THz spectroscopy of superconductors

Low-energy excitations in solids are among the most fascinating fields in condensed matter physics, albeit one of the most challenging too. Superconductors were probably the first systems extensively studied by probing with electromagnetic radiation in the terahertz (THz) frequency range; and after 50 years, the THz spectroscopy of superconductivity is still the most active field ranging from fundamental questions to commercial applications. Here, we highlight some aspects of the electrodynamics of the superconducting state explored in recent years. The experiments we will concentrate on have been conducted by quasi-optical methods using a continuous wave THz spectrometer, with backward wave oscillators as coherent and tunable radiation sources.