Emissivity measurement of high-Tc superconductive materials using terahertz passive imaging apparatus with background-limited sensitivity

We have carried out a terahertz (THz) passive measurement of high-T_c superconductive materials down to liquid nitrogen temperature at 1.5-2.5 THz using a 4 K-cryocooled stressed Ge:Ga photoconductive detector. Using the calibration line, which gives the relation between THz signal emitted from sample, emissivity and temperature, we have derived the temperature-dependent THz emissivity of superconductive materials. The result shows that emissivity of superconductive materials steeply change around each superconductive transition temperature. We also demonstrated the THz passive imaging of superconductive wires and successfully obtained THz passive images before and after superconductive transition.