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.