  Among various plasmonic waveguides, the metal-insulator-metal (MIM) type is
the most promising for true subwavelength photonic integration. To date, many
photonic devices based on MIM waveguides have been investigated, including
resonators. However, most of the reported MIM ring resonators suffer from low
extinction ratios. In this paper, we present a comprehensive analysis of the
intrinsic reasons for the low performance of MIM ring resonators, and give the
analytical transmission relation for a universal all-pass ring resonator which
has coupling loss. Based on the analysis we propose the plasmonic racetrack
resonators in MIM waveguides and show that the performance can be greatly
improved.
