  In transmission-mode terahertz time-domain spectroscopy (THz-TDS), the
thickness of a sample is a critical factor that determines an amount of the
interaction between terahertz waves and bulk material. If the interaction
length is too small, a change in the transmitted signal is overwhelmed by
fluctuations and noise in the system. In this case, the sample can no longer be
detected. This article presents a criterion to determine the lower thickness
boundary of a free-standing film that can still be detectable by free-space
transmission-mode THz-TDS. The rigorous analysis yields a simple proportional
relation between the sample optical length and the system SNR. The proposed
criterion can help to decide whether an alternative terahertz thin-film sensing
modality is necessary.
