| THz is 1012 Hz, THz electromagnetic wave is also called THz radiation, usually point to bandwidth on frequencies 0.1 to 10 THz. The bandwidth centers on the frequencies 0.3 to 3 THz has been usually studied,THz wave is situated between infrared and microwave. Absorption coefficient and index of refraction of the LiNbO3(LN) crystal have been obtained by terahertz time domain spectroscopy (THz-TDS). Three samples, LN: clear, LN: Fe(0.08%), LN: Fe(0.1%), are cut along (001) plane. Detected the T-ray pulse of the reference time domain waveform and the signal time domain waveform permeated the sample in the THz-TDS. The frequency domain waveforms, include amplitude and phase, are obtained after Fourier transform. Than absorption coefficient, index of refraction and dielectric constant can be calculated by the process. The index of refraction is 6.3-6.6, the dielectric constant is 39.9-43.6 and the absorption coefficient is 6.62-88.2cm-1 of the clear LN sample in the range of 0.4-1.6 Thz.The absorption coefficients of the three samples are approximately equal in the range of 0.4-1.6THz, they all increase with the frequencies. It indicates that the absorption of the samples is not affected with Fe doped. The stronger vibration mode of LN comes from the vibration of [NbO6] octahedron, it is also the deformation vibration of O-Nb-O aggregate. Use the method of least square to fit the absorption coefficient, it matches with quadric polynomial quite well. The coefficient of the quadric polynomial is defined as structure index of the sample. There are little differences in the 3 samples, it indicates the discrepancies of the samples'structure when doped with Fe. Absorption intensity depends on the density of states of phonon: g(ω), the density of states is in inverse proportion to the energy grads in the k-space: dk/dω. We set up one dimension Nb-O dual atom chain mode. There are 2 lengths of bond in the [NbO6] octahedron, the longer length is 0.211nm and the shorter is 0.189nm, bond force constants of Nb-O are calculated according to the length of bond, they are 491 N/m and 683N/m respectively. The optical branch and acoustic branch of the lattice vibration and the polariton are calculated in the theory. The dk/dωof the Lower polariton branch increases with the frequencies, which is similar to absorption coefficient, the line is similar to hyperbolic curve type, it indicates that the absorb comes from polartion.The other sample LiNbO3: Fe(0.08%) is cut along (100) plane. The absorption coefficient and index of refraction are obtained too. The absorption coefficient has little difference with the sample cut along (001) plane, but the discrepancy of the index of refraction is obvious, a giant birefringence is observed in the LN crystal, and the average index of refraction difference between the ordinary wave and the extraordinary wave no-ne reach up to about 1.6.Transmission and reflection spectrum of the LN samples from the infrared light–visible light-ultraviolet light (wavelength: 200nm-2000nm) have been also measured. The absorption coefficient can be calculated by transmission and reflection spectrum. When known the reflection spectrum, the optical constants, such as the dielectric, conductivity, absorption coefficient and the refraction of index, can be calculated in terms of the Kramers-Kronig relations.
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