Abstract

The absolute photometric measurements in terahertz (THz) frequency range are hampered by the lack of the etalon sources. Another problem is the limitation in the conventional techniques which either require cryogenic cooling, such as calibrated bolometers, or lack the sensitivity required, e.g. Golay Cells. But it is still attractive to possess a room-temperature scheme for measuring the values of spectral brightness of terahertz wave radiation (TR). The difficulties in calibration may be overcome utilizing a nonlinear-optical terahertz detection scheme. The origin of this approach comes from the work of David N. Klyshko where the zero-vacuum fluctuations were proposed to be used as a reference for the absolute calibration of the optical brightness [1]. Recently we have shown that while considering the terahertz range, this method should be modified due to significance of the thermal fluctuations compared to the vacuum ones at THz frequencies [2]. For measuring the TR spectral brightness it is necessary to detect both the signals of TR conversion to the optical range and the noise signal generated due to conversion of thermal and vacuum fluctuations at the same THz frequency, known as the spontaneous parametric down-conversion (SPDC) process.

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