Fundamental aspects of surface plasmon polaritons at terahertz frequencies

We present in this chapter an introduction to the field of terahertz (THz) plasmonics. The characteristics of surface plasmon polaritons (SPPs) are determined by the complex permittivity of conductors. Therefore, we introduce the Drude model to describe the permittivity of conductors at THz frequencies. The large absolute value of the permittivity of metals leads to a weaker confinement of SPPs in these materials compared to semiconductors. We illustrate the characteristic lengths of SPPs on flat surfaces by calculating them on an InSb–SiO2 interface at different temperatures. These characteristic lengths are the wavelength, propagation and confinement lengths. The tunability of the permittivity of semiconductors by controlling the free charge carrier concentration, for example, with temperature, opens the possibility of active THz plasmonic components. We also describe long-range and short-range SPPs in thin layers of semiconductors surrounded by a homogeneous dielectric. These surface modes arise from the coupling of SPPs at the opposite sides of the thin film. New trends in the field of THz plasmonics, comprising designed and localized surface plasmon polaritons, are also discussed in this chapter.