Fig. 1. The basic structure of terahertz time domain spectroscopy system

Fig. 2. Time-domain spectra (a) and power spectra (b) of six samples

Fig. 3. Refractive index (a), absorption coefficient (b) and dielectric constant (c) of six samples

Fig. 4. Euclidean distance dendrogram (a) and PC1 score histogram (b) with the input of refractive index

Fig. 5. Euclidean distance dendrogram (a) and PC1 score histogram (b) with the input of absorption coefficient