## 2 Experiments  
A schematic diagram of our time-domain THz tomography system is shown in Figure 1. The photoconductive antenna emitter and detector are irradiated by the optical pulse trains from the pump and probe lasers, respectively, which make up the ECOPS laser system. The THz pulse train output from the emitter is transmitted through the silicon beam splitter and then steered by a two-dimensional (2D) galvanometer scanner with an optical angle (\(\theta\)) ranging from \(-20^{\circ}\) to \(+20^{\circ}\) in both the X and Y directions. Through a telecentric f-\(\theta\) Teflon lens, the THz beam is focused on and normally incident onto the flat focal plane regardless of the optical angle. The lens was designed to have a focal length of 146.9 mm and the beam size on the focal plane was measured to be about 4 mm [16; 18]. A sample is placed on the flat focal plane. The reflected THz wave from the sample propagates back to the beam splitter along the same path. The reflected THz wave from the beam splitter is detected by the detector.  
A 7.8-mm A-scan range, corresponding to a time delay window of 52 ps, can be scanned by ECOPS at a scan rate of 1 kHz. The telecentric f-\(\theta\) lens was designed to have a C-scan area of 100 \(\times\) 100 mm\({}^{2}\). As a result, a volume of 100 \(\times\) 100 \(\times\) 7.8 mm\({}^{3}\) can be scanned in 40 s, producing 200 \(\times\) 200 \(\times\) 725 voxels. It is possible to further reduce the total scan time by decreasing the number of transverse pixels or by increasing the A-scan rate while reducing the A-scan range.  
Our frequency-domain THz tomography system that uses the high-speed broadband frequency sweep of CW THz radiation is illustrated in Figure 2. To achieve the high-speed broadband THz frequency sweep, an optical beat source with a kHz beat-frequency sweep rate is used in THz photomixing. The optical beat source consists of a DFB-LD and a WSL. The DFB-LD operates at a fixed wavelength of 1545 nm, and the output wavelength of the WSL is swept from 1544 to 1558 nm at a 1 kHz sweep rate. Frequency-domain THz data are measured in the frequency range up to 1.5 THz. The optical output of the WSL is amplifiedby an optical fiber amplifier and then combined with that of the DFB-LD via a 3 dB fiber coupler. One of the two output arms of the coupler is connected to a THz CW transmitter module and the other to a THz CW receiver module via a variable optical delay line.  
Figure 1: Schematic diagram of our time-domain THz tomography system. PD: photodetector, NC: nonlinear optical crystal, EM: THz emitter, DT: THz detector, OAPM: off-axis parabolic mirror, BS: Si beam splitter, GS: 2D galvanometer scanner, DPG: digital delay/pulse generator, WG: waveform generator, Amp: current preamplifier.  
Figure 2: Schematic diagram of our frequency-domain THz tomography system. WSL: wavelength-swept laser, OFA: polarization-maintaining optical fiber amplifier, DFB-LD: distributed feedback laser diode, PM fiber: polarization-maintaining fiber, THz-Tