  Inefficient coupling between dielectric guided mode and plasmon mode has been
overlooked in the past. The mechanism in it is essentially different from the
conventional coupling between dielectric modes. Based on qualitative
theoretical analysis, we proposed two methods to strengthen the coupling
between dielectric waveguide modes and exterior plasmon whispering gallery
modes. One is using a U-shaped bent waveguide to break the adiabatic mode
conversion process, and the other is to render the dielectric mode of
higher-order to reach phase matching with plasmon mode. Both the transmission
spectrum of waveguide and the energy spectrum of cavity demonstrate that the
coupling efficiencies can be greatly improved. These simple configurations are
potential for wide applications, for example, tunable integrated optical
devices, nanolasers and sensors.
