  Plasmonics offers an enticing platform to manipulate light at the
subwavelength scale. Currently, loss represents the most serious challenge
impeding its progress and broad impact towards practical technology. In this
regard, silver (Ag) is by far the preferred plasmonic material at optical
frequencies, having the lowest loss among all metals in this frequency range.
However, large discrepancies exist among widely quoted values of optical loss
in Ag due to variations in sample preparation procedures that produce
uncontrollable grain boundaries and defects associated with additional loss. A
natural question arises: what are the intrinsic fundamental optical properties
of Ag and its ultimate possibilities in the field of plasmonics? Using
atomically-smooth epitaxial Ag films, we extracted new optical constants that
reflect significantly reduced loss and measured greatly enhanced propagation
distance of surface plasmon polaritons (SPPs) beyond what was previously
considered possible. By establishing a new benchmark in the ultimate optical
properties of Ag, these results will have a broad impact for metamaterials and
plasmonic applications.
