We exploit a plasmon mediated two-step momentum downconversion scheme to
convert low-energy tunneling electrons into propagating photons. Surface
plasmon polaritons (SPPs) propagating along an extended gold nanowire are
excited on one end by low-energy electron tunneling and are then converted to
free-propagating photons at the other end. The separation of excitation and
outcoupling proofs that tunneling electrons excite gap plasmons that
subsequently couple to propagating plasmons. Our work shows that electron
tunneling provides a non-optical, voltage-controlled and low-energy pathway for
launching SPPs in nanostructures, such as plasmonic waveguide