THz imaging system for industrial material investigations

Goal of this project is the development of a specific THz-imaging system for material investigations. This system in reflexion geometry uses pulsed THz radiation and, besides conventional cross-sectional imaging, will allow us to record transversal images parallel to the sample surface, and to exploit the polarization properties of the THz radiation. The corresponding concepts have been developed for transversal and polarization-sensitive (PS) optical coherence tomography (OCT) and will be investigated and adopted for THz-measurements for the first time. For transversal scanning, a galvano-scanner unit will be applied. An advantage is the increased imaging speed, since no mechanical translation of the sample is required, as used in most of the to-date THz systems. Furthermore, instead of the entire THz waveform only one single data point at a defined optical depth within the sample is recorded, and therefore depth-scanning of a delay line is omitted. By exploiting the polarization properties of the THz radiation, depth-resolved information about birefringence (and consequently about internal stress or anisotropies) shall be obtained. As the effect of birefringence on THz imaging is a field nearly unexplored so far, different concepts for PS-THz imaging will be investigated, adopted and evaluated. The developed THz system will in particular be applied and tested for industrial purposes, such as analysis of plastics, polymers and composite materials. As future area of applications, contactless and non-destructive quality control is targeted, which will profit from the large penetration depths of THz radiation. With regard to these applications, the suitability of the transversal PS-THz system for defect detection as well as its potential for the investigation of stress will be investigated. Therefore, the properties of various relevant materials (penetration depth, scattering properties, birefringence) will be analyzed. Further issue is the comparison between THz imaging and OCT. These are closely related methods with similar applications. However, OCT works on a different scale with penetration depths of only a few millimetres and a depth-resolution of a few microns. By measurements of identical samples, the strengths and limitations of the respective methods in particular for industrial problems will be revealed. With regard to future industrial applications, the THz system will be available for interested companies for test measurements, such as of helicopter rotor blades or of epoxy high-voltage isolators. The project will be performed by the Department of Contactless Sensor Technology of the Upper Austrian Research GmbH (UAR), Linz, in collaboration (within the frame of the D-A-CH agreement) with the Terahertz Systems Group (TSG) of Prof. Koch the Institute for High-Frequency Engineering (Technical University of Braunschweig, Germany). The experimental setup and the THz investigations will be done in the labs of UAR. Role of TSG is the development of improved THz emitters and of novel PS-detector structures on one chip.