Figure 2.

Propagation constants k_z of the three lowest eigenmodes of an infinitely long gold wire as a function of wire radius R. Displayed are the real (solid blue lines) and imaginary (dotted red lines) parts of k_z pointing along the taper axis for angular momenta n = 0,1,2 (panels a)–c)). The SPP frequency corresponds to a vacuum wavelength of 800 nm, as used in the experiment. For n = 0, both Re(k_z) and Im(k_z) diverge for small radii. For n = 1, k_z approaches the light line, k₀ = ω/c₀, for small radii. A cutoff behavior at approximately 600 nm is observed for n = 2. Panel d) summarizes these findings and schematically illustrates the mode profiles.