  We report on terahertz frequency-domain spectroscopy (THz-FDS) experiments in
which we measure charge carrier dynamics and excitations of thin-film
superconducting systems at low temperatures in the THz spectral range. The
characteristics of the set-up and the experimental procedures are described
comprehensively. We discuss the single-particle density of states and a theory
of electrodynamic absorption and optical conductivity of conventional
superconductors. We present the experimental performance of the setup at low
temperatures for a broad spectral range from 0.1 - 1.1 THz by the example of
ultra-thin films of weakly disordered superconductors niobium nitride (NbN) and
tantalum nitride (TaN) with different values of critical temperatures.
Furthermore, we analyze and interpret our experimental data within the
framework of conventional Bardeen-Cooper-Schrieffer (BCS) theory of
superconductivity.
