Fraunhofer Institute for Industrial Mathematics ITWMOne of the established measurement techniques in the terahertz spectral range is terahertz time domain spectroscopy (TDS). It is based on the generation of broadband electromagnetic radiation by ultrashort femtosecond (fs) laser pulses and on detection with the pump-probe principle. The advantages here are a coherent detection of the terahertz waves and thus a high-resolution amplitude and phase recording of the terahertz electric field in the time domain. This measurement technique suppresses incoherent radiation, i.e. there is no interference from room temperature and ambient light.
Keramikreaktor
Visualizing the Invisible
With the help of Terahertz time-domain spectroscopy (TDS) we can characterize materials such as dry chemicals, solids and liquids with a non-contact and non-destructive method.
© Fraunhofer ITWM
Terahertz Time-Domain Spectroscopy
Main Applications
The most important application focus is on time-of-flight measurements, where the time-of-flight of the reflected signals is analyzed. In this way, distances and coating thicknesses can be determined very accurately. This approach underlies the industrially used paint thickness measurement.
Terahertz time-domain spectroscopy is a reliable method for the non-contact and non-destructive characterization of materials, e.g. powders, solids or liquids and gases. Many molecules show characteristic signatures in their absorption spectra in this spectral range – a chemical fingerprint.
Gases show rotational spectra with narrow absorption lines. Only crystalline solids show broad absorption bands due to the phonon vibrations of the crystal. Therefore, powdery or solid substances must have at least a crystalline component to be identified by these absorption bands. Liquids therefore do not show distinct absorption bands. Spectra are evaluated using chemometric methods for reliable substance identification under real operating conditions. The Postscanner is based on the combination of terahertz spectroscopy and chemometric evaluation for the identification of drugs and explosives in mail.
Unlike IR and Raman spectroscopy – which are sensitive to intramolecular vibrational and rotational motions – terahertz spectroscopy provides information on intermolecular motions. Thus, in addition to the mere detection of macromolecules, statements can be made about the state of aggregation, polymorphic structures as well as the crystallinity of the substances.