Terahertz spectroscopy

Christoph Miethaner, July 2017

Terahertz spectroscopy is a non-destructive testing method which uses Terahertz radiation to determine elements and their positions inside components. It can detect defects of size less than one millimetre inside materials.^[1] Terahertz radiation is a non-ionizing radiation and therefore it doesn’t have hazardous effects on tissue.

Physical fundamentals

Terahertz radiation is part of the electromagnetic spectrum. The position and proximate fields can be seen in figure 1. Terahertz radiation couldn’t be used for decades because of a lack of applicable transmitters and receivers. In the last two decades developments in transmitters and receivers made applications with Terahertz radiation possible. Terahertz is produced by frequency multiplication or the difference in frequency of two lasers. Quantum cascade laser, free-electron laser and backward-wave oscillators are examples of applicable producers of Terahertz radiation.

Characteristics

Terahertz spectroscopy uses electromagnetic radiation with a frequency of 0.1 - 10 THz. Physical properties are given below for a frequency of 1 THz.^[2][3] For more information regarding Terahertz radiation see Terahertz Technologie: Funktionsweise und Einsatzmöglichkeiten

Physical properties

These values can be computed with the following formulas:

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With

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With

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With

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With

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$//<![CDATA[ \begin{array}{l}\text {k = Boltzmann constant}\end{array} //]]>$ ^[4]

Advantages of Terahertz spectroscopy

• Non-destructive
• Contactless
• Non-ionizing radiation
• Applicable for hollow containers
• Measurement of reflecting and transmitting radiation ^[5][6]

Method principles

A sample is irradiated by Terahertz radiation. The incoming wave gets partly reflected on the surfaces. The rest of the radiation will be transmitted through the component or gets absorbed by the component. If there are impurities inside the component, part of the wave will be reflected again (see figure 2).

Terahertz radiation will be transmitted by non-metals. Contrary to this metals will reflect most of the injected radiation and water will absorb most of it. In order to determine defects inside components, the transmitted radiation is measured. There are two ways to measure the transmitted radiation: One can measure either directly the transmitted radiation or the injected and the reflected radiation. A spectroscopy disperses the measured radiation and analyses its spectrum.

Measurement in transmission (absorption spectroscopy)

The measuring system only measures the transmitted radiation. Hollows and impurities will absorb, reflect and scatter the incoming radiation. From the measurement of the transmitted radiation the location of defects can be calculated. Differences in thickness can be calculated by the measurement of time the radiation needs to propagate through the component. Therefore a reference sample can provide a precise time, the radiation needs to pass the component.^[2]

Measurement in reflection (reflection spectroscopy)

For measurements in reflection the measuring system only counts for reflected radiation. Part of the radiation will be reflected on every boundary layer. In that manner hollows and impurities will cause additional reflected radiation. Defects can be located by the additional time measurement the radiation needs to the hollow and back.

A combined measurement of reflected and transmitted radiation can be used for volume inspections. The transmitted radiation provides information concerning the 2-D geometry. If the propagation time through the material is measured too, additional information of the overall thickness or density is achieved. Measurement in reflection generates information about the depth in addition.

For both measurement methods the spatial resolution is limited by the wavelength of Terahertz radiation. The wavelength is approximately 300 µm, so the resolution can be a little bit less than one millimetre.^[2]

Spectroscopy systems

Depending on the spectroscopy systems it is possible to detect defects, hollows, impurities, porosities and differences in thickness ^[7]. The two most developed systems TDS and FMCW are explained in the two following topics.

TDS (time domain spectroscopy)

With TDS very short Terahertz pulses of less than 1 ps are emitted by a laser. These pulses have a wide range of frequencies from 0.1 to 4 THz. The advantage is that with one single measurement a component can be analysed with a wide range of frequencies at once. Furthermore the boundary layer thickness can be determined very precise by these short pulses. Therefore a minimum thickness of 10 µm is required, then the system can determine the thickness with a precision of 1 µm.^[2]

FMCW (frequency modulated continuous wave)

FMCW is mostly used as an imaging technique. The frequency band is modulated in such a way, that only one wave length of Terahertz radiation is emitted. With a fast voltage controlled oscillator (VCO) up to 1000 measurements per second are possible. The disadvantage of FMCW is that - because of the use of only one single wavelength - the images have little depth resolution.^[2]

Examples for applications

The field of possible applications is wide. Boundary thickness of lacquer can be determined. Enclosed air bubbles or other impurities can be detected. Defective adhesive joints etc. can be examined.^[8]

Concentrations of filler material and moisture content can be determined as well as directions of fibres inside materials.^[9]

Differences to other methods

In comparison to X-ray techniques Terahertz radiation provides a better image contrast in plastic material ^[10]. The high amount of hydrogen in plastics interacts better with Terahertz radiation. Furthermore X-ray is an ionizing radiation and will cause harm to any tissue.

