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We report thorough measurements of surface plasmon polaritons (SPPs) runningalong nearly perfect air-gold interfaces formed by atomically flat surfaces ofchemically synthesized gold monocrystals. By means of amplitude- andphase-resolved near-field microscopy, we obtain their propagation length andeffective mode index at visible wavelengths (532, 594, 632.8, 729, and 800 nm).The measured values are compared with the values obtained from the dielectricfunctions of gold that are reported in literature. Importantly, a reporteddielectric function of monocrystalline gold implies $\sim 1.5$ times shorterpropagation lengths than those observed in our experiments, whereas adielectric function reported for properly fabricated polycrystalline gold leadsto SPP propagation lengths matching our results. We argue that the SPPpropagation lengths measured in our experiments signify the ultimate limit ofoptical losses in gold, encouraging further comprehensive characterization ofoptical material properties of pure gold as well as other plasmonic materials.

Ultimate limit for optical losses in gold, revealed by quantitative near-field microscopy