Far infrared measurements on YBCO films

We present far infrared measurements on YBCO films deposited on Si substrate. In particular, we measured the radiation intesity transmitted in the spectral range 50-550 cm/sup -1/ through two samples composed by YSCO films of different thickness deposited on the same Si substrate. With this procedure we derived the extinction coefficient of YBCO at temperatures above and below the critical temperature Tc. Above Tc the-data may be described by using -the Drude model. Below Tc the data are consistent with the results of the Mattis-Bardeen theory for the superconducting phase, by assuming an energy gap of about 250 cm/sup -1/.