  Quantum manifestations of various properties of metallic thin films by
quantum size effect (QSE) have been studied intensively. Here, using
first-principles calculations, we show quantum manifestation in dielectric
properties of Al(111) ultrathin films. The QSE on the dielectric function is
revealed, which arises from size dependent contributions from both intraband
and interband electronic transitions. More importantly, the in-plane interband
transitions in the films thinner than 15 monolayers are found to be smaller
than the bulk counterpart in the energy range from 1.5~eV to 2.5~eV. This
indicates less energy loss with plasmonic material of Al in the form of
ultrathin film. Our findings may shed light on searching for low-loss plasmonic
materials via quantum size effect.
