Anisotropy in the transparency of HTS films at millimeter and submillimeter microwave radiation

The propagation of microwave radiation through YBa₂Cu ₃O_7-δ and Tl₂Ba₂Cu₁O₆ films deposited on Al₂O₃, MgO, LaAlO₃ and LaSrAlTaO single crystal substrates, as well as through the bare substrates, has been studied in the 60-600 GHz frequency range. The angular dependence of the transmitted radiation was found to have a rather complicated structure. The dependence is affected by anisotropy of the substrate due to birefringence, resonance in substrate modes, non-homogeneity and the granular structure of the HTS film. To reduce reflections from the surface of the substrate, quarter-wave matching layers were used at selected frequencies. After correction for stray effects, the common feature is the presence of four maxima with a period of π/2 with four minima in between that can be due to the energy gap anisotropy in d-wave superconductors