Coherent THz Radiation from Charge Transfer between Monolayers of Transition Metal Dichalcogenides

We report the observation of coherent electromagnetic radiation from ultrafast charge transfer in MoS₂/WS₂ vertical heterostructures excited by an above-band-gap femtosecond laser. The emitted electric field profile is a near-half-cycle pulse in the THz frequency regime. The field polarities are consistent with the charge transfer direction expected for the type-II band alignment, and are opposite for heterostructures with opposite stacking order. The field strength scales linearly with excitation power and absorbance of the heterostructure. The observations are reproduced in samples made with different growth and fabrication methods. Using a high-bandwidth setup we obtain an upper limit of net charge transfer time constant of ~100 fs.