Limitation in thin-film detection with transmission-mode terahertz time-domain spectroscopy

TL;DR

This paper establishes a criterion for the minimum detectable thickness of thin films using transmission-mode terahertz time-domain spectroscopy, aiding in the assessment of detection limits based on system noise and sample properties.

Contribution

It introduces a simple proportional relation between sample optical length and system SNR to determine the lower thickness boundary in THz-TDS detection.

Findings

Derived a criterion for minimum detectable film thickness

Established a proportional relation between optical length and SNR

Guided the choice of sensing modalities based on detection limits

Abstract

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.