Cryogenic blackbody calibration source for superconducting terahertz detectors

Abstract

Blackbody radiation source has been widely used as a calibration source for terahertz (THz) radiometers in recent decades with the applications of THz detection technology in the fields of aerospace, astronomy and remote sensing. We develop a THz blackbody calibration source capable of working in the cryogenic environment and having adjustable radiation power for the calibration of THz superconducting detectors. The ideal blackbody source has an emissivity and absorptivity of 1 and the reflectance coefficient is used to indirectly characterise the performance of the developed blackbody source. In this work, we use a mixture of epoxy, catalyst, carbon black and glass beads as blackbody absorbing material. The real part and imaginary part of the complex dielectric constant of Berkeley blackbody material are extracted from the THz time-domain spectra, and its reflection coefficient is measured. We use this material to design a conical blackbody radiation source , and simulate it as well. The simulation result show that it has low reflectivity below –35 dB in a frequency range of 0.2–0.5 THz. We fabricate a conical blackbody radiation source that is mounted in a dilution refrigerator, and use filters and light-guiding systems to make the detector for measuring the radiation by the THz light of a specific wavelength. The radiation power can be tuned by changing its temperature. The relationship between radiation power and temperature shows a power tuning range of 10<sup>–12</sup>–10<sup>–9</sup> W in the frequency range of 0.2–0.5 THz with a minimum power value of 2.13 × 10<sup>–12</sup> W. The designed blackbody radiation source can meet the calibration requirements of THz superconducting detectors, and will contribute to the development and application of highly sensitive THz radiometers.