Fig. 1. Principle of operation. (a) Flowchart of time-domain THz-DCS. (b) Acquisition of the temporal waveform using the adaptive sampling method.

Fig. 2. Configuration of comb-mode-resolved adaptive sampling THz-DCS. SFG-X, sum-frequency-generation cross-correlator; BBO, beta-barium borate crystal; PC-THz comb, photocarrier THz comb; M, double-balanced mixer; FM, frequency multiplier (frequency multiplication factor N=40); PCA, photoconductive antenna; and AMP, current preamplifier.

Fig. 3. Performance of the dual-comb fiber laser. (a) Output spectrum of the laser. (b) RF spectrum of the dual-comb pulses. (c) Fluctuations of frep1, frep2, and Δfrep. (d) Measured frequency instability of frep1 and Δfrep.

Fig. 4. (a) Comparison of the temporal waveforms averaged 100,000 times obtained using different sampling clocks. Inset: a zoomed-in plot of the main THz pulse. (b) Comb-mode-resolved THz spectrum through air at room pressure. Inset: a zoomed-in plot around 0.5672 THz.

Fig. 5. Comb-mode-resolved THz spectroscopy of a mixture gas sample of CH3CN and air with a total pressure of 360 Pa. Absorption spectra of CH3CN within the frequency range of (a) 0.2 to 0.72 THz and (b) 0.31 to 0.37 THz. The red stars indicate the manifolds of the rotational transitions for the vibrationally excited states. (c) Comparison of the absorption spectra between the database fitting and the experimental data and their residual and (d) the corresponding zoomed-in plot around 0.3310 and 0.3677 THz. (e) Absorption spectra around 0.3310 THz and (f) 0.3677 THz.

Fig. 6. Mode-resolved absorption characterization of CH3CN around 0.331 THz at (a) 430 Pa, (b) 330 Pa, (c) 280 Pa, (d) 256 Pa, (e) 149 Pa, and (f) 115 Pa.

Fig. 7. Pressure broadening characteristic of CH3CN/air gas.

Table 1. Quantitative analysis of CH3CN/air-mixed gas.