This paper demonstrates optical control of time-reversal symmetry breaking and the realization of the topological Haldane model in a laser-dressed two-dimensional insulating crystal of monolayer hexagonal boron nitride (hBN). By tailoring the spatial symmetry of the light waveform to match the hBN lattice and twisting this waveform, the researchers achieved ultrafast switching between band structure configurations, controlling the bandgap's magnitude, location, and curvature. This led to an asymmetric valley population and a measurable valley Hall current, detectable via optical harmonic polarimetry. The method's universality and robustness offer a pathway for on-the-fly valley-selective bandgap engineering and few-femtosecond quantum switches.

Sambit Mitra, Álvaro Jiménez-Galán, Mario Aulich, Marcel Neuhaus, Rui E. F. Silva, Volodymyr Pervak, Matthias F. Kling, Shubhadeep Biswas