Subcycle Observation of Lightwave-Driven Dirac Currents in a Topological Surface Band

Employing the carrier wave of light as an AC bias may pave the way to electronics at optical clock rates. Lightwave-driven currents play a key role in high-harmonic generation in solids [1-3], attosecond-streaking experiments [4], and atomic-scale ultrafast microscopy [5]. In conventional semiconductors and dielectrics, however, the finite effective mass and ultrafast scattering limit the electron's ballistic excursion and velocity.