ir surface-plasmon attenuation coefficients for Ge-coated Ag and Au metals

The surface-plasmon attenuation coefficients have been measured for bare and coated Ag and Au metals in the 10-{μ}m wavelength region. For the bare metal agreement is obtained with the Drude model predictions. Next the surface-plasmon propagation characteristics of these same films are studied as a function of Ge overcoating thickness. The predicted linear dependence of the attenuation coefficients with Ge thickness is observed but the measured slopes, which are directly related to the plasma frequency of the metals, are almost a factor of 2 larger than theory can predict. This discrepancy does not appear to be caused by absorption in the Ge, interface states, surface roughness, anomalous skin effect, or transverse zero sound.