Extraordinary SPP propagation distance in epitaxially grown silver film*

We measured greatly enhanced propagation distances of surface plasmon polaritons (SPPs) on an atomically-smooth epitaxial silver \textit{(Ag}) film beyond what was previously considered possible. These extraordinary propagation lengths approach the fundamental limit determined by the new optical constants measured in these films. We excited and detected the SPPs in reflection geometry. Light incident on a single groove launched the SPPs, which were subsequently detected at a series of output coupling slits with increasing distance from the launching site. We used incident wavelengths of 632nm and 880nm and extracted propagation distances of 22$\mu$m and 42$\mu$m, respectively. Calculations using the optical constants measured on the same film predict distances of 42$\mu$m at 63nm and 155$\mu$m at 880nm. The discrepancy is mainly due to scattering from the 1-2 monolayer fluctuations at the \textit{Ag} surface. The propagation distance extrapolated from our measurements far exceeds the speculated theoretical limit ($\sim$ 5X) in template stripping \textit{Ag} films.