T. Probst

Compact and low-cost THz QTDS system.

We present a terahertz quasi time domain spectroscopy (QTDS) system setup which is improved regarding cost and compactness. The diode laser is mounted directly onto the optical delay line, making the optical setup more compact. The system is operated using a Raspberry Pi and an additional sound card. This combination replaces the desktop/laptop computer, the lock-in-amplifier, the stage controller and the signal generator. We examined not only a commercially available stepper motor driven delay line, but also the repurposed internal mechanics from a DVD drive. We characterize the performance of the new system concept.

Alignment and illumination issues in scaled THz RCS measurements

Radar cross section (RCS) measurements are widely employed for the target identification of objects such as aircraft. The RCS of large, complex geometries is usually determined in a well defined, scaled experimental setup. In such a setup, the measuring frequency equates the frequency of interest multiplied by the scaling factor of the model. For high scaling factors, frequencies well above 100 GHz result so that conventional microwave equipment reaches its technological limits. In this paper, we discuss the use of a fiber coupled terahertz time domain spectroscopy system for angle dependent RCS measurements. Several measurements on simple objects like metal plates with corresponding simulations are performed. We also investigate how angular misalignment and displacement of the scatterer effects the measurement results. Moreover, we discuss the influence of non planar incident wavefronts and compare experimentally obtained data to simulations using an enhanced physical optics model.

Cost-efficient delay generator for fast terahertz imaging

We present a fast and low-cost delay generator for terahertz (THz) waves that transfers a rotational motion of a transparent dielectric cube into an effective THz delay. The device is easily implemented in the THz beam path and allows for coherent sampling over 40 ps with a scan rate of hundreds of hertz. Furthermore, we show that our approach is particularly suitable for fast THz imaging.

Terahertz transceiver concept

We present a photoconductive terahertz transceiver based on a modulation of the optical pulses used for generation and detection at different rates. External modulation of the THz pulses is not required as opposed to previously reported approaches. Devices from fiber-optic technology are used, providing flexibility and stability to the system. Imaging and thickness measurement experiments are carried out to demonstrate the performance of the transceiver.

Terahertz plastic compound lenses

We present terahertz (THz) lenses made of highly refracting polymeric compounds which provide a better focusing performance and an increased functionality in comparison to conventional THz lenses. Using mixtures consisting of polypropylene (PP) and alumina as well as PP and zinc sulfide allows a significant increase of the refractive index while simultaneously keeping a low extinction and dispersion. With these new material combinations, lenses with an increased focusing capability are realized. This is evaluated by focal plane measurements using a fiber coupled THz time-domain spectrometer.

Fiber-coupled terahertz transceiver heads for reflection measurements

We present two fiber-coupled terahertz (THz) time-domain transceiver heads that are well suited for zero degree reflection measurements and allow for flexible handling. One consists of only one photoconductive switch that is capable of emitting and detecting at the same time and the other operates with two antennas and a beam splitter made from a polypropylene-titanium (PP-TiO2) compound.

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.

Cure monitoring of two-component epoxy adhesives by terahertz time-domain spectroscopy

We report nondestructive measurements of the properties of two-component epoxy adhesives at terahertz wavelengths using a transmissive time-domain spectroscopy system. The results show that the different epoxies have measurably different THz properties, that the changes which occur during the curing process can be monitored by measurements of their refractive index and absorption at terahertz wavelengths, and that this technique is sensitive enough to record changes in the optical material parameters during the postcuring process.

Z-scan based fiber-coupled coherent cw THz imaging system

We present a fiber-coupled cw THz system for fast and accurate imaging in the field of non-destructive testing. Fibercoupled THz antennas make this system flexible and robust. The time delay is not realized via an optical delay line but by moving the detector antenna along the optical axis (Z-Scan).