A short flash of light hits a graphene sheet, and at the point of impact, an energy gap momentarily opens, as if a pocket were cut into a continuous fabric. This pocket lasts only an instant, but it can be observed with a new instrument: a terahertz scanning tunneling microscope (which combines a tip that probes the surface atom by atom with light pulses oscillating trillions of times per second).

Usually, such gaps are detected by how electrons pass through the entire material. Here, the microscope's tip approaches a chosen spot and measures the current, like a finger checking whether an iron has heated up. If light has opened a gap at that location, the current changes. This allows mapping out exactly where light most strongly rearranges the electronic levels.

The edges of graphene ribbons are especially interesting. Under illumination, conducting pathways emerge along which electrons run in only one direction, like an escalator. If the ribbon is too narrow, the protection of these pathways breaks down — visible as smeared spots on the current map. The authors calculated the width at which this occurs.

Another trick: an impurity that twists electrons in one direction leaves a characteristic pattern on the map. From this, one can tell where exactly electrons are flowing along the edge. Such a microscope could become a tool for testing materials intended for ultrafast electronics.