Low-energy plasmons in quantum-well and surface states of metallic thin films

Abstract

We studied low-energy plasmons in ultrathin films of silver in the thickness regimes where the surface states as well as quantum-well states must play significant roles. Realistic band structure was adopted for assessing the quantum-mechanical effect on the low-energy charge dynamics. In addition to the expected quasi-twodimensional plasmon mode, we find the modes that resemble an acoustic surface plasmon on semi-infinite metal surfaces and an additional plasmon mode related to the interband transitions between the two slab-split surface states. It is found that the dispersion of the latter mode is almost identical to the acoustic surface plasmon except the energy offset at small momenta values. The peaks in the surface response function related to interband transitions between the surfacelike states and bulklike states are identified as well. The present work elucidates the role of quantized electronic states and surface states on the plasmonic excitations in the ultimately thin films potentially used in the future nano-optics devices.

The influence of electron confinement, quantum size effects, and film morphology on the dispersion and the damping of plasmonic modes in Ag and Au thin films

“…Apart from bulk noble-metal systems, which are not the focus of the present review, ASP has been predicted also for ultrathin films of silver [67,77]. The most prominent feature in the surface loss function of ultrathin films is a strongly dispersing peak, corresponding to the conventional antisymmetric plasmon mode in thin films, whose energy depends on the thickness d as:…”

“…In addition to the expected quasi-2D plasmon mode, Silkin et al [77] find modes resembling ASP on semi-infinite metal surfaces. In particular, it is shown [77] that the quantization of electron states within the Ag adlayer implies the occurrence of low-energy collective electronic modes.…”

“…In particular, it is shown [77] that the quantization of electron states within the Ag adlayer implies the occurrence of low-energy collective electronic modes. The presence of Ag QWS in ultrathin films gives rise to an acoustic plasmon, which becomes the ASP at higher Ag thickness.…”

“…Adapted from Ref. [77]. atomic steps or molecular structures, can allow coupling between ASP and light.…”

“…In addition, the typical energy of the ASP (from zero up to 1 eV) makes it a channel of dissipation for the excess energy released in surface reactions. Thus, the observation of ASP in noble-metal thin films [77] opens perspectives in energy conversion, transport, and confinement far below optical frequencies.…”