Introducing an intermediate section between two metal-insulator-metal plasmonic waveguides with unequal width considerably enhances the transmission spectra of a direct junction. In this paper, we design various junctions based on analytic optimization to obtain maximum power transfer through a junction. Using advantages of quasi-static approximation for subwavelength devices, a pure analytic expression is derived, which leads to broader bandwidth and higher transmittance at given frequency. We achieve zero reflection from 125 to 25 nm width MIM junctions by inserting transition sections consisting of quarter-wavelength and tapered structures between two waveguides. Our analysis and optimization results are numerically validated by the finite-difference time-domain simulation.
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