The article presents the results of experimental studies of attenuation of terahertz (THz) surface plasmon polaritons (SPPs) on sputtered gold as a function of the radiation wavelength ( \(\lambda\) ). The experiments were carried out with the Novosibirsk free electron laser radiation in the range \(\lambda\) = 51–357 μm (0.84–6 THz). It has been found that for a real metal-air interface (with inhomogeneities), the dependence of the THz SPP propagation length on \(\lambda\) differs significantly from the quadratic dependence calculated for an ideally smooth and homogeneous interface using the Drude model for the bulk metal permittivity \({\varepsilon }_{1}^{\text{Drude}}\) . Our results convincingly indicate that the main cause of this difference are the intense radiative losses of THz SPPs caused by the SPP scattering on surface inhomogeneities rather than the difference between the effective permittivity \({\varepsilon }_{1}^{\text{eff}}\) of the metal skin layer and \({\varepsilon }_{1}^{\text{Drude}}\) . Moreover, the role of the radiative losses in the SPP attenuation grows with increasing wavelength much faster than the role of the ohmic losses does. This effect is a consequence of the declining difference between the refractive indices of SPPs and a bulk wave in air with increasing \(\lambda\) . Besides, it has been shown that the radiative losses can be suppressed dozens of times by means of dielectric coatings with thickness of λ/350, which enables using THz SPPs in various THz applications.
