  We propose a closed form formulation for the impedance of the
Metal-Insulator-Metal (MIM) plasmonic transmission lines by solving the
Maxwell's equations. We provide approximations for thin and thick insulator
layers sandwiched between metallic layers. In the case of very thin dielectric
layer, the surface waves on both interfaces are strongly coupled resulting in
an almost linear dependence of the impedance of the plasmonic transmission line
on the thickness of the insulator layer. On the other hand, for very thick
insulator layer, the impedance does not vary with the insulator layer thickness
due to the weak-coupling/decoupling of the surface waves on each
metal-insulator interface. We demonstrate the effectiveness of our proposed
formulation using two test scenarios, namely, almost zero reflection in
Tee-junction and reflection from line discontinuity in the design of Bragg
reflectors, where we compare our formulation against previously published
results.
