Steffen Wietzke

Terahertz imaging: applications and perspectives

Terahertz (THz) spectroscopy, and especially THz imaging, holds large potential in the field of nondestructive, contact-free testing. The ongoing advances in the development of THz systems, as well as the appearance of the first related commercial products, indicate that large-scale market introduction of THz systems is rapidly approaching. We review selected industrial applications for THz systems, comprising inline monitoring of compounding processes, plastic weld joint inspection, birefringence analysis of fiber-reinforced components, water distribution monitoring in polymers and plants, as well as quality inspection of food products employing both continuous wave and pulsed THz systems.

Terahertz time-domain spectroscopy as a tool to monitor the glass transition in polymers.

We demonstrate the suitability of terahertz time-domain spectroscopy as a non-destructive, contact-free tool to monitor the glass transition in polymers--a core feature of the amorphous phase. Below the glass transition temperature T(g), segmental motions along the polymer chain are frozen due to the lack of free volume between neighboring macromolecules. We show that this transition also reflects in the temperature dependence of the refractive index at terahertz frequencies. Two domains can be identified, which differ in their sensitivity to temperature changes. To verify the proposed approach, we determine the glass transition temperature T(g) of semi-crystalline poly(oxymethylene) (POM) with terahertz time-domain spectroscopy and validate the results by destructive differential scanning calorimetry (DSC) measurements.

Modelling heterogeneous dielectric mixtures in the terahertz regime: a quasi-static effective medium theory

We present a quasi-static effective medium theory for heterogeneous dielectric mixtures consisting of particles embedded in a host matrix. The model remains valid for all mixture concentrations and can also consider particle shape distributions, thus overcoming the restrictions of previous approaches. The theory is experimentally validated by highly accurate terahertz time domain spectroscopy measurements on three different polymeric compound systems. A comparison with commonly applied theories is given.

Mechanically flexible polymeric compound one-dimensional photonic crystals for terahertz frequencies

We describe the fabrication and characterization of fully flexible one-dimensional photonic crystals for terahertz frequencies based on alternating layers of a highly refracting polymericcompound and a pure polymer. Due to the high permittivity contrast and low intrinsic material absorption, even a low number of layers yields structures with a pronounced artificial band gap, with a center frequency reflectivity close to unity. Besides the high filter performance, the polymericcompound one-dimensional photonic crystals can be employed for beam shaping by applying a curvature to the mechanically flexible structures, since the spectral characteristics remain nearly unchanged when the structure is flexed.

Thermomorphological study of the terahertz lattice modes in polyvinylidene fluoride and high-density polyethylene

Semicrystalline polymers possess a complex morphology comprising both crystalline and amorphous domains. We investigate two representatives from this group, polyvinylidene fluoride and high-density polyethylene, with terahertz time-domain spectroscopy. Intriguingly, the thermal gradient of lattice modes arising from the crystalline phase changes at the glass transition. Thus, terahertz spectroscopy can identify the glass transition temperature Tg even in highly crystalline polymers, where conventional methods such as differential scanning calorimetry fail. Furthermore, these findings provide experimental evidence of the strong interactions between the amorphous and the crystalline domains, as the glass transition is an exclusive feature of the mobile amorphous fraction.

Industrial applications of THz systems

Terahertz time-domain spectroscopy (THz TDS) holds high potential as a non-destructive, non-contact testing tool. We have identified a plethora of emerging industrial applications such as quality control of industrial processes and products in the plastics industry. Polymers are transparent to THz waves while additives show a significantly higher permittivity. This dielectric contrast allows for detecting the additive concentration and the degree of dispersion. We present a first inline configuration of a THz TDS spectrometer for monitoring polymeric compounding processes. To evaluate plastic components, non-destructive testing is strongly recommended. For instance, THz imaging is capable of inspecting plastic weld joints or revealing the orientation of fiber reinforcements. Water strongly absorbs THz radiation. However, this sensitivity to water can be employed in order to investigate the moisture absorption in plastics and the water content in plants. Furthermore, applications in food technology are discussed. Moreover, security scanning applications are addressed in terms of identifying liquid explosives. We present the vision and first components of a handheld security scanner. In addition, a new approach for parameter extraction of THz TDS data is presented. All in all, we give an overview how industry can benefit from THz TDS completing the tool box of non-destructive evaluation.

