Christian Jördens
Braunschweig University of Technology
Network
Latest external collaboration on country level. Dive into details by clicking on the dots.
Publication
Featured researches published by Christian Jördens.
Applied Optics | 2010
Christian Jansen; Steffen Wietzke; Ole Peters; Maik Scheller; N. Vieweg; Mohammed Salhi; N. Krumbholz; Christian Jördens; Thomas Hochrein; Martin Koch
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.
Optical Engineering | 2008
Christian Jördens; Martin Koch
We demonstrate the detection of metallic and nonmetallic foreign bodies in chocolate using pulsed terahertz imaging. Investigating the shape of the temporal waveform allows for the discrimination between wanted ingredients like nuts on one hand and foreign bodies like stone, glass, or plastic particles on the other hand. Yet, the intensity image alone does not provide enough information to evaluate the quality of the chocolate bar. To achieve a low false-alarm rate it is important to measure the height profile of the sample and to include the measured results in the image-processing step. Our results show that terahertz imaging can be used for the detection of contaminations in chocolate bars. Furthermore, other kinds of dry food can be investigated with our technique.
Optics Express | 2011
Benedikt Scherger; Christian Jördens; Martin Koch
We present a variable focus lens for the THz range. The focal length can be changed by pumping a medical white oil in and out of the lens body. Due to the optical transparency of the liquid and a similar refractive index in the visible frequency range, the THz beam path can be aligned using conventional optical light sources. This type of lens might find applications in terahertz based quality control, stand-off detection and wireless communication systems.
Journal of Biological Physics | 2009
Christian Jördens; Maik Scheller; B. Breitenstein; Dirk Selmar; Martin Koch
We present an electromagnetic model of plant leaves which describes their permittivity at terahertz frequencies. The complex permittivity is investigated as a function of the water content of the leaf. Our measurements on coffee leaves (Coffea arabica L.) demonstrate that the dielectric material parameters can be employed to determine the leaf water status and, therefore, to monitor drought stress in plant leaves. The electromagnetic model consists of an effective medium theory, which is implemented by a third order extension of the Landau, Lifshitz, Looyenga model. The influence of scattering becomes important at higher frequencies and is modeled by a Rayleigh roughness factor.
Applied Physics Letters | 2006
Thomas Kleine-Ostmann; Christian Jördens; Kai Baaske; Thomas Weimann; M. Hrabe de Angelis; Martin Koch
We investigate the conductivity of single-stranded and double-stranded herring deoxyribose nucleic acid (DNA) in buffer solution spotted and dried on Au nanocontacts. We find an exponential increase of the conductivity with increasing humidity that is identical for single- and double-stranded DNA within the measurement accuracy. While the small conductivity of dry DNA is comparable to that of a large band-gap semiconductor, we attribute the increase at high humidity levels to water molecules accumulated at the phosphate backbone. For high humidities we observe s-shaped current-voltage characteristics that can be well explained by the dissociation of water attached to the DNA molecules.
Applied Optics | 2009
Christian Jördens; Maik Scheller; Matthias Wichmann; Martin Mikulics; Karin Wiesauer; Martin Koch
A terahertz time-domain spectrometer is employed to study different birefringent samples. We develop a method based on the temporal waveform and the impulse response of a sample to map the anisotropy of their inner structure. To validate our algorithm, we study the polarization-affecting structure of various classes of materials such as crystals, plastics, and natural products. Among all samples we observe the largest birefringence for a rutile crystal with Deltan=3.3 at 1 THz.
Optics Express | 2010
Maik Scheller; Christian Jördens; Martin Koch
We report on one-dimensional photonic crystals designed to exhibit a pronounced form birefringence at terahertz frequencies. The crystals can be used as volumetric quasioptical elements for a broad frequency range. Theoretical simulations of the dielectric parameters of these structures are presented as well as measurement results of a polymeric crystal that exhibit a birefringence of 0.25 at 300 GHz. As a potential application, the device is exemplarily used as terahertz wave plate.
International Symposium on Photoelectronic Detection and Imaging 2009: Terahertz and High Energy Radiation Detection Technologies and Applications | 2009
Steffen Wietzke; Christian Jansen; Christian Jördens; N. Krumbholz; N. Vieweg; Maik Scheller; Mohammad Khaled Shakfa; D. Romeike; Thomas Hochrein; M. Mikulics; Martin Koch
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.
ieee international conference on solid dielectrics | 2010
Steffen Wietzke; Christian Jansen; N. Krumbholz; Ole Peters; N. Vieweg; Christian Jördens; Maik Scheller; D. Romeike; T. Jung; Marco Reuter; S. Chatterjee; Martin Koch; Fachbereich Physik; Benjamin Baudrit; T. Zentgraf; Thomas Hochrein; Martin Bastian
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.
international conference on infrared, millimeter, and terahertz waves | 2009
Maik Scheller; Steffen Wietzke; Christian Jansen; Christian Jördens; Marcus Lehnhardt; Martin Koch
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.