Surface plasmon polaritons are highly localised field oscillations that propagate along interfaces between a dielectric and a metal. Their localised fields make them potential signal carriers between integrated circuits, replacing electronic signals, to allow wider bandwidths and less power dissipation. End-fire coupling is a simple, compact method for exciting surface plasmon polaritons with potential for high coupling efficiency. However, the parameters that determine the mechanism's optimisation are not well understood.
We present a semi-analytical model of the end-fire mechanism, in which a free-space incident beam excites a surface plasmon polariton in a configuration with a single transverse dimension. We first model coupling into a surface plasmon polariton on an air-lossless metal interface. Our projection method, a key modelling component, uses an energy conservation expression which includes contributions from both propagating and evanescent fields, which become significant for near-field sources. By optimising the incident beam width and position, we find maximum coupling efficiencies of up to ~90% over the wavelength range of λ∈[0.5,1.8] μm, averaging ~80%. We also find that each excitation produces not only the primary surface plasmon polariton, but also a transversely propagating secondary surface plasmon polariton.
We then adapt our method to simulate lossy plasmonic materials, replacing the lossless metal with silver, gold, titanium nitride and zirconium nitride using their values from literature. By optimising the incident beam parameters, we find that the coupling efficiencies of all four materials reach 87% over the range λ∈[0.38,2] μm. These high coupling efficiencies are explained using an impedance-matching argument, concluding that most of the surface plasmon polariton field is in the dielectric, not the plasmonic material. Using a simple analytical model, we show that there is a universal relationship between the optimum beam width and position.