High Resolution Spectroscopic Study in the Terahertz

Spectroscopic measurements with a 1 GHz resolution were performed to examine fine spectral absorbance details in the signature of materials in the terahertz domain. The existence of fine spectroscopic details in the terahertz region can contribute to the understanding of chemical structure and its dynamics. Another important application in consequence of such spectroscopic findings is its potential to serve as a technique to discriminate among materials also in a concealed form, using remote sensing techniques. In this paper, we describe a new measurement method to extract the spectral absorption cross-section of materials from the inevitable presence of a standing wave pattern in a standard optical setup, based on frequency multiplication of a monochromatic RF source. This method demonstrates the benefit of tracking the standing wave phase to simultaneously measure both the absorption and spectral refractive index of low absorptive samples, such as polyethylene pellets. The spectral phase shifts are translated to variations in the material index of refraction and compared with that evaluated by the Kramers-Kronig analysis. The absorption cross sections are extracted by fitting the optical density of several different pellets having various concentrations of the materials to the prediction of the Beer-Lambert law. Periodical spectral artifacts resulting from the interference of multiple reflections from sample surfaces are removed by setting the incidence angle to the Brewster angle, in respect to the p-polarization of the source. The spectroscopic study in this paper is limited to the spectral range of 300-650 GHz, where atmospheric transmission allows remote sensing to achieve reasonable ranges.