  We demonstrate an unexpectedly strong surface-plasmonic absorption at the
interface of silver and high-index dielectrics based on electron and photon
spectroscopy. The measured bandwidth and intensity of absorption deviate
significantly from the classical theory. Our density-functional calculation
well predicts the occurrence of this phenomenon. It reveals that due to the low
metal-to-dielectric work function at such interfaces, conduction electrons can
display a drastic quantum spillover, causing the interfacial electron-hole pair
production to dominate the decay of surface plasmons. This finding can be of
fundamental importance in understanding and designing quantum nano-plasmonic
devices that utilize noble metals and high-index dielectrics.
