Plasmon coupling and finite size effects in metallic multilayers

This work uses electron energy loss spectroscopy (EELS) to study the excitation of interface and bulk plasmons in artificial, nonmagnetic metallic multilayers. We obtain EELS spectra for W/Si and Ta/Si multilayers with layer periodicity d using an off‐axis acquisition technique and compare the results with dispersion relations and energy loss probability distributions predicted from dielectric theory. Our energy loss spectra, taken for a fixed out‐of‐plane wavevector k z = π/ d and low in‐plane wavevectors k y < 1/ d , contain features which are identified as coupled interface modes by solving the appropriate superlattice dispersion relation. These interface modes are accompanied by bulk plasmons of the individual media. We find that dielectric theory greatly overemphasizes these bulk modes. This behaviour is interpreted phenomenologically as a finite size effect and is modelled by introducing a cut‐off for the long‐wavelength bulk modes.