End-fire coupling efficiencies of surface plasmons for silver, gold, and plasmonic nitride compounds

We present a semi-analytical study exploring the end-fire coupling of a free-space incident beam into a surface plasmon propagating along the interface of a dielectric and a lossy plasmonic material. Using a projection method that calculates the surface plasmon coupling efficiencies for this configuration, we explain in detail how the challenges of including loss, and thus having complex permittivities, have been overcome. A comparison is made of the coupling efficiencies of four plasmonic materials: Ag, Au, TiN, and ZrN. We show that TiN has a consistently higher coupling efficiency than the other three materials for wavelengths between 0.5 and 2.0 μm. All four materials are capable of providing end-fire coupling efficiencies above 87% within this wavelength range.