Over the past three decades a new spectroscopic technique with unique possibilities has emerged. Based on coherent and time-resolved detection of the electric field of ultrashort radiation bursts in the far-infrared, this technique has become known as terahertz time-domain spectroscopy (THz-TDS). In this review article the authors describe the technique in its various implementations for static and time-resolved spectroscopy, and illustrate the performance of the technique with recent examples from solid-state physics and physical chemistry as well as aqueous chemistry. Examples from other fields of research, where THz spectroscopic techniques have proven to be useful research tools, and the potential for industrial applications of THz spectroscopic and imaging techniques are discussed.

Terahertz spectroscopy and imaging – Modern techniques and applications

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Philosophical Transactions of the Royal Society of London. for the Year Mdcccxxvii. Part I. Volume Cxvii:355–388, 1827

We present a polarization-sensitive optical coherence-domain reflectometer capable of characterizing the phase retardation between orthogonal linear polarization modes at each reflection point in a birefringent sample. The device is insensitive to the rotation of the sample in the plane perpendicular to ranging. Phase measurement accuracy is ±0.86°, but the reflectometer can distinguish local variations in birefringence as small as 0.05° with a distance resolution of 10.8 μm and a dynamic range of 90 dB. Birefringence-sensitive ranging in a wave plate, an electro-optic modulator, and a calf coronary artery is demonstrated.

Polarization-sensitive low-coherence reflectometer for birefringence characterization and ranging

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.

Terahertz imaging: applications and perspectives

Bibliometric data set the scene by illustrating the growth of terahertz work and the present interest in terahertz science and technology. After locating terahertz sources within the broader context of terahertz systems, an overview is given of the range of available sources, emphasizing recent developments. The focus then narrows to terahertz sources that rely on surface phenomena. Three are highlighted. Optical rectification, usually thought of as a bulk process, may in addition exhibit a surface contribution, which, in some cases, predominates. Transient surface currents, for convenience often separated into drift and diffusion currents, are well understood according to Monte Carlo modelling. Finally, terahertz surface emission by mechanical means—in the absence of photoexcitation—is described.

https://iopscience.iop.org/article/10.1088/0022-3727/47/37/374001/pdf

A review of terahertz sources

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.

Detection of foreign bodies in chocolate with pulsed terahertz spectroscopy

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.

Evaluation of leaf water status by means of permittivity at terahertz frequencies.

Investigation of the water absorption in polyamide and wood plastic composite by terahertz time-domain spectroscopy

We present the first investigation of plastic weld joints using terahertz waves. Terahertz time-domain spectroscopy clearly reveals contaminations like metal or sand within the weld joint of two high-density polyethylene sheets. Furthermore, areas can be identified where the welding process has failed and the parts to be joined are separated by a small air gap. We show that a three layer structure of polyethylene-air-polyethylene has a characteristic, frequency-dependent transmission behaviour. This allows for a distinction between welded and non-welded material as well as for the calculation of the air layer thickness from the relative transmission spectrum. Consequently, terahertz time-domain spectroscopy provides a promising new non-destructive and even contactless technique, which is desired by the plastics industry for detecting a variety of deviations from the ideal welding process.

http://www.jeos.org/index.php/jeos_rp/article/download/07013/189

Christian Jördens

Papers

Terahertz imaging: applications and perspectives

Detection of foreign bodies in chocolate with pulsed terahertz spectroscopy

Evaluation of leaf water status by means of permittivity at terahertz frequencies.

Investigation of the water absorption in polyamide and wood plastic composite by terahertz time-domain spectroscopy

Terahertz imaging: a new non-destructive technique for the quality control of plastic weld joints