Studies of the residual absorption of HTSC at submillimeter wavelengths by means of photothermal interference spectroscopy

The determination of the residual, low temperature absorption of high temperature superconductors is of interest for applications of this new material at submillimeter wavelengths and of basic interest. The photothermal interference spectroscopy allows us to measure the residual, low temperature absorption of a HTSC. For the determination of the residual absorption of a superconductor a far-infrared beam is periodically modulated and focused on the sample. Absorption leads to a periodic change of the temperature of the sample surface and, due to heat diffusion, also in the gas volume adjacent to the sample. This temperature change in the gas is detected via the refractive index change using a two beam interferometer. We studied the residual losses of YBaCuO thin films on various substrates and of BiSrCaCuO (2212) single crystals at submillimeter wavelengths. We find that the frequency dependence of the absorptivity, which shows a frequency squared behavior at microwave frequencies, is less than quadratic at THz-frequencies. The YBaCuO thin films show a plateau between 0.6 THz and 4 THz with an absolute value of the absorptivity of about 10 ^-2. A BiSrCaCuO single crystal shows a plateau between 1 THz and 4 THz with an absorptivity in the order of 10 ^-3.