Ralf Gente

Monitoring leaf water content with THz and sub-THz waves

Terahertz technology is still an evolving research field that attracts scientists with very different backgrounds working on a wide range of subjects. In the past two decades, it has been demonstrated that terahertz technology can provide a non-invasive tool for measuring and monitoring the water content of leaves and plants. In this paper we intend to review the different possibilities to perform in-vivo water status measurements on plants with the help of THz and sub-THz waves. The common basis of the different methods is the strong absorption of THz and sub-THz waves by liquid water. In contrast to simpler, yet destructive, methods THz and sub-THz waves allow for the continuous monitoring of plant water status over several days on the same sample. The technologies, which we take into focus, are THz time domain spectroscopy, THz continuous wave setups, THz quasi time domain spectroscopy and sub-THz continuous wave setups. These methods differ with respect to the generation and detection schemes, the covered frequency range, the processing and evaluation of the experimental data, and the mechanical handling of the measurements. Consequently, we explain which method fits best in which situation. Finally, we discuss recent and future technological developments towards more compact and budget-priced measurement systems for use in the field.

Scaled Bistatic Radar Cross Section Measurements of Aircraft With a Fiber-Coupled THz Time-Domain Spectrometer

The knowledge of the radar cross section (RCS) of aircraft and other objects is of great interest both for civil and military applications. Scaled setups are often used in order to facilitate RCS measurements in a well-defined laboratory environment. As radar frequencies steadily increase, for high scaling factors these measurements have to be carried out in the THz regime. In this paper, we propose an experimental setup consisting of a fiber-coupled THz time-domain spectrometer integrated with a two circle goniometer, which enables bistatic scaled RCS measurements. To assess the accuracy of the setup, measurements on reference objects as well as on scale model aircraft are performed. The measured data of the reference objects is compared to the theoretical predictions. As for the aircraft, the comparison between a Panavia 200 Tornado and a Lockheed F117 Nighthawk is made and the influence of individual components like bombs on the overall RCS is evaluated.

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.

Quality Control of Sugar Beet Seeds With THz Time-Domain Spectroscopy

Sugar beet seeds have a comparably high rate of defective seeds of about 35%-40% which are discarded before dissemination in the field. The separation of proper and defective seeds is a complex and labor-intensive process. We show how THz time-domain spectroscopy can be used for the examination of the seeds including an algorithm for automated classification of the seeds.

Ultra-broadband THz time-domain spectroscopy of common polymers with THz air-photonics

Several common polymers are characterized in the ultra-broadband Terahertz frequency window 2-15 THz using a THz time-domain spectrometer solely based on air-photonics. The spectral features relevant for materials science and THz photonics are revealed.

Plant water status monitoring with THz QTDS

We demonstrate how water status measurements on plants can be performed using terahertz quasi time domain spectroscopy (THz QTDS). Compared with conventional THz TDS, this approach enables much more cost-efficient measurement setups and thus brings the technology closer to its application in the field. Exemplary data is obtained from examining the drought stress response of rye and soy.

Freestanding terahertz metamaterials fabricated by laser beam machining

We demonstrate freestanding terahertz metamaterials, fabricated by laser-beam machining of commercial Aluminum-foil. This method allows for cost-efficient and rapid prototyping. Low insertion losses and outstanding transmission characteristics render the samples as high performance filters.

Water status in plants monitored by THz time-domain spectroscopy

We present a method to determine the water content of vegetative tissue from terahertz transmission spectroscopy based on effective medium theory.