Plasmon polaritons in conducting-metal-oxide films

We report the evolution with thickness of p-polarized reflectance spectra of indium tin oxide (ITO) films deposited on BK-7 glass. ITO is one of the most common examples of the class of conducting metal oxides. Due to the low charge carrier density, $\sim $10-21/cm$^3$, the spectral features of the plasmon are observed in the near infrared. The spectra are dominated by two plasmon polariton structures, which we show are associated with the screened bulk plasmon (SBP) for very thin films and the surface plasmon for thick films. The conductor skin depth is the cut-off between the thin and thick film behavior. Remarkably, all features of these complex spectra are accurately described using only the three-phase model and the Drude free-electron representation of the dielectric function of the films. This first observation of the SBP is made possible by the unique features of these films, which include continuity for even the thinnest films and an absence of complications from bulk absorption in the spectral region of interest. The observation of the SBP is possible due to the fact that ITO behaves as a free electron conductor. Specifically, ITO has no band-to-band transitions that would obscure the intrinsic screed bulk plasmons.