For researching dielectrics, various spectroscopic analyses must be conducted. Optical absorption spectroscopy is one of such spectroscopic measures. Fourier-transform mid-infrared (FT-MIR) absorption spectroscopy, which uses MIR photons, is very popular and can detect intramolecular vibrations or vibrations of a relatively small chemical group occurring inside a molecule. In contrast, FT-far-IR (FT-FIR) absorption spectroscopy is suitable for the detection of vibrations of an entire molecule or those of a big chemical group. Furthermore, there is another piece of emerging absorption spectroscopy, called THz absorption spectroscopy. The THz spectroscopy is believed to be able to detect intermolecular vibrations. The pros and cons of THz and FT-FIR absorption spectroscopy as a characterization tool for dielectric materials research are discussed in comparison to FT-MIR spectroscopy, by showing several typical examples of experimental results acquired in the authors' laboratory.
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