We have developed a far-infrared-absorption-measurement method using an optically pumped far-infrared laser. The temperature dependence of the light absorption by high-T, superconductors has been investigated for the photon energy between 7.6 and 17.6 meV in the temperature region between 12 and 160 K. A well-defined transition temperature was obtained from the absorption measurement. With decreasing temperature, the absorption decreased, but did not reach zero even at su%ciently low temperatures.
The temperature dependence of the absorption of the thick niobium nitride films was measured using an AC far-infrared laser calorimeter. Moreover the temperature and frequency dependences of the absolute transmission and reflection of same thin niobium nitride frims were measured from 5 to 20 K and from 10 to 200 cm ' using a fourier transform infrared spectrometer. The temperature dependences of the skin depth and the absorptance determined independently by both methods are compared. However the skin depths and the absorptances agree well each other near and above T, they begin to deviate from each other with decreasing temperature below T.
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