Terahertz time-domain spectroscopy of sulfur-containing biomolecules

Abstract

We present frequency-dependent absorption coefficients and refractive indices of sulfur-containing biomolecules in the far-infrared region measured by terahertz (THz) time-domain spectroscopy (TDS). The THz radiation 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 vibrational modes.