For laser-welded polymers, final inspection of the weld is often associated with the costly destruction of the product since current nondestructive 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—especially visually nontransparent materials such as polypropylene, polyamide, and fiber-reinforced polymers.
While terahertz imaging is very promising for the nondestructive testing of polymer components because of its high transmissivity through most polymers, its long wavelength limits its spatial resolution through far-field approaches to roughly 1–2 mm, which is inadequate for inspection considering that typical air-void defects in polymer-laser welds have diameters in the range of 50–100 μm. To circumvent this problem, researchers at Protemics and the Fraunhofer Institute for Laser Technology (Fraunhofer ILT), both in Aachen, Germany, are using terahertz microprobes close to the surface of a polymer weld rather than far-field detection in order to uncover micron-scale voids and defects.1,2
Terahertz scattering signatures
In contrast to far-field approaches, microprobe-based detection efficiently measures the light scattered from local inhomogeneities in a weld. A terahertz plane-wave pulse incident from underneath a weld seam propagating toward an air void generates a second spherical wave that propagates away from the defect into every 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 runs in front of the plane wave, and both waves are subject to interference effects (see Figs. 1 and 2).
A 100 fs, 80 MHz pulsed 780 nm erbium fiber laser optically excites a photoconductive terahertz emitter and probe in a classic pump/probe scheme to analyze 380-μm-diameter lateral width welds in opaque polypropylene sheets with a material thickness of 1000 μm per joining partner. To measure the transmitted field in the time domain, the tip of the microprobe is scanned across a virtual plane at a distance of a few tens of micrometers above the weld surface.
The terahertz microprobe consists of a tapered pair of electrodes patterned on a 1.3-μm-thick gallium-arsenide cantilever. Recorded terahertz field images clearly indicate voids when the peak amplitude of the plane wave just reaches the microprobe; scattering continues as the wave propagates. Measured differences between the imaged spots are attributed to different-sized air voids.
Near-surface detection is superior to standard far-field detection because the information from the radial scattered wave 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 the Mie scattering theory, the extinction efficiency is a maximum for particle sizes close to the wavelength of the incident light, a further reason why terahertz light is especially attractive for this application.
"Measuring the terahertz light immediately at the surface is the key enabling the inspection of micron-scale structures and defects," says Protemics CTO Christopher Matheisen. "We found that this is still true even when the microstructures are buried under a few millimeters of plastic as in this case. By probing the near-field light scattering, we can see structures that are much too small to be recognized with standard far-field systems. We believe that the technology will find a great demand in the growing sector of laser-based manufacturing where micron-scale resolution is extremely important."
REFERENCES
1. M. Nagel et al., "THz microprobe system for contact-free high-resolution sheet resistance imaging," 28th EUPVSEC Conference, 856–860, Villepinte, France (2013).