Ultrasound can be easily disturbed by the boundary layers in glass fibre reinforced composites. Every boundary layer causes much scattering for ultrasound techniques. Terahertz techniques are in this case more useful.^[11]

Literature

1. Kaiserslautern, Fraunhofer IPM, 2017, Zerstörungsfreie Prüfung mit Terahertz. [Online] Available: https://www.ipm.fraunhofer.de/de/gf/materialcharakterisierung-terahertz/anwendungen/zerstoerungsfreie-pruefung-mit-terahertz.html.
2. Kaiserslautern, Fraunhofer IPM, 2017, Jonuscheit, J., Berührungslos und zerstörungsfrei – Keramiken charakterisieren mit Terahertz-Wellen. [Online] Available: https://www.ipm.fraunhofer.de/content/dam/ipm/de/PDFs/Artikel/Keramiken_charakterisieren_Terahertz_cfi.pdf.
3. Würzburg, SKZ, 2017, Terahertz. [Online] Available: https://www.skz.de/de/forschung/geschaeftsfelder/schwerpunkt-zerstoerungsfreie-pruefung-zfp/terahertz-thz/3654.Terahertz.html.
4. Kaiserslautern, Fachbereich Physik Technische Universität Kaiserslautern, 2015, THz Physik: Grundlagen und Anwendungen. [Online] Available: https://www.physik.uni-kl.de/fileadmin/beigang/Vorlesungen/WS_15_16/THz_WS15_16_2016_02_02_V15.pdf.
5. Kaiserslautern, Fraunhofer ITWM, 2017, TECHNISCHE KERAMIKEN: ZERSTÖRUNGSFREIE PRÜFUNG MIT TERAHERTZ-MESSTECHNIK. [Online] Available: https://www.itwm.fraunhofer.de/content/dam/itwm/de/documents/MC_Infomaterial/mc_flyer_technische-keramiken.pdf.
6. ZfP Zeitung Februar 2014, C. M. Sabine Wohnsiedler, Zerstörungsfreie Prüfung von Verbundwerkstoffen Zerstörungsfreie Prüfung von Verbundwerkstoffen mit Terahertz-Technik im Vergleich zu etablierten Prüfverfahren. [Online] Available: http://www.ndt.net/article/dgzfp/pdf/FB%20Terahertz%20138.pdf.
7. Kaiserslautern, Fraunhofer ITWM, 2017, VERBUNDWERKSTOFFE: ZERSTÖRUNGSFREIE PRÜFUNG MIT TERAHERTZ-MESSTECHNIK. [Online] Available: https://www.itwm.fraunhofer.de/content/dam/itwm/de/documents/MC_Infomaterial/mc_flyer_verbundwerkstoffe.pdf.
8. Kaiserslautern, Fraunhofer ITWM, 2017, Terahertz-Sensoren zur zerstörungsfreien Prüfung von Flugzeugbauteilen: EU-Projekt DOTNAC. [Online] Available: https://www.ipm.fraunhofer.de/de/gf/materialcharakterisierung-terahertz/anwendungen/zerstoerungsfreie-pruefung-mit-terahertz/dotnac.html.
9. Würzburg, SKZ, 2017, Evaluierung der THz-Technologie zur Prüfung von Kunststoffbauteilen. [Online] Available: https://www.skz.de/de/forschung/geschaeftsfelder/schwerpunkt-zerstoerungsfreie-pruefung-zfp/terahertz-thz/3690.Evaluierung-der-THz-Technologie-zur-Pruefung-von-Kunststoffbauteilen.html.
10. Kaiserslautern, Fraunhofer ITWM, 2017, TECHNISCHE KUNSTSTOFFE: ZERSTÖRUNGSFREIE PRÜFUNG MIT TERAHERTZ-MESSTECHNIK. [Online] Available: https://www.itwm.fraunhofer.de/content/dam/itwm/de/documents/MC_Infomaterial/mc_flyer_technische-kunststoffe.pdf. Accessed on: Jul. 03 2017.
11. Kaiserslautern, Fraunhofer ITWM, 2017, TIEFER BLICK INS BAUTEIL: Terahertz-Sensoren prüfen moderne Flugzeugwerkstoffe. [Online] Available: https://www.itwm.fraunhofer.de/content/dam/itwm/de/documents/MC_Infomaterial/mc_info_materialpruefung-terahertz-sensoren.pdf.