  We study a plasmonic coupler involving backward (TM_01) and forward (HE_11)
modes of dielectric waveguides embedded into infinite metal. The simultaneously
achievable contradirectional energy flows and codirectional wavevectors in
different channels lead to a spectral gap, despite the absence of periodic
structures along the waveguide. We demonstrate that a complete spectral gap can
be achieved in a symmetric structure composed of four coupled waveguides.
