N. Vieweg

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.

Liquid crystal based electrically switchable Bragg structure for THz waves.

We demonstrate the first electronically switchable Bragg structure for THz frequencies. The structure works as stop-band filter and as mirror. It exhibits the 60 GHz broad stop-band around 300 GHz which can be removed by reorienting liquid crystal molecules in an external electric field. Our first proof-of-principle experiments agree very well with transfer matrix calculations.

Paper terahertz wave plates

We present a low-cost terahertz wave plate based on form birefringence fabricated using ordinary paper. Measurements of the transfer function of the wave plate between polarizers closely agree with predictions based on the measured complex indices of refraction of the effective medium. For the design frequency, the dependence on wave plate angle also agrees with theory.

Impact of the contact metallization on the performance of photoconductive THz antennas

Both AuGe based alloys and Ti/Au metal layer stacks are widely used as ohmic metal contacts for photoconductive THz antennas made of low temperature grown GaAs. Here, we present the first systematic comparison between these two metallization types. A series of antennas of both kinds is excited by femtosecond laser pulses and by the emission from two diode lasers, i.e. we test the structures as pulsed THz emitters and as photomixers. In both cases, coherent and incoherent detection schemes are employed. We find that the power emitted from the antennas with AuGe metallization is 50% higher than that of antennas with a Ti/Au metal layer. From a comparison with a photomixer model we conclude that the higher output power results from a lower contact resistance of the AuGe contacts leading to an increased current flow. However, Ti/Au contacts have a higher thermal stability which might be advantageous if high system stability is called for.

Ultrabroadband terahertz spectroscopy of a liquid crystal

Liquid crystals (LCs) are becoming increasingly important for applications in the terahertz frequency range. A detailed understanding of the spectroscopic parameters of these materials over a broad frequency range is crucial in order to design customized LC mixtures for improved performance. We present the frequency dependent index of refraction and the absorption coefficients of the nematic liquid crystal 5CB over a frequency range from 0.3 THz to 15 THz using a dispersion-free THz time-domain spectrometer system based on two-color plasma generation and air biased coherent detection (ABCD). We show that the spectra are dominated by multiple strong spectral features mainly at frequencies above 4 THz, originating from intramolecular vibrational modes of the weakly LC molecules.

Polarization sensitive terahertz imaging: Detection of birefringence and optical axis

We present a practicable way to take advantage of the spectral information contained in a broadband terahertz pulse for the determination of birefringence and orientation of the optical axis in a glass fiber reinforced polymer with a single measurement. Our setup employs circularly polarized terahertz waves and a polarization-sensitive detector to measure both components of the electromagnetic field simultaneously. The anisotropic optical parameters are obtained from an analysis of the phase and frequency resolved components of the terahertz field. This method shows a high tolerance against the skew of the detection axes and is also independent of a reference measurement.

Terahertz properties of liquid crystals with negative dielectric anisotropy

We present what is believed to be the first terahertz time-domain study of a set of liquid crystals (LCs) with negative dielectric anisotropy. From the measured data, refractive indices, and absorption coefficients for ordinary and extraordinary polarization are extracted. We find that the investigated materials exhibit a much smaller absorption than LCs with positive dielectric anisotropy. Thus, these materials are more useful for switchable terahertz devices. Moreover, the LC 1808 shows what is to our knowledge the largest terahertz birefringence reported so far.

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.

Terahertz and optical properties of nematic mixtures composed of liquid crystal isothiocyanates, fluorides and cyanides

Several simple nematic eutectic liquid crystal mixtures composed of different two and three ring isothiocyanato compounds as well as mixtures consisting of analogous cyano and fluoro compounds were formulated. The optical properties of the thereby obtained mixtures, i.e. refractive indices, birefringence and absorption coefficients αe and αo are measured in the frequency range between 0.5 and 2.5 THz and in the visible range at 589 nm. Our results show that two mixtures can be identified, which seems particularly promising for applications in the terahertz (THz) range: those consisting only of isothiocyanates show the highest birefringence at THz frequencies while those consisting of isothiocyanates and cyanides have the smallest dichroic ratio over a broad THz range. We also found that in most cases, the absorption coefficients of the isothiocyanato compounds fulfill the relationship αe > αo while for cyano and fluoro compounds, αo > αe is observed as a dominating trend. Furthermore, we show that by carefully selecting constituents of both isothiocyanato and cyano compounds, we can achieve a mixture for which the values of αo are in the same order as for αe for THz frequencies below 2 THz.

Efficient terahertz en-face imaging

In this work, we develop a pulsed terahertz imaging system in reflection geometry, where due to scanning of the terahertz beam neither the sample nor the emitter and detector have to be moved. We use a two mirror galvanoscanner for deflecting the beam, in combination with a single rotationally symmetric focusing lens. In order to efficiently image planar structures, we develop an advanced scanning routine that resolves all bending effects of the imaging plane already during measurement. Thus, the measurement time is reduced, and efficient imaging of surfaces and interfaces becomes possible. We demonstrate the potential of this method in particular for a plastic-metal composite sample, for which non-destructive evaluation of an interface is performed.