  Optical dynamics in van der Waals heterobilayers is of fundamental scientific
and practical interest. Based on a time-dependent adiabatic GW approach, we
discover a new many-electron (excitonic) channel for converting photoexcited
intralayer to interlayer excitations and the associated ultrafast optical
responses in heterobilayers, which is conceptually different from the
conventional single-particle picture. We find strong electron-hole interactions
drive the dynamics and enhance the pump-probe optical responses by an order of
magnitude with a rise time of ~300 fs in MoSe$_2$/WSe$_2$ heterobilayers, in
agreement with experiment.
