Terahertz(THz) wave, which lies between infrared and microwave in electromagnetic spectrum, belongs to
far-infrared band. THz-wave covers the frequency range from 0.1THz to 10THz. The physical mechanism of
terahertz generation was discussed in this paper. It provided theoretical foundation for designing
terahertz-generator of LiNbO3 crystal and optimizing output property of terahertz through the analysis of internal
mechanism. The polariton dispersion characteristic was mainly reflected by its dispersion curve. Density functional
theory (DFT) was used to calculate Raman spectrum and dispersion curve for LiNbO3 in this paper. The polariton
dispersion curve of LiNbO3 indicated that the frequencies between 0THz and 7.5THz (the wave numbers were
0cm-1 250cm-1) were generated from LiNbO3 crystal when the scattering angle were about 0 - 5 degree. This
predicted theoretically that terahertz was generated from LiNbO3. It was very obvious that the Raman vibration
modes of crystal model were more than molecular structure. Intermolecular vibrations were depicted by molecular
structure and parts of vibration modes were simulated. But whole vibration-rotation of unit cell was depicted by
crystal model and all atoms were concerned with vibration. It was more accurate that the prediction about
generated THz-wave and the range of THz-wave by crystal model than by molecular structure based on the
analysis of polariton dispersion curves because crystal model was close to the experimental sample.
The physical mechanism of terahertz-wave generated from nonlinear crystal GaP was discussed based on polariton. The
method which predicted to generate terahertz-wave in nonlinear crystal were reported. The polariton dispersion curve
was physical basis of which terahertz-wave was generated from nonlinear crystal. The polariton dispersion curve of GaP
crystal was depicted by using density functional theory and the polariton dispersion relation. The corresponding
frequency of part polariton dispersion curve for GaP were between 2.5 THz-9.1 THz when small scattering angle about
2-11 degree were required. Namely, it showed theoretically that terahertz-wave with a range of 2.5-9.1 THz were
generated from GaP crystal.
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