Nondestructive Material Characterization in the Terahertz Band by Selective Extraction of Sample-Induced Echo Signals

Abstract

Terahertz time-domain spectroscopy (THz-TDS) allows broadband noninvasive measurement of the optical parameters of various materials in the THz domain. The measurement accuracy of these parameters is highly influenced by the difficulty in distinguishing THz signals from unwanted signals such as noise, signal fluctuation, and multiple echoes, which directly affects material identification and characterization efficiency. We introduce a novel method that provides effective extraction and separation of THz signals from such undesired effects. The proposed algorithm was assessed through experiments that presented enhancement in material parameter evaluation, such as the decomposition of the sample-induced echoes (SIEs) from the complex THz sample signal with near-zero extraction error. Improved precision (±0.05 µm) was achieved in the determination of the sample thickness compared to that of the mechanical method (±10 µm). Furthermore, we could infer from the component concentration measurement results of a compound sample (44.2 % decrease in the root mean square concentration error) that the material parameter calculation accuracy had improved, proposing a means to enhance the ultimate nondestructive material evaluation performance.

A THz semiconductor hybrid plasmonic waveguide with fabrication-error tolerance

“…The structure under study outperforms existing HTPWGs, with the former having twice and half the propagation length and mode area, respectively, of those reported in Ref. 8, thus allowing it to be used in applications such as biomolecular tweezers 17,18) and high-density integrated circuits. 3,8) We have carried out a detailed mode analysis using COMSOL commercial finite element method (FEM) software to investigate the effects of structural parameters on modal properties to achieve an improved trade-off by adopting two important evaluation factors (i.e., propagation length and mode area) to describe the propagation loss and mode confinement, respectively.…”

THz-TDS parameter extraction: empirical correction terms for the analytical transfer function solution

“…A Gouy phase shift correction can be applied to THz-TDS data to correct for the non-plane wave-front of a focused Gaussian THz beam [32,33] which has been demonstrated with 2D semiconductor materials to increase the accuracy of extracted charge carrier parameters [34]. The parameters extracted by THz-TDS are extremely dependent on an accurate independent measure of sample thickness; to this end an extraction technique has been proposed in which multiple internal pulse echos within the sample are measured and the input sample thickness varied until the extracted parameters from each echo result in the same refractive index [35]. Additionally it is possible to use these internally reflected echo pulses to self-reference the THz signal and remove the need for an independent air reference scan, which is shown to reduce variation between repeated TDS measurements [36].…”

Subwavelength InSb-based Slot wavguides for THz transport: concept and practical implementations

“…Therefore the THz technology is now receiving an extensive attention with great promises for wide applications such as material characterization15, study of electronic coherence of SCs16, biochemical sensing17 and highly integrated THz circuits (ITCs)18. Similar to IPCs, the development of ITCs leads to a new generation of fast, low-loss and on-chip subwavelength THz devices.…”