Continuous wave THz spectroscopy of solids

Low-energy excitations in solids are among the most fascinating fields in condensed matter physics, albeit one of the most challenging. Here we highlight some aspects explored in recent years, e.g. collective phenomena in broken symmetry ground states, electrodynamics of the superconducting state, localization effects due to electronic and magnetic interactions. The experiments have been conducted by quasi-optical methods using a continuous wave THz spectrometer, with backward wave oscillators as coherent but tunable radiation sources.