Terahertz spectroscopy: A powerful tool for the characterization of plastic materials

Terahertz (THz) spectroscopy holds a high potential as non-destructive, contact-free testing tool for the analysis of macromolecules, the monitoring of plastic processing and the inspection of plastic components. Even molecular properties, such as the glass transition temperature of highly-crystalline polymers, can be derived from THz measurements. Furthermore, we have identified a plethora of emerging applications in the plastics industry. To appeal to these upcoming challenges, we developed the first THz time-domain spectroscopy (TDS) system for the inline monitoring of compounding processes, which was awarded the Otto von Guericke Prize 2009. It has been demonstrated that the filler content at the end of the extrusion line could be precisely determined by time-of-flight measurements. In addition, THz technology is capable of measuring the water content sensitively, THz birefringence measurements reveal the fiber orientation in reinforced plastics and the quality of plastic weld joints or adhesive bonds can be inspected by the interference evaluation of the THz data. Due to the outstanding dielectric contrast of materials at THz frequencies, even contaminations invisible to x-rays or ultrasonic measurements can be revealed by THz TDS imaging opening the door for a new generation of non-destructive, contact-free quality control systems.

Investigating Material Characteristics and Morphology of Polymers Using Terahertz Technologies

The high transmittance of polymers in the terahertz (THz) frequency range is certainly the main reason for the pronounced interest this class of materials receives from the community. Polymers are commonly applied as base material for optical components, sample carriers, or simply test samples, in most fields of THz spectroscopy and imaging. On the other hand, polymers offer an important opportunity for demonstrating the potential of THz technologies as versatile tool for analytical investigations. In this paper, we summarize some recent technical advances underlining the suitability of THz spectroscopy and imaging for material characterization and contact-free testing. The absorption coefficient and refractive index of several biodegradable polymers before and after aging in water are reported. Furthermore, the influence of additive agglomeration in elastomers complements the evaluation of this review of the analytical potential of THz technologies with respect to polymers.

Applications for effective medium theories in the terahertz regime

We investigate various composite systems in the terahertz frequency regime and apply different effective medium theories to model their dielectric response. A broad variety of composites is considered, ranging from polymeric compound mixtures to biological samples. For each case, the effective medium theory of choice is illuminated in detail.

Applications of terahertz spectroscopy in the plastics industry

The worldwide production volume of polymers is still rising exponentially and the number of applications for plastic components steadily increases. Yet, many branches within the polymer industry are hardly supported by non-destructive testing techniques. We demonstrate that terahertz (THz) spectroscopy could be the method of choice to ensure high-quality polymer products. Applications range from the in-line monitoring of extrusion processes and the quality control of commodities in a mass production up to a total inspection of high-tech safety relevant products. Furthermore, we present an extension to THz time-domain spectroscopy in the form of a new data extraction algorithm, which derives the absorption coefficient, the refractive index and the thickness of a sample with very high precision in a single pass. Apart from that, we discuss the ability of THz systems for quality control of polymeric compounds. Here, it is essential to monitor the additive content as well as additive inhomogeneities within the mixture. Recently, we built a fiber-coupled THz spectrometer for in-line monitoring of compounding processes. Additionally, we demonstrate the potential of THz systems for the non-destructive and contactless testing of structural components. THz imaging is capable of analyzing material thicknesses, superstructures, the quality of plastic weld joints, and of detecting flaws in components. Plastics and THz form a very fruitful symbiosis. In return, plastics industry can provide THz systems with custom-tailored components, which have very attractive properties and extremely low costs. Examples of this development are photonic crystals or polymeric Bragg filters, which have recently been demonstrated.