Coherent Phonons in van der Waals MoSe2/WSe2 Heterobilayers

The increasing roleof two-dimensional (2D) devices requires thedevelopment of new techniques for ultrafast control of physical propertiesin 2D van der Waals (vdW) nanolayers. A special feature of heterobilayersassembled from vdW monolayers is femtosecond separation of photoexcitedelectrons and holes between the neighboring layers, resulting in theformation of Coulomb force. Using laser pulses, we generate a 0.8THz coherent breathing mode in MoSe2/WSe2 heterobilayers,which modulates the thickness of the heterobilayer and should modulatethe photogenerated electric field in the vdW gap. While the phononfrequency and decay time are independent of the stacking angle betweenthe MoSe2 and WSe2 monolayers, the amplitudedecreases at intermediate angles, which is explained by a decreasein the photogenerated electric field between the layers. The modulationof the vdW gap by coherent phonons enables a new technology for thegeneration of THz radiation in 2D nanodevices with vdW heterobilayers.