## 2 Materials and Methods  
The schematic of our THz A-scan measurement system is shown in Figure 1. The frequency of the CW THz radiation generated by THz photomixing corresponds to the optical frequency difference between the DFB-LD and the WS semiconductor laser. The high-speed broadband THz frequency sweep is implemented by the optical frequency sweep of the WS semiconductor laser. The DFB-LD is operated at a fixed wavelength of 1545 nm, and the output wavelength of the WS semiconductor laser is swept from 1544.5 to 1560 nm at a repetition rate of 18 kHz. The optical frequency step of the WS semiconductor laser can be calibrated using a fiber-coupled H\({}^{13}\)C\({}^{14}\)N gas cell [34]. Frequency-domain THz signals are measured in the frequency range of up to 1.88 THz at the same repetition rate using coherent homodyne detection. A longer A-scan range can be inspected due to the longer coherence time of the WS semiconductor laser [32, 33]. The optical output of the WS semiconductor laser is amplified by an optical attenuator and an optical fiber amplifier and is then combined with that of the DFB-LD by a 3 dB fiber coupler. One of the two optical outputs of the coupler is input to a THz CW transmitter (THz-Tx) module and the other is input to a THz CW receiver (THz-Rx) module via a variable optical delay line. No polarization controller is used since all the optical components are connected using polarization-maintaining fibers. Figure 1: Schematic of our THz A-scan measurement system. WSL: wavelength-swept semiconductor laser, Gas cell: fiber-coupled H\({}^{13}\)C\({}^{14}\)N gas cell, PD: high-speed photodetector, OA: optical attenuator, OFA: optical fiber amplifier, DFB-LD: distributed feedback laser diode, WG: waveform generator, THz-Tx: THz CW transmitter, VODL: variable optical delay line, THz-Rx: THz CW receiver, BS: thin-film THz beam splitter, OAPM: off-axis parabolic mirror, and Amp: current preamplifier. The THz wave emitted by the THz-Tx is focused on a sample after passing through a thin-film THz beam splitter with a thickness of about a few micrometers [35]. The reflected THz wave from the sample is reflected by the beam splitter and focused onto the THz-Rx. The photocurrent output from the THz-Rx is amplified using a current preamplifier with a bandwidth of 14 MHz and then recorded using a 100 MS/s digitizer. The A-scan range is limited to about 400 ps by \(\tau=f_{d}/(df_{\text{THz}}/dt)\), where \(\tau\) and \(f_{d}\) are the time delay and the detection frequency limited by the 14 MHz bandwidth, respectively, and \(df_{\text{THz}}/dt\) is the sweep speed of the THz frequency, which is equal to 3.5 \(\times\) 10\({}^{4}\) THz/s at a sweep rate of 18 kHz [32]. The sweep start trigger signal output from the WS semiconductor laser triggers a waveform generator to provide the THz-Tx with a bias voltage modulated at half the repetition rate. At the same time, data traces are consecutively acquired at the repetition rate by the digitizer triggered by the sweep start trigger signal. The signal-to-noise ratio of THz data can be enhanced by subtracting noise traces acquired with the bias off from THz data traces carrying noise acquired with the bias on and averaging the resulting THz data traces. The A-scan data are obtained by FFT of the measured frequency-domain data in the frequency range of 0.5-1.88 THz [32]. When previously using the WS fiber laser, we measured the temporal variation of the optical frequency of the WS fiber laser (and thus that of the THz frequency) during the frequency sweep period and converted the time to the THz frequency using the temporal variation of the THz frequency to obtain frequency-domain THz data [32]. In addition, the frequency-domain THz data were interpolated at regularly spaced frequencies for their FFT. However, in the case of the WS semiconductor laser, the process is not needed due to its high frequency-sweep linearity. In Figure 1, the yellow highlighted area illustrates our THz A-scan setup.