Origin of pseudo-dispersion in non-dispersive media by terahertz time-domain spectroscopy.

We report the false appearance of dispersion in non-dispersive materials when measured by terahertz time-domain spectroscopy. This occurs when the material is measured in reflection geometry and has a bulk metal interface opposite to the incident interface, for example, when a substrate is supported by a metal stage with the THz light incident on it from above. We explain this effect in terms of the frequency-dependent response of the material when it is represented by a shorted transmission line model. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement