Particularly in safety-critical components there is an ongoing trend to hundred percent complete inspections which can only be achieved through non-destructive testing methods. So far, a final inspection of the weld is often associated with the costly destruction of the product since current non-destructive testing methods such as optical microscopy (based on light from the visible or infrared region) are restrained by the large absorption or scattering found in many plastics. Hence, for certain visually non-transparent materials such as polypropylene, polyamide and fiber reinforced polymers the currently available non-destructive testing methods are unable to uncover defects and inhomogeneity in laser welds. Accordingly, process and quality control are impaired due to insufficient feedback information.
Terahertz based non-destructive testing of polymers
Radiation from the Terahertz (THz) frequency range is very promising for the non-destructive testing of polymer components because of its high transmissivity through most polymers – essentially regardless of color or composition. Consequently, THz imaging has already been tested for polymer sorting applications  and the analysis of large-scale polymer weld joints  using classic diffraction-limited far-field transmission schemes. Typical air void defects in polymer laser welds have diameters in the range of ca. 50-100 µm. Unfortunately, due to the large wavelength of THz rays the spatial resolution which can be achieved through far-field approaches is insufficient to recognize such microscopic defects or inhomogeneities found at laser-generated welds.
THz light scattering from defects
Using microprobes as a key component for THz light detection close to the sample surface instead of far-field detection enables the monitoring of micron-scale buried air voids and inhomogeneity in laser welds, as demonstrated now in a study undertaken by Protemics GmbH  and the Fraunhofer ILT , both located in Aachen, Germany. The reason for the much better recognition performance of the microprobe-based detection in contrast to earlier approaches is given by the ability to measure very efficiently the light scattering generated by the local inhomogeneities in the welds. The process of scattered light generation at a buried air void in a polymer bulk material is sketched in Fig. 2 (a)-(c). The structure under test is shown in cross-sections at different times of plane wave excitation. A THz plane wave pulse is incident from the bottom side and propagating towards the air void (Fig. 2 (a)). The scattering interaction between the incident plane wave and the air void is generating a second spherical wave propagating away from the defect into every room direction. Due to the decreased phase retardation within the air void in comparison to the bulk material the undeflected forward propagating part of the scattered wave is running in front of the plane wave. Both waves are subject to interference effects which is also visible in the more detailed field simulation shown in Fig. 3. As in Fig. 2 the incident plane is propagating into the upper direction, too. The shown THz field amplitude refers to the field vector component in transversal direction.
The reason, why the surface-near detection is superior to standard far-field detection becomes very apparent from these simulation results. The main signal contribution caused by the presence of the void is the scattering light, however, because of the radial divergence of the scattered wave this information is almost completely lost at larger distances from the sample where the far-field is detected. Likewise, interference effects are much more pronounced at closer distances. As described by theory for Mie scattering  the extinction efficiency is maximum for particle sizes close to the wavelength of the incident light, which is giving a further reason why the THz light is especially attractive for this application.
The samples investigated in this work have been processed at the Fraunhofer – Institute for Laser Technology ILT in Aachen, Germany. They are based on a polypropylene opaque to visible light with a material thickness of 1000 µm per joining partner. A buried laser weld with a lateral width of ca. 380 µm has been generated between both parts.
Using microprobes to pick up the scattering light
The measurement system used for the tests in this work has been described in detail in an earlier publication . A THz plane wave pulse is transmitted through the sample under test (SUT). In order to measure the transmitted field in the time-domain the tip of the microprobe is scanned across a virtual plane in a distance of a few tens of micrometers above the SUT. Fig. 4 (a) is showing the recorded THz field image at the time when the peak amplitude of the plane wave has just reached the microprobe. The image exhibits a large reddish colored background area (corresponding to the peak of the THz plane wave) including an L-shaped lighter area, which refers to the laser-welded area. Within this area there are 6 prominent spots generated by air voids showing strongly decreased THz field amplitudes reaching even negative values. The measured differences between the spots are attributed to size differences of the air voids. Fig. 4 (b) is showing a further image taken at a time-delay of 230 fs later than Fig. 4 (a). Here, the propagation of the spherical waves scattered from the air voids is clearly visible.
The Terahertz microprobing technique is a highly attractive novel approach for the inspection of laser welds in polymers. The new method is paving the way for the non-destructive inspection of critical types of polymers such as polypropylene or fiber reinforced polymers which can only be analyzed by destructive methods, so far.
 A. Maul, M. Nagel, “Polymer identiﬁcation with terahertz technology,” OCM 2013-Optical Characterization of Materials-conference proceedings, 265 (2013).
 S. Wietzke, C. Jördens, N. Krumbholz, et al. „Terahertz imaging: a new non-destructive technique for the quality control of plastic weld joints,” Journal Of The European Optical Society – Rapid Publications, 2 (2007).
 H. C. van de Hulst, Light Scattering by Small Particles (John Wiley & Sons, Inc., 1957).
 M. Nagel, A. Safiei, S. Sawallich, C. Matheisen, T. M. Pletzer, A. A. Mewe, N. J. C. M. van der Borg, I. Cesar, H. Kurz „THz microprobe system for contact-free high-resolution sheet resistance imaging,” 28th European Photovoltaic Solar Energy Conference and Exhibition, pp. 856-860 (2013).