Tomographic imaging in speed-of-light contrast

We report on tomographic measurements in the terahertz spectral range and subsequent image reconstruction of a phantom subject. At terahertz wavelengths we abandon the traditional hard-field tomography approach to use attenuation measurements for imaging, because of the strong beam steering at the periphery of objects and the existence of a substantial diffusely scattered component. Under these conditions we focus on imaging from the measurements of the terahertz pulse propagation delay, taken with a system for time-domain terahertz spectroscopy, and reconstruct the subject´s optical density instead of e.g. material density as in x-ray tomography. The novelty is our experimental approach to retain, by spatial and temporal filtering, only the photons that propagate along the shortest straight paths allowing hard-field tomography, approach for the image reconstruction. We show that pulse delay measurements result in path integrals of the light´s group velocity in the subject and therefore the image reconstruction yields the spatial distribution of the real part of the refractive index. Furthermore, we suggest a procedure whereby the pulse delay time is calculated from the time-domain waveform with a better precision, taking into account the physics of THz generation by ultrashort pulses in a biased-gap antenna.