Lightwave electronics in quantum materials – from Floquet band engineering to attoclocking

The concept of ‘lightwave electronics’ has pushed quantum control of condensed matter to unprecedented time scales: By harness­ing the carrier wave of intense light pulses as an alternating voltage, electrons can be driven faster than a cycle of light, unlocking a fascinating coherent quantum world. In the unique environ­­ment of the topological surface state on bulk Bi₂Te₃, terahertz light fields can accelerate electrons like relativistic particles to cover large distan­ces without scattering and heating. This motion leads to a new quality of non-integer high-harmonic generation, whose polarization reveals topologically non-trivial electron trajectories. By advancing angle-resolved photoelectron spectroscopy (ARPES) to subcycle time scales, we can now even visualize the lightwave-driven acceleration of Dirac electrons, the transient formation of Floquet-Bloch states and the non-perturbative interplay of inter- and intraband dynamics in actual subcycle band-structure movies. Our results shed new light on the process of high-harmonic generation and open novel possibilities for ultrafast band-structure engineering.

Also electron–hole pairs in atomically thin quantum materials can be accelerated and collided by strong lightwaves, giving rise to high-order sideband generation. By clocking electron-hole recollisions with attosecond precision, we can proceed beyond the single-particle picture and directly capture how many-body correlations affect the motion of Bloch electrons. Strong Coulomb correlations in atomically thin WSe₂ are found to shift the optimal timing of recollisions by up to 1.2 fs compared to the bulk material. Attosecond chronoscopy of delocalized electrons could become a powerful tool in exploring unexpected phase transitions and emergent many-body quantum-dynamic phenomena.