We study the coupling mechanism between a high refractive index contrast waveguide and a plasmonic thin-film waveguide in the IR range. We also propose a novel design of a vertical coupler based on loading the plasmonic waveguide with a high refractive index contrast medium on each side. We achieve a coupling efficiency and an insertion loss of about 95% and 0.2 dB, respectively, with a coupling length of only 2.85 μm at the working wavelength of 1.55 μm.