  The short wavelength of graphene plasmons relative to the light wavelength
makes them attractive for applications in optoelectronics and sensing. However,
this property limits their coupling to external light and our ability to create
and detect them. More efficient ways of generating plasmons are therefore
desirable. Here we demonstrate through realistic theoretical simulations that
graphene plasmons can be efficiently excited via electron tunneling in a
sandwich structure formed by two graphene monolayers separated by a few atomic
layers of hBN. We obtain plasmon generation rates of $\sim10^{12}-10^{14}/$s
over an area of the squared plasmon wavelength for realistic values of the
spacing and bias voltage, while the yield (plasmons per tunneled electron) has
unity order. Our results support electrical excitation of graphene plasmons in
tunneling devices as a viable mechanism for the development of optics-free
ultrathin plasmonic devices.
