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Emissivity measurement of cold objects down to liquid-nitrogen temperature with a 4K-cryocooled terahertz photoconductive detector

We have carried out the passive measurement of cold objects down to liquid nitrogen temperature at 1.5-2.5 THz using a 4 K-cryocooled terahertz (THz) photoconductive detector. We measured output signals of THz radiation from five materials of a 2 mm-thick acrylic plate, a 0.1 mm-thick pure H2O, a 2 mm-thick high-resistivity silicon substrate, a #2010 black velvet coated plate, and a gold plate as a function of temperature. By assuming emissivities of the #2010 black velvet coated plate and gold plate are 1 and 0 respectively, we calculated the calibration lines of THz output signal as functions of emissivity and temperature. Using the calibration lines, we have derived the temperature-dependent THz emissivity of three samples.