Coherent Phonons\nin van der Waals MoSe₂/WSe₂ Heterobilayers

Abstract

The increasing role\nof two-dimensional (2D) devices requires the\ndevelopment of new techniques for ultrafast control of physical properties\nin 2D van der Waals (vdW) nanolayers. A special feature of heterobilayers\nassembled from vdW monolayers is femtosecond separation of photoexcited\nelectrons and holes between the neighboring layers, resulting in the\nformation of Coulomb force. Using laser pulses, we generate a 0.8\nTHz coherent breathing mode in MoSe₂/WSe₂ heterobilayers,\nwhich modulates the thickness of the heterobilayer and should modulate\nthe photogenerated electric field in the vdW gap. While the phonon\nfrequency and decay time are independent of the stacking angle between\nthe MoSe₂ and WSe₂ monolayers, the amplitude\ndecreases at intermediate angles, which is explained by a decrease\nin the photogenerated electric field between the layers. The modulation\nof the vdW gap by coherent phonons enables a new technology for the\ngeneration of THz radiation in 2D nanodevices with vdW heterobilayers.