Fig. 1. Schematic of coherent detection THz-FDS, including transmission and reflection systems

Fig. 2. Original water vapor signal measured by transmission THz-FDS, where the inset shows enlarged peak and valley signals from 0.8990 to 0.9010 THz

Fig. 3. Transmission THz-FDS. (a) Optical path; (b) physical map

Fig. 4. THz transmission spectra of lactose monohydrate obtained with different data processing methods. (a) Envelope of reference calculated by three methods; (b) enlarged envelope of reference at about 0.48 THz

Fig. 5. Atmospheric transmission spectra ( relative humidity is 7.5% and temperature is 24.8 ℃). (a) Transmission spectra from 0.05 to 2.00 THz; (b)－(d) transmission spectra in three different frequency bands

Fig. 6. Variation of THz wave transmittance with relative humidity at different frequency values

Fig. 7. Calculated transmission coefficient, refractive index, real part and imaginary part of permittivity of lactose monohydrate with different data analysis methods. (a) Transmission coefficient; (b) refractive index; (c) real part of permittivity; (d) imaginary part of permittivity

Fig. 8. Picture and structure schematic of sample container. (a) Picture; (b) structure schematic

Fig. 9. Measured refractive index of cyclohexane and n-hexane. (a) Cyclohexane; (b) n-hexane

Fig. 10. Optical path and photo of reflection THz-FDS. (a) Optical path; (b) photo

Fig. 11. Schematic of self-referenced reflection structure

Fig. 12. Measured permittivity of methanol and ethanol, where solid and dotted lines are measurement and Debye fitting results, respectively. (a) Methanol; (b) ethanol

Fig. 13. Normalized A(β) calculated by SSKK

Fig. 14. Measured refractive index and absorption coefficient of doped silicon. (a) Refractive index; (b) absorption coefficient