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Terahertz (THz) waves, or submillimeter/far-infared waves, refer to electromagnetic radiation in the frequency interval from 0.1 to 10 THz. The terahertz regime of the electromagnetic spectrum lies between the mid-infared and microwave bands, which represents an important transition from quantum-level to bulk transport physics.
As a useful reference, the following quantities and units are related:
| Freq |
Period |
Wavelength |
Wavenumber |
Energy |
Temperature |
| 1 THz |
1 ps |
300 μm |
33 cm-1 |
4.1 meV |
46.7 K |
This spectral domain hosts low-frequency crystalline lattice vibrations (phonon modes), hydrogen bonding stretches, and other intermollecular vibrations of molecules in many chemical and biological materials, including explosives and related compounds (ERCs), drugs and other biomolecules. Many polar gases also have distinctive spectroscopic fingerprints in the THz range. The transmitted and reflected THz spectra of these materials contain THz absorption fingerprints characterizing these vibrational modes, providing information not readily available in other parts of the electromagnetic spectrum.
Advantages of THz Waves
THz waves have low photon energies (1 THz = 4.1 meV), one million times weaker than x-rays, and will not cause harmful photoionization in biological tissues. This has advantages both for imaging biological materials, and in operational contexts where an operator or other subjects may be exposed to THz radiation. This is good, considering a substantial portion of room temperature blackbody radiation can be found in the terahertz band.
In addition to being considered "safe" for most applications, THz radiation can penetrate through many commonly used nonpolar dielectric materials such as paper, cardboard, textiles, plastics, wood, leather and ceramic with moderate attenuation. This allows THz waves to be used in non-invasive and non-destructive inspection using spectroscopy and imaging techniques (see our applications section for more information).
Limitations unique to THz Waves
A substantial limitation to THz applications is the absorption of THz waves by water, whether it is found in biological tissue or as vapor in ambient air.
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| Top figure shows relative absorbance of ambient air out to 20 THz, while the bottom figure shows there are significant gaps in the absorption spectra of air in the operating range of common terahertz instruments available today. Figure appears courtesy of the Center for Terahertz Research, Rensselaer Polytechnic Institute |
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