Lightwave controlled band engineering of quantum materials

Stacking and twisting atom-thin structures with matching symmetry creates new superlattice structures with emergent quantum properties [1]. In parallel, coherent electron motion in solid can also be manipulated with strong light fields, leading to potential applications in quantum electronics for ultrafast switches of quantum properties at room temperature [2].

We have implemented a tailored lightwave-driven analogue to twisted layer stacking in a hexagonal boron nitride monolayer [3, 4]. We tailored the symmetry of the light waveform to that of the crystal lattice (C₃), which allowed for sub-femtosecond control over time-reversal symmetry breaking and thereby band structure engineering. In this way, for the first time, we have realized the light-analogue of the topological Haldane model [5] in an insulating material, controlling its parameters with sub-femtosecond precision. Twisting the lightwave relative to the lattice orientation enables switching between band configurations, providing unprecedented control over the magnitude and location of the band gap, and curvature. This also allows us to establish a new regime of valleytronics [6] that uses non-resonant light fields and allows valley polarization control on femtosecond timescales.