Spectroscopic imaging using terahertz time-domain signals

Imaging systems based on terahertz time-domain spectroscopy offer a range of unique modalities due to the broad bandwidth, sub-picosecond duration, and phase-sensitive detection of the terahertz pulses. Furthermore, an exciting possibility exists to combine spectroscopic characterization and/or identification with imaging because the radiation is broadband in nature. In order to achieve this, novel methods for real-time analysis of terahertz waveforms are required. Unfortunately, both the absorption and the phase delay of a transmitted terahertz pulse vary exponentially with the sample´s thickness. We describe a robust algorithm for extracting both the thickness and the complex index of refraction of an unknown sample. In contrast, most spectroscopic transmission measurements require accurate knowledge of the sample´s thickness to determine the optical parameters. We also investigate the limits of our method