One Kelvin means 21 GHz: Probing superconductors with low-frequency optics

Superconductors are characterized by several energy scales such as the critical temperature or the superconducting energy gap that characterizes the Cooper-pair formation. These energy scales can be probed by low-energy optical spectroscopy at cryogenic temperatures, but such experiments are challenging due to the spectral regime that spans from the THz range down to GHz frequencies. We describe THz and GHz experiments that elucidate different relevant energy scales of diverse low-temperature superconductors, including the `superconducting domes' in the phase diagrams of granular aluminum and Nb-doped SrTiO
₃
.