  Temperature dependence of far-infrared transmission of NbN thin films
deposited on MgO and Si substrates was measured at several frequencies from 0.4
up to 4.3 THz. Exponential increase of relative penetration depth at low
temperatures and a peak in transmission near Tc was observed for frequencies
below the optical gap. On the contrary, transmission measured at frequencies
above the gap exhibits only flat, almost linear temperature dependence. This
behavior is consistent with the BCS theory of superconductivity. Similar
measurements were performed also on YBaCuO thin films deposited on MgO and
sapphire substrates. Low-temperature variation of transmission is linear or
constant and the peak below Tc predicted by the BCS theory is not observed.
Flat temperature dependence at higher frequencies indicates that the photon
energy was sufficient for excitation over the optical gap. The s-wave BCS
theory is adequate for NbN films but not for the YBCO materials.
