Ultrafast Pulsed Terahertz Sensing for Process Monitoring and Industrial Imaging

We present here a recently developed approach of Terahertz Time Domain Spectroscopy (THz-TDS) and its application on rapid and precise Non-Destructive Testing (NDT) of composites and concealed objects. Electromagnetic radiation of THz frequencies can penetrate dielectric materials without ionizing effect; it allows the contact-free testing of dielectric volumes or buried metallic surfaces and combinations of them. THz-TDS is a particular powerful tool for NDT as it provides a multitude of information useful at the same time for imaging, material characterisation and spectroscopic analysis. Amplitude and time of flight of the ultrashort THz pulse in the time domain, as well as the spectrally resolved amplitude and phase information available by FourierTransformation, enable the analysis of complex objects. In many cases, three dimensional monitoring is possible with one-sided access to the object. The single shot THz pulsed sensor developed at the Laboratory of Physical Chemistry reduces the acquisition time drastically relative to the available repetitive solutions of THz-TDS: the ultrashort detection window of some tens of picoseconds is recorded within one single laser shot. The measurements are performed at high stability and acquisition of kHz rates, without moving parts. With this leap in technology, the potential of terahertz radiation can be further exploited in industrial environments such as production lines. We recently analysed an object and the integrity of its structural sub-units during free fall. The here presented applications illustrate further the performance of the technology, particular its ability to determine the thickness of substructures with micrometre precision, to rebuild the 3D form of concealed objects, to localize defects and differentiate between metals, air and dielectrics. Often the defects can be further characterized. It is shown that the high contrast of ultrafast THz pulses on dielectrics enables the localization and analysis of polymers that are buried in another polymer.