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Detector standard for terahertz radiation

Especially interesting for
  • metrology institutes
  • manufacturers and users of THz radiation sources

The first capacity worldwide for the precise calibration of the power responsivity of detectors for terahertz (THz) radiation has been developed at PTB in the form of a novel detector standard. Its spectral responsivity was traced back to the International System of Units (SI) by means of visible laser light. This was achieved with a standard measurement uncertainty of less than 2 % for all emission lines of a molecular gas laser in the spectral range from 1 THz to 5 THz.

Terahertz radiation has wavelengths that are nearly 1000 times longer than those of visible light. The radiation's diffraction is stronger, so that it is more difficult to focus it compared to visible light. The aperture of a THz detector must therefore be large enough to detect all the radiation without losses due to diffraction. Also, the wavelength-dependent absorption of the detector absorber has to be taken into account.

The absorber and detection medium for the new THz standard is a special neutral-density glass plate of optical quality. It is mounted into a conventional measuring instrument for laser radiant power. The optical losses could be determined accurately because, in addition, the rear side of the glass plates was coated with a thin layer of gold. The specular reflection of the front surface is thus the sole significant detection loss. This could be measured accurately – namely both in the THz and in the visible spectral ranges where the absorptivity of the neutral-density glass is similar. It was thus possible to trace back the spectral responsivity of the THz detector to the International System of Units (SI) at the wavelength of a red helium-neon laser with the lowest possible uncertainty and to extend this precision into the THz range. A consistent THz scale is essential for risk assessment in potential new applications of THz radiation, which is invisible and penetrates numerous substances. Examples are security applications, medical diagnostics and the non-destructive testing of materials.

PTB is the first metrology institute to be able to calibrate THz detectors. This service immediately sparked interest worldwide: the first calibration orders were placed by customers from Israel, the USA, France and Canada. Further customers in Japan, Russia and China were served with THz detectors developed in collaboration with a German manufacturer and calibrated by PTB.


Andreas Steiger
Department 7.3 Detector Radiometry and Radiation Thermometry
Phone : +49 (0)30 3481-7532
Opens window for sending emailandreas.steiger(at)ptb.de

Scientific publication

R. Müller, W. Bohmeyer, M. Kehrt, K. Lange, C. Monte, A. Steiger: Novel detectors for traceable THz power measurements. Journal of Infrared, Millimeter, and Terahertz Waves 35, 8, 659–670 (2014)