Surface plasmon polaritons and screened plasma absorption in indium tin oxide compared to silver and gold

This study addresses the ability of a conducting metal oxide to act as a material capable of supporting surface plasmon polaritons (SPPs). By use of a two-phase Fresnel model that provides insight into the difference between polariton-induced and absorptive decreases in reflectivity, indium tin oxide (ITO) is compared to the noble metals gold and silver, which are widely used as materials for surface plasmon resonance (SPR) detection of analytes. The study builds on application of the Drude free electron model that provides an explanation for the observed optical extinctions as a function of ITO film thickness, including the dependence of wavenumber on angle, by use of only two adjustable parameters, the plasma frequency cop and the damping F. Herein, models of the dispersion and absorption include both the real and imaginary parts of the dielectric function to obtain the plasma absorption spectra and dispersion curves for ITO, Ag, and Au. ITO is found to have surface plasmon dispersion curves and plasma absorption free from interband transitions as observed in Ag but not Au. Au has significant contribution from bound electrons that damps the plasma absorption and broadens the dispersion curve of the SPP. Ag is more like a free-electron conductor than An but still has some absorption in the frequency range above the observed surface plasmon. ITO is a nearly ideal free-electron conductor.