  Measurement of the penetration depth as a function of temperature using
millimeter wave transmission in the range 130-500GHz are reported for three
YBCO thin films. The experiment provides the absolute value for the penetration
depth at low temperature: the derivation from the transmission data and the
experimental uncertainty are discussed. We find a zero temperature penetration
depth of 199+/-20nm, 218+/-20nm and 218+/-20nm, for YBCO-50nm/LaAlO3
(pristine), YBCO-130nm/MgO and YBCO-50nm/LaAlO3 (irradiated) respectively. The
penetration depth exhibits a different behavior for the three films. In the
pristine sample, it shows a clear temperature and frequency dependence, namely
the temperature dependence is consistent with a linear variation, whose slope
decreases with frequency: this is considered as an evidence for the scattering
rate being of the order of the measuring frequency. A two fluids analysis
yields 1.7 10^{12} s^{-1}. In the two other samples, the penetration depth does
not display any frequency dependence, suggesting a significantly larger
scattering rate. The temperature dependence is different in these latter
samples. It is consistent with a linear variation for the YBCO/MgO sample, not
for the YBCO/LaAlO3 irradiated one, which exhibits a T^2 dependence up to 40K.
We have compared our data to the predictions of the d-wave model incorporating
resonant scattering and we do not find a satisfactory agreement. However, the
large value of the penetration depth at zero Kelvin in the pristine sample is a
puzzle and sheds some doubt on a straightforward comparison with the theory of
data from thin films, considered as dirty d-wave superconductors.
