Two-color light steers electrons through graphene's transient topological state

This research paper, published in Nature Physics, details the creation and manipulation of a fleeting, electronic state in graphene known as a Floquet topological insulator. Researchers at several German, Austrian and Israeli institutions demonstrated that exposing graphene to two distinct colors of light can alter its electronic properties, giving rise to a new, temporary state.

The investigators used a laser field to temporarily modify graphene's electronic band structure, causing electrons to adopt circular orbits that repeat in time and form a Floquet state. This state has unique properties not found in graphene's equilibrium state. By applying a second laser field with double the frequency of the first, the scientists were able to control the flow of electrons within the Floquet state.

Through measuring the resulting photocurrents and applying varying conditions to the two light fields, the researchers observed intriguing behaviors. These included circular dichroism, where the current direction depended on the rotation of the second light field, and an all-optical anomalous Hall effect, which deflected electrons sideways without an applied magnetic field. The team also noted valley-polarized currents, indicating that one electronic valley contributed more strongly to the current than the other.

The researchers found that the strength and direction of photocurrents were sensitive to changes within a single light cycle, suggesting the potential for fine-tuning such electronic properties. This control over electrons within a light-induced band structure marks a step towards engineering hybrid light-matter states with unique properties, particularly topologically protected currents, in 2D materials.

These findings could pave the way for novel electronics, sensors, and spectroscopy tools, and could bridge ideas from the study of ultracold atoms to solid-state physics.