strength and direction of the resulting photocurrents. They measured these currents using the gold electrodes connected to the graphene strip. The team observed two key effects: photocurrent circular dichroism, where the current depended on the rotation direction of the second light field, and an all-optical anomalous Hall effect, where electrons were deflected sideways without the need for a magnetic field. Their measurements, supported by theoretical calculations, showed that the photocurrents were influenced by changes occurring within a single cycle of light. This indicated that one of the two electronic "valleys" in the graphene contributed more strongly to the current than the other. The researchers believe that using a second optical field to control electrons in the Floquet state is a significant advancement. This hybrid state, which combines the properties of the material and light, opens up new possibilities for engineering materials with unique properties, such as topologically protected currents. The experimental techniques used in this study could also be applied to other materials to create similar light-induced topological states.