September 24, 2008--For the first time, the spectral responsivity of terahertz (THz) radiation detectors has been accurately characterized over a large wavelength range; this was accomplished by the Physikalisch-Technische Bundesanstalt (PTB; Braunschweig), the German national metrology institute.
Terahertz radiation may lead to easier and more thorough techniques for security checks of people and their belongings at airports and other transport stations. Before THz radiation can be used for this and other purposes, however, it has to be measured quantitatively, so that damage to health caused by radiation can be ruled out.
Radiation in the THz range (with wavelengths from 30 µm to 3000 µm and frequencies from 0.1 THz to 10 THz) penetrates clothing and many other organic materials, and furthermore offers spectroscopic information on safety-relevant materials such as explosives and pharmacological substances. The broad spectrum of the possible applications extends from security checks to the investigation of the spatial and/or time structure of the electron packages in storage rings for synchrotron radiation production and in free-electron lasers--for which the receivers of the German Electron Synchrotron DESY (Hamburg and Zeuthen) characterized by the PTB are used.
Complete information on THz spectra can only be determined using detectors of known spectral responsivity. Up to now, the integral responsivity of THz detectors and their spectral distribution have been largely unknown. The PTB has now for the first time determined the spectral responsivities of two THz detectors in the wavelength range from 50 µm to 600 µm with the aid of cavity radiators.
To make spectral radiation fluxes available in the THz range, calculable according to Planck's radiation law, the PTB uses two THz cavity radiators at different temperatures in connection with THz band and longwave-pass filters. The interior surfaces of the radiators are lined with a coating that possesses a known, high emissivity in the THz range, enabling the intensity of the radiation incident on the detector to be calculated. To obtain a sufficiently spectral purity of the THz radiation, a suppression of the infrared radiation of more than nine orders of magnitude is necessary.
By using an FT-IR spectrometer, the transmittance for all filter combinations was accurately determined in the wavelength range from 0.8 µm to 1700 µm. Due to the known amount of radiation from the cavity radiators and the known transmittance of the filters, it was possible to accurately determine the spectral radiation fluxes and thus determine the absolute spectral responsivities of THz receivers. Such absolutely characterized receivers will in the future be used at the PTB's Metrology Light Source to investigate its characteristics in the THz range, and to investigate the effect of THz radiation on biological cell cycles.