Quantum Tuning of Plasmons in Ultrathin Metal Films

The surface plasmon is a coherent charge density oscillation localized at a metal surface. It can couple with light and the resulting plasmon-polariton hybrid mode is confined to volumes that are much smaller than the classical diffraction limit of light. Nano-plasmonics is a rapidly evolving field where light manipulation at the nanoscale may lead to novel applications. However, as the size of plasmonic devices approaches the quantum-size regime, the macroscopic picture of plasmon may no longer be valid. To elucidate the influence of the discretization of the single particle spectrum on the collective plasmon response, we performed a systematic study of plasmons in ultrathin metal films, using reflection electron energy loss spectroscopy (REELS). We selected two metal systems, Mg(0001) and Pb(111), grown epitaxially on Si(111) and Ge(111) substrates, respectively. The plasmon response of bulk Mg can be captured within jellium theory, while that of bulk Pb is dominated by band structure effects. Surprisingly, the plasmon response of a 4.5 monolayer (ML) thick epitaxial Mg(0001) film on Si(111)-(7 x 7) already resembles that of the semi-infinite jellium model. However, we unveil a direct correlation between the thickness-dependent oscillatory charge spilling of the quantum well states and the linear dispersion coefficient of the monopole surface plasmon. In addition, the spectral intensities of photoemission threshold excitation and multipole surface plasmon follow a similar quantum oscillatory pattern. These results are attributed to the quantum size effects on the surface charge density profile. The 2 eV excitation associated with an interband transition in bulk Pb redshifts to 0.3 eV in ultrathin films. This excitation is attributed to the symmetric surface plasmon branch, which is rarely seen in metallic films. Its appearance may be related to the perfect interfaces in our studies, in conjunction with the quasi one-dimensional nature of the screening response in Pb(111) films. Both the multipole mode in Mg films and the interface mode in Pb films are highly relevant for plasmonics. The fundamental insight gleaned from these studies may thus have practical relevance as nano-plasmonic feature sizes enter the realm of quantum size physics.