Researchers have for the first time directly mapped the Berry curvature produced when circularly polarized light turns a gapless electronic system into a light-driven topological state called a Floquet-Chern insulator. Berry curvature is a geometric property of electron bands that acts like a magnetic field in momentum space and controls how electrons respond to forces. The experiment shows how this geometric response appears and changes when a Dirac-like surface state is driven periodically by light.

The team used circular-dichroism time- and angle-resolved photoemission spectroscopy (Tr-ARPES). Time- and angle-resolved photoemission measures the energy and momentum of electrons that are ejected by a short probe pulse. Circular dichroism means they compared signals produced when the mid-infrared pump light had left- versus right-handed polarization (handedness is called helicity). By subtracting those two measurements, they isolated the geometric, Berry-curvature part of the light-induced response of the Floquet-Bloch states on the surface of Bi2Se3.

In their data the researchers saw alternating-sign features of Berry curvature at the light-induced gaps (Floquet gaps). These features repeat at energy intervals set by the pump photon energy, consistent with the expected pattern of Floquet replicas — copies of the electronic bands spaced by the photon energy. When they changed the strength of the pump field, the pattern of Berry curvature shifted in a way that agrees with calculations for a periodically driven Dirac Hamiltonian, a simple model for electrons near a Dirac point.

An important caveat is that the Berry-curvature signal and the Floquet Dirac gap weakened faster with decreasing drive strength than the spectral intensity of the replicas. In other words, replica bands in the spectra can remain visible even after coherent Floquet hybridization and the induced geometric response have largely decayed. The measurements were performed on the surface states of Bi2Se3 and are interpreted with supporting calculations; they do not by themselves demonstrate bulk transport effects or device performance.