T. Hasek

Terahertz birefringence of liquid crystal polymers

The authors present terahertz time-domain spectroscopy (TDS) measurements on injection molded liquid crystalline polymers (LCPs). Pure LCP yields a strong birefringence of Δn=0.2. They find that the direction of the optical axis varies within the injection molded sample. By terahertz (polarization) imaging domains of certain directions of the optical axis could be observed. These strongly depend on the mold geometry. Since the alignment of particles and rodlike polymer molecules tend to follow the melt flow during injection molding, terahertz TDS can be used to analyze injection molded LCP parts and control the process performance.

Photonic crystals for fluid sensing in the subterahertz range

The authors present experimental results on photonic crystals in the subterahertz range for fluid-sensing applications. Using a standard machining process, a two-dimensional photonic crystal consisting of a triangular array of air holes in the dielectric medium was produced. The insertion of fluids with different refractive indices into the active region affects the transmission profile in the stop band. The experiment agrees well with theoretical calculations.

Fiber-coupled THz spectroscopy for monitoring polymeric compounding processes

We present a compact, robust, and transportable fiber-coupled THz system for inline monitoring of polymeric compounding processes in an industrial environment. The system is built on a 90cm x 90cm large shock absorbing optical bench. A sealed metal box protects the system against dust and mechanical disturbances. A closed loop controller unit is used to ensure optimum coupling of the laser beam into the fiber. In order to build efficient and stable fiber-coupled antennas we glue the fibers directly onto photoconductive switches. Thus, the antenna performance is very stable and it is secured from dust or misalignment by vibrations. We discuss fabrication details and antenna performance. First spectroscopic data obtained with this system is presented.

A fiberstretcher operating as an optical delay line in a fiber-coupled THz spectrometer

We demonstrate the use of a fiber stretcher as an optical delay line in a fiber-coupled THz spectrometer. No measurable dispersion effects occur when extending the fiber. Measurements of the terahertz pulses at a stretching frequency of 20 Hz show the great potential of such a device for future all-fiber terahertz systems.

Sub-terahertz 2D Photonic Crystal Waveguides for Fluid Sensing Applications

We present experiments on 2D photonic crystal waveguides for fluid sensing applications. Using a standard machining process, photonic crystal waveguides for the sub-terahertz (0.1 THz) frequency range are fabricated. The insertion of fluids into the active region of the crystal leads to a change of the photonic band gap properties. We present results for different non-polar liquids, which are in the good agreement with simulations.

Towards Real-Time Terahertz Quality Assurance of Chocolate Products

We evaluate the potential of terahertz imaging for quality control in the food industry, in particular for monitoring chocolate fabrication. To increase the imaging speed only a few points of the THz waveform are measured.

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.

A fluid sensor based on a sub-terahertz photonic crystal waveguide

We present two-dimensional photonic-crystal waveguides for fluid-sensing applications in the sub-terahertz range. The structures are produced using a standard machining processes and are characterized in the frequency range from 67 to 110 GHz using a vector network analyzer. The photonic crystal consists of an air-hole array drilled into a high-density polyethylene block. A waveguide is introduced by reducing the diameter of the holes in one row. The holes can be loaded with liquid samples. For all structures we observe photonic band gaps between 97 and 109 GHz. While the pure photonic crystal shows the deepest stop band (28 dB), its depth is reduced by 5 dB when inserting a waveguiding structure. The depth of the photonic band gap is further reduced by several decibels depending on the refractive index of the liquid that is inserted. With this type of fluid sensor we can clearly distinguish between cyclohexane and tetrachloromethane with refractive indices of 1.42 and 1.51, respectively. The results are in good agreement with theoretical calculations based on the 2D finite-difference time-domain (FDTD) method.

Mail inspection using THz imaging : a comparison of three different systems

To evaluate the potential of THz imaging systems for mail and luggage inspection we study a set of letters containing different hazardous items. The samples are investigated with three different THz systems available in our group: A microwave based system working around 100 GHz, a THz time-domain system and a THz gas laser. We provide a comparative discussion on our results and the advantages and disadvantages of each system.

THz spectroscopy for the inline control of polymeric compounding processes

We present non-destructive and contactless inline measurements on the molten polymer polypropylene and on the polymeric compound polypropylene-CaCO3 carried out with a THz spectrometer. The effect of temperature and pressure on the THz properties of the molten polymeric material is investigated. Additionally, we analyze the additive content of the compound during the extrusion processing and compare this data with offline results. Further measurements on wood plastic composites demonstrate the capability of detecting the degree of dispersion of compounds with THz-spectroscopy.