Many basic condensed matter properties and phenomena are governed by the spectrum of electronic and vibrational degrees of freedom at low energies, comparable to the Boltzmann thermal energy (kBT). They determine both the material’s ground state at finite temperature and its dynamical response to electromagnetic or electronic perturbations. Optical access to these low-energy excitations is the realm of terahertz (THz) spectroscopy: an electromagnetic wave of frequency ν = 1 THz = 1012 Hz corresponds to a photon energy hν ≈ 4.136 meV. As a result, measurement of the THz electromagnetic response in the ≈0.1–50 THz range yields