Fig. 1. Terahertz time-domain spectroscopy system device

Fig. 2. Optical path diagram of transmission terahertz time-domain spectroscopy system

Fig. 3. Time-domain waveform of terahertz wave propagation in nitrogen

Fig. 4. Time-domain waveform and fitting curve of each sample in group 4

Fig. 5. Terahertz spectra of experimental sample groups

Fig. 6. Terahertz spectra of experimental sample groups (from -382 ps to -376 ps)

Fig. 7. Fitting results of maximum value of time-domain waveform and heating temperature

Fig. 8. Fitting results of peak-to-peak value of time-domain waveform and heating temperature

Fig. 9. Frequency spectra of nitrogen and each sample group

Fig. 10. Terahertz absorption spectra of sample groups

Fig. 11. Linear fitting results of sample absorption coefficient and heating temperature