Terahertz and infrared spectroscopy of superconductors

In the talk, results of experimental investigations of terahertz and infrared electrodynamics of different families of superconductors will be reviewed. In the conventional BCS-superconductors, a pronounced gap feature dominates the terahertz response below T_c. In the cuprates, entering into the superconducting state is accompanied by developing of a narrow zero-frequency centered mode in the real part of the in-plane complex conductivity. In the multi-gap superconductors (such as MgB₂), opening of the gap on each sheet of the Fermi surface contributes to the changes of dynamic complex conductivity.
A particular stress will be given on electron-doped cuprates, the electrodynamical response of which was found to have a remarkable dependence on the doping level. Significant changes in the gap anisotropy or even in the gap symmetry may account for this dependence.
The majority of the experiments have been performed on a home-made spectrometer, which utilizes backward-wave oscillators as sources of coherent and frequency-tunable terahertz radiation. An interferometer arrangement of the spectrometer has allowed both, amplitude and phase shift measurements, thus permitting direct (without using the Kramers-Kronig relations) determination of the optical constants.