  The spectrum width can be narrowed to a certain degree by decreasing the
coupling strength for the two-level emitter coupled to the propagating surface
plasmon. But the width can not be narrowed any further because of the loss of
the photon out of system by spontaneous emission from the emitter. Here we
propose a new scheme to construct a narrow-band source via a one-dimensional
waveguide coupling with a three-level emitter. It is shown that the reflective
spectrum width can be narrowed avoiding the impact of the loss. This approach
opens up the possibility of plasmonic ultranarrow single-photon source.
