Terahertz time-domain spectroscopy of sulfur-containing biomolecules

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We present frequency-dependent absorption coefficient and refractive indices of sulfur-containing biomol-ecules in the far-infrared region measured by terahertz(THz)time-domain spectroscopy(TDS). The THzra-diation was generated by a surface photocurrent method with a(100)InAs wafer as a THz emitter, and its intensity was enhanced by applying a magnetic field of 1.6 T. The THz radiation was detected by the electro-optic sampling method with a ZnTe crystal. The characterization of the apparatus of THz TDS is discussed in detail. The spectral features in the THz region are different among sulfur-containing biomolecules of L,L-cystine,L-cysteine,L-methionine, and oxidized glutathione in powder. These results demonstrate that THz TDS is a promising method for studies on the low-frequency spectra of complex molecules and identification of materials. We present a calculation method to analyze the THz spectra based on a model function for dynamics of the dipoles, which allows us to obtain spectral information characterizing the low-frequency responses of the molecules. The experimental results are compared with ab initio calculations to discuss assignment of the vi-brational modes.