Terahertz Technology

Your contact: Sandrine Van Frank

 


Terahertz (THz) Technology is a highly topical field of research. The THz-frequency domain has become accessible more easily during the last 20 years due to the development of efficient emitters and detectors, strongly coupled with the availability of powerful femtosecond-lasers. THz-radiation features some very attractive properties that open a broad field of applications.

 
Abb.: Terahertz-Frequency domain [Ref: Tatoute]

 

 

Interesting physical properties of THz-radiation:

  • THz-radiation is located in between the infrared and the microwaves in the electromagnetic spectrum, whereas 1 THz (=10^12Hz) corresponds to a wavelength of 300 µm. THz-radiation penetrates many materials (advantage compared to infrared) and on the other hand exhibits a better spatial resolution than microwaves.

  • The penetration depth of THz-waves is relatively large for many non-conductive materials, such as plastics, paper, cardboard, textiles, etc. This allows the use of THz-radiation for investigating the inner structure of samples, like e.g., defects, inclusions, etc. by THz-imaging.

  • Several substances show characteristic fingerprints at THz-frequencies due to collective molecular excitations. This allows the identification and characterisation of substances such as pharmaceuticals, drugs, explosives, etc. by THz-spectroscopy.

  • THz-radiation exhibits very low energies (1 THz corresponds to an energy of about 4meV, in comparison: room temperature about 25 meV) and is consequently non-ionizing.

Measuring principles of THz-time-domain spectroscopy:

 

A commonly used method is so called THz-time-domain spectroscopy (THz-TDS). This method can be used for imaging as well as for spectroscopy and employs pulsed THz-radiation. As the THz-pulses have durations of only a few picoseconds, a direct electronic measurement of the pulses is not possible to-date. Therefore, the THz-pulses are sampled in a time-resolved wave. The most important parts of a standard THz-TDS setup are as following:


 
Schematic setup of a THz-TDS-System(top), time resolved THz-pulse (bottom right) and THz-spectrum computed by FFT (bottom left).

  • A femtosecond laser emits very short laser pulses, which only last a few tens of femtoseconds (femto = a millionth of a billionth). The fs-laser pulses are divided by a beam splitter and are sent to
  • a THz-emitter and a THz-detector, respectively, where they excite THz-radiation or are used for detecting the THz-pulse. In our case, the THz-emitter and THz-detector are photoconductive antennas, which consist of semiconductor material with a metallic antenna structure.
  • A mechanical delay line introduces a variable time delay between excitation and detection of the THz-pulse, enabling time resolved sampling.
  • The signal from the THz-detector needs to be amplified (using a current amplifier and a lock-in-amplifier), so that the very small THz-Pulse can be digitized and displayed on a computer.
  • The THz-Pulse is transformed into a THz-spectrum by Fourier transformation (Fast Fourier transformation; FFT). The short duration of the pulse leads to a broad THz-spectrum, which can directly be used for spectroscopy.

Works at RECENDT GmbH:

 

The research area “THz-Technology” at RECENDT GmbH focuses on methods for THz-spectroscopy (time-domain spectroscopy, TDS) and THz-imaging for the investigation and characterisation of materials with the following main topics:

  • THz-Technology for industrial problems
  • Investigation and developement of scanning THz-Systems
  • Polarisation-sensitive (PS) maesurements for the analysis of orientations
  • High-speed and spatially resolved THz-spectroscopy
Currently, 3 THz-TDS laboratory setups with the following properties are available: Spectroscopy:

    Spectroscopy:
    • Measurements in transmission and reflection
    • With nitrogen purging
    • Point-wise and spatially resolved THz-Spectroscopy

    Imaging:
    • Measurements in transmission and reflection
    • Special scanning THz-system for industrial samples
    • PS-THz for the investigation of orientations

Examples of applications:

 

  • Qualitycontrol: Detecting defects in a sheet of glue

     

     
    [Ref: S. Katletz, M. Pfleger, H. Pühringer, N. Vieweg, B. Scherger, B. Heinen, M. Koch, and K. Wiesauer, „Efficient Terahertz En-face Imaging“, Opt. Express 19, 23042–23053 (2011)]

  • Inner structures: “inner life” of a toy figure

     


  • Hyper-spectral imaging (HSI): space-resolved spectroscopy on lateral inhomogeneous samples

     

     
    [Ref: H. Pühringer, J.Kasberger, M. Pawliczek, S. Katletz, M. Pfleger, H. Lohninger, and K. Wiesauer, "Terahertz Hyperspectral Imaging: Comparison of Different Evaluation Methods", International Forum on Terahertz Spectroscopy and Imaging, 06.-07.03.2012, Kaiserslautern (Germany)]

Find more information about this technology in our project sheets "THz" and "THz industrial"!


 




Industrial Applications:



 

Tribological coatings

Metallic components are often coated dielectrically in order to optimize the operating characteristics (such as tribology).
With Terahertz-Technology (THz) such coatings can be efficiently measured inline.



Hidden (adhesive-) layers 

With the Terahertz-Technology (THz) you can also look through relatively thick plastic layers and test hidden layers non-destructively (e.g. adhesive layers)! The used terahertz waves are not harmful for your health (no ionizing radiation) but still make it possible to look into many optically non-transparent materials.


 

Spectroscopic imaging

The spectroscopic control of chemical compositions, e.g., of pills can be achieved by different technologies (NIR, MIR, Raman, THz). Also spatially resolved spectroscopy is possible – moreover you can get a picture of, for instance, the API-distribution in the product!


 

Spectroscopy in the product development, production and QA

For the production monitoring / quality control or the product development spectroscopic technologies (NIR, MIR, Raman, THz) can play a significant role. These methods make it possible to get very precise measurements of mixing ratios, compositions, ingredients, reaction progresses and similar chemical information without sample preparation and costly laboratory analyses.
Such real-time measured data provide an ideal basis for your process optimization!



Multilayer pipe extrusion (Co-extrusion)

With THz-Technology, individual layer thicknesses can be measured in-line in the multi-layer pipe extrusion process (Co-extrusion). This allows a precise regulation and optimization, energy and raw material savings and quality assurance.



Fiber directions in composite materials

Do you want to gain deeper insights and understandig of interior properties of your fiber-reinforced plastics? Are you interested in the anisotropies of your components or do you need to determine the orientation of the fibers inside a CFRP-injection moulded component?
With our Terahertz-Technology (THz), OCT-Technology or Laser Ultrasonics Technology we can assist!



Explosives or drugs identification

With spectroscopic methods the identification of substances is easily realizable. With these methods it is possible to identify explosives and to distinguish between non-hazardous materials and hazardous or illegal substances.
Moreover, with Terahertz-Spectroscopy such analysis can also be performed for substances inside closed and non-transparent containers / objects (e.g. letters, cardboard boxes).


 

Security checks

Terahertz-imaging received large attention in the media in connection with the so called “body scanners” for security checks at the airport. Indeed, it is possible to detect the THz radiation, which is emitted of every body, through clothing / jackets by THz-technique. These images can help to detect shadows like hidden weapons.




Contact:

van Frank Sandrine, PhD
Position:
Research Scientist
Adresse:
Altenberger Straße 69, 4040 Linz
Tel.:
+43 (0)732/2468-4640
E-Mail:
 

Rankl Christian, PhD
Position:
Area Manager Optics
Adresse:
Altenberger Straße 69, 4040 Linz
Tel.:
+43 (0)732/2468-4644
E-Mail:
 

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