The use of spontaneous parametric down-conversion for calibration of the terahertz wave brightness

Among different approaches for compact table-top terahertz wave (THz) sources and detectors, the nonlinear-optical frequency-conversion schemes have been in the forefront during the last decades. They produce conversion of optical energy of visible and NIR lasers into the THz gap under THz generation, and can be used for THz detection using advanced optical technologies via up-conversion of the THz radiation into the optical range. Nevertheless, the problem of absolute measurement of THz fields is very important for such devices, as well as for other THz schemes, but still remains as not properly solved. We propose to use the quantum-optical effect of spontaneous parametric down-conversion for calibration of the spectral brightness of CW or quasi-CW radiation in the course of its non-linear-optical detection. It will be shown how the effective brightness of the quantum vacuum fluctuations (1 photon per mode in quantum units) can serve as a brightness reference in the terahertz spectral range.