Enhanced Free-Electron-Photon Interactions at the Topological Transition in van der Waals Heterostructures.

Heterostructures composed of graphene and molybdenum trioxide (MoO) can support in-plane hybrid polaritons in the infrared. The isofrequency contour for these subwavelength polaritons can exhibit a quasi-flat region when the topological transition occurs as the doping level of graphene is tuned. Such a topological transition can be useful for optical sensing and imaging at nanoscale. Here, by analyzing electron energy-loss spectroscopy (EELS), we theoretically demonstrate that free-electron-photon interactions in the heterostructure can be enhanced due to this quasi-flat region. Moreover, the free-electron-photon interaction is sensitive to the electron trajectory and is robust against certain types of defects in the structure. Furthermore, we show that the free-electron-photon interaction can undergo an ultrafast subpicosecond modulation by optical pumping and heating of graphene. Our findings may pave the way toward dynamical electron beam shaping, free-electron-based quantum light sources, and quantum sensing.