  In the framework of full Maxwell equations, we systematically study the
electromagnetic radiation, namely the transition radiation, when a swift
electron perpendicularly crosses a monolayer graphene. Based on the plane wave
expansion and the Sommerfeld integration, we demonstrate the spatial
distribution of this free-electron radiation process in the frequency domain,
which clearly shows the broadband excitation of both the photons and graphene
plasmons. Moreover, the radiation spectra for the excited photons and graphene
plasmons are analytically derived. We find that the excitation of photons and
graphene plasmons favours different velocities of the swift electron. To be
specific, a higher electron velocity can give rise to the excitation of photons
with better directivity and higher intensity, while a lower electron velocity
can enable the efficient excitation of graphene plasmons in a broader frequency
range. Our work indicates that the interaction between swift charged particles
and ultrathin 2D materials or metasurfaces is promising for the design of
terahertz on-chip radiation sources.
