Review of THz spectroscopy, its uses and mechanism

1. Terahertz Spectroscopy
Revi Muharam Fadli
23321005
9/12/2021

2. Terahertz Spectrum

3. Background and History
 The first image generated using THz radiation in 1960.
 In 1995, THz image was generated using Terahertz time-domain spectroscopy, generated a great deal of interest, and sparked a rapid growth in the field of terahertz technology.
 In 2002, the European Space Agency (ESA) star Tiger Team at Rutherford Appleton laboratory produced first passive THz image of a hand.
 By 2004, ThruVision Ltd, demonstrated the world’s first compact THz camera for security screening application. This system successfully imaged guns and explosive under clothing

4. Background and History
 In mid-2007, scientists at the U.S. Department of Energy's
Argonne National Laboratory, along with collaborators in Turkey
and Japan, announced the creation of a compact device that
can lead to portable, battery-operated sources of T-rays, or
terahertz radiation.
 In 2008, engineers at Harvard University demonstrated that
room temperature emission of several hundred nanowatts of
coherent terahertz radiation could be achieved with a
semiconductor source

5. Background and History
 In 2009, it was shown that T-waves are produced when unpeeling adhesive tape. The observed spectrum of this terahertz radiation exhibits a peak at 2 THz and a broader
peak at 18 THz. The radiation is not polarized. The mechanism of terahertz radiation is tribocharging of the adhesive tape and subsequent discharge.
 In 2011, Japanese electronic parts maker Rohm and a research team at Osaka University produced a chip capable of transmitting 1.5 Gbit/s using terahertz radiation.
 In 2013, researchers at Georgia Institute of Technology's Broadband Wireless Networking Laboratory and the Polytechnic University of Catalonia developed a method to
create a graphene antenna: an antenna that would be shaped into graphene strips from 10 to 100 nanometers wide and one micrometer long. Such an antenna would
broadcast in the terahertz frequency range.

6. Terahertz Spectroscopy

7. Types of THz generation
 Surface emitters
 A semiconductor material is hit by laser with energy above its energy band-gap, which photogenerates mobile carriers.
 Photoconductive emitters
 An incident laser pulse abruptly changes the antenna from an insulating state into a conducting state. Due to an electric bias applied
across the antenna, a sudden electric current transmits across the antenna. This changing current lasts for about a picosecond, and
thus emits terahertz radiation since the Fourier transform of a picosecond length signal will contain THz components.
 Optical rectification
 A nonlinear-optical process, where an appropriate crystal material is quickly electrically polarized at high optical intensities. This
changing electrical polarization emits terahertz radiation.

8. Terahertz Time-Domain Spectroscopy
 Ultrashort laser pulses(<1 picosecond) are split in two, one towards a variable delaying stage,
another towards the antenna, generating terahertz pulses
 THz pulses are focused on the sample, then combined with laser from the delay stage via a
pellicle mirror.
 Both beams hit an electrooptic crystal, which modulates the laser according to the THz signal
 Different polarizations of the modulated laser are split with a Wollaston prism
 Both beams go into the detector

9. Terahertz Time-Domain Spectroscopy

10. Fourier Transform

11. Computational Reference
Vibrational Spectroscopy
 Fourier transform
peaks are compared
with vibrational
frequencies in
simulated material
 Only applicable to
crystalline materials

12. Measuring Aging Damage in
Polyethylene(PE)
 Samples that have been exposed to
ᵧ-Co60 at different doses & annealed
at 398 K for 31 days show different
peak strengths in the THz spectrum,
indicating reduction in crystallinity, as
well as chain scissions, crosslinking
formations, and oxidation

13. Measurement of Primary and
Secondary Glass Transition
 Dielectric losses
caused by absorption
is measured on
different temperatures
 Applicable to
amorphous materials

14. Advantages
 Many materials(including clothing) are transparent to THz radiation
 It is non-ionizing, thus safe for biological tissues
 Safer than x-ray
 Many materials, such as explosives and illegal drugs, exhibit unique THz fingerprint
 Many biological macromolecules exhibit rotation and vibration in THz-band level
 Advantage over FTIR, since it detects time-domain phenomena such as relaxation, instead of just
average amplitude over time
Human tooth
Wood structure and fiber
isotropy distribution

15. Limitations
 Unable to penetrate metal, which reflects
almost 100% of the THz radiation(though this
can be exploited to detect hidden weapons)
 Strongly absorbed by water, which also imprints
its signature THz characteristics. Can be
mitigated by calibrating with control water-only
samples with similar dimensions, then subtract
the measurement from actual sample
measurements

16. References
 Sethy, P.K., Mishra, P.R. and Behera, S., 2015, February. An introduction to terahertz technology, its history, properties and application. In International conference on computing and communication.
 Uddin, J. ed., 2017. Terahertz Spectroscopy: A Cutting Edge Technology. BoD–Books on Demand.
 Reid, M.E., Hartley, I.D. and Todoruk, T.M., 2013. Terahertz applications in the wood products industry. In Handbook of Terahertz Technology for imaging, sensing and communications (pp. 547-578). Woodhead Publishing.
 Ruggiero, M.T., Sibik, J., Zeitler, J.A. and Korter, T.M., 2016. Examination of L-glutamic acid polymorphs by solid-state density functional theory and terahertz spectroscopy. The Journal of Physical Chemistry A, 120(38), pp.7490-7495.
 Beckmann, J., von der Ehe, K., Jaunich, M., Wolff, D. and Schade, U., 2018, September. THz-and mid IR Fourier Transform Spectroscopy on Physical Aged Polyethylene. In 2018 43rd International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) (pp. 1-1). IEEE.
 Capaccioli, S., Ngai, K.L., Thayyil, M.S. and Prevosto, D., 2015. Coupling of caged molecule dynamics to JG β-relaxation: I. The Journal of Physical Chemistry B, 119(28), pp.8800-8808.
 Sibik, J. and Zeitler, J.A., 2016. Direct measurement of molecular mobility and crystallisation of amorphous pharmaceuticals using terahertz spectroscopy. Advanced drug delivery reviews, 100, pp.147-157.

17. Thank You

18. This work is licensed under a Creative Commons
Attribution-ShareAlike 3.0 Unported License.
It makes use of the works of Mateus Machado Luna.