Measurements of the residual absorption of high T_c superconductors at THz-frequencies using the photothermal interference spectroscopy

For the measurement of the absorptivity of a sample a modulated FIR beam is focused on the sample leading due to absorption to a periodic modulation of the temperature of the sample and of a layer of the adjacent gas. This temperature change is detected by the use of a two beam interferometer. We report about measurements of the low temperature residual absorption of thin YBaCuO films on SrTiO₃ and MgO and of a BiSCCO single crystal. We find that the absorptivity at low temperature, which follows a frequency squared behaviour at microwave frequencies up to 100 GHz, shows a plateau in the frequency range between 0.6 THz and 4 THz.