Abstract-Absolute brightness measurements in the terahertz frequency range using vacuum and thermal fluctuations as references

My Note: This is slightly dated, but I wanted to include.

G. Kh. Kitaeva, 

P. V. Yakunin, 

V. V. Kornienko, 

A. N. Penin

http://link.springer.com/article/10.1007/s00340-014-5779-0

A total procedure for the terahertz wave brightness calibration using the quantum fluctuations at terahertz frequencies as a reference is formulated. An experimental proof for its theoretical background is provided. Klyshko method for the brightness calibration using spontaneous parametric down-conversion is modified, considering the case when the idler wave hits the terahertz frequency range and relative additives to the background signals induced by an external terahertz radiation can be measured both in Stokes and anti-Stokes ranges. It is shown at different temperatures that the thermal-fluctuation-induced signals have the same spectral and angular shapes, as the spontaneous parametric down-conversion signals, while the thermal-induced signal amplitudes are proportional to the number of photons in the thermal equilibrium modes