Lightwave-controlled band engineering in quantum materials

Abstract In recent years, the stacking and twisting of atom-thin structures with matching crystal symmetry has provided a unique handle to create new superlattice structures where new properties emerge 1,2 . In parallel, control over the temporal characteristics of strong light fields has allowed to manipulate coherent electron transport in such atom-thin structures on sub-laser-cycle timescales 3,4 . Here, we demonstrate a tailored lightwave-driven analogue to twisted layer stacking. Tailoring the spatial symmetry of the light waveform to that of the lattice of a hexagonal boron nitride monolayer, and twisting this waveform results in optical control of time-reversal symmetry breaking 5 , and the realization of the topological model of Haldane 6,7 in the laser-dressed 2D insulating crystal. Further, the parameters of the effective Haldane-type Hamiltonian are controlled by the rotating light waveform, enabling ultrafast switching between band structure configurations and unprecedented control over the magnitude, location, and curvature of the band gap. A resultant asymmetric population at complementary quantum valleys leads to a measurable valley Hall current 8 , detected via optical harmonic polarimetry. The universality and robustness of our scheme opens the way to band engineering on the fly, unlocking the possibility to create few-femtosecond switches of quantum degrees of freedom.