Lower bound of sample thickness in terahertz time-domain spectroscopy

Dielectric films with a thickness much less than a wavelength pose a challenge in transmission-mode terahertz time-domain spectroscopy (THz-TDS). A small signal change induced by such films is likely to be obscured by system uncertainties. In this abstract, several possible thin-film measurement procedures are carefully considered. It is found that an alternating sample and reference measurement approach is most sensitive for thin-film sensing. Importantly, a closed-form equation is developed to determine a lower bound of sample thickness as a function of the refractive index and system uncertainties. An experimental validation shows that typical THz-TDS can detect polymer films with a thickness of a few microns. The given criterion can be used to evaluate the system performance with respect to thin-film sensing.