Material parameter estimation with terahertz time-domain spectroscopy

Imaging systems based on terahertz (THz) time-domain spectroscopy offer a r ange of unique modalities owing to the broad bandwidth, subpicosecond durat ion, and phase-sensitive detection of the THz pulses. Furthermore, the poss ibility exists for combining spectroscopic characterization or identificati on with imaging because the radiation is broadband in nature. To achieve th is, we require novel methods for real-time analysis of THz waveforms. This paper describes a robust algorithm for extracting material parameters from measured THz waveforms. Our algorithm simultaneously obtains both the thick ness and the complex refractive index of an unknown sample under certain co nditions. In contrast, most spectroscopic transmission measurements require knowledge of the sample's thickness for an accurate determination of its o ptical parameters. Our approach relies on a model-based estimation, a gradi ent descent search, and the total variation measure. We explore the limits of this technique and compare the results with literature data for optical parameters of several different materials. (C) 2001 Optical Society of Amer ica.