Publications: Science Omega Review UK Issue 1

Generation THz - an article from Dr Axel Zeitler

Semiconductor testing
There is no commonly agreed definition of the upper and lower frequency limits of terahertz (THz) radiation. Its spectral range overlaps with the far infrared at the higher frequency end and the microwave region at lower frequencies.
Dr Axel Zeitler
Dr Axel Zeitler, of the University of Cambridge’s Department of Chemical Engineering and Biotechnology, looks at the dramatic progress made in terahertz research in recent years…

Terahertz radiation has excellent potential to help with the understanding of fundamental and exciting new challenges at the interface between physics, materials chemistry and the life sciences. Examples include fields as diverse as non-destructive testing of composite materials such as wind turbine blades, semiconductor quality control and intraoperative probes for breast cancer surgery. However, light located in this range of the electro-magnetic spectrum was very difficult to generate until quite recently, and so the full potential of these exciting applications is only just starting to emerge.

There can be no doubt that the main reasons for the surge in interest in performing spectroscopy at terahertz frequencies were the development of ultrafast lasers and the discovery of the Auston switch. These technologies made it possible to provide light at terahertz frequencies (a frequency of 1THz equals a wavelength of 0.3mm) in a relatively simple way. They enabled the development of a new generation of spectrometers in the early 1990s that were able to generate and detect pulses of coherent terahertz radiation with previously unprecedented ease and sensitivity. Today, most of the research in terahertz spectroscopy and imaging is carried out using such time-domain spectrometers. Terahertz time-domain spectroscopy (THz-TDS), therefore, is currently the main focus of research activities in the terahertz community.

There is no commonly agreed definition of the upper and lower frequency limits of terahertz (THz) radiation. Its spectral range overlaps with the far infrared at the higher frequency end and the microwave region at lower frequencies. The term thus refers to a relatively narrow part of the electro-magnetic spectrum. Despite this narrowness, which it shares, for example, with visible light, terahertz radiation is of great importance in terms of fundamental research as well as in technology and the life sciences. And yet, whilst nobody would question the importance of research involving radiation such as visible light, until recently research into terahertz radiation has been relatively obscure.

Terahertz radiation has unique properties in that it easily penetrates through most polymeric and ceramic materials and is therefore an exciting new tool to study such materials, which are often opaque at visible frequencies. This transparency of terahertz radiation to non-polar and non-metallic materials motivates the use of terahertz radiation in security screening and industrial quality control applications. As well as being a non-destructive probe of materials in organic molecular crystals such as drug molecules, terahertz radiation has the important property of interacting with vibrational modes that extend across large domains of a crystal lattice. This makes terahertz spectroscopy unique: even though it is possible to excite molecules using a variety of energies, it is only through the careful selection of the low energy in the terahertz range that it is possible to selectively excite crystal lattice vibrations and study in a unique way the presence and nature of interactions between molecules.

A wide range of further significant microscopic physical phenomena can be found in the terahertz regime. Bulk dielectric relaxations and intermolecular motions occur in this spectral range. Critical frequencies for Debye relaxation processes in many liquids fall into the terahertz regime. Pure rotational transitions occur when polar gases are stimulated by terahertz radiation. Moderately doped semiconductors have their plasma frequencies and damping rates defined between 0.1 and 2THz.

Terahertz research continues to mature at rapid pace and commercial instrumentation is now readily available. This has made it possible for the field to expand from a niche technology in semiconductor physics to an exciting mainstream research and sensing platform with a broad variety of applications, from fundamental research to industrial process control.

Dr Axel Zeitler
Terahertz Application Group
Department of Chemical Engineering and Biotechnology
University of Cambridge


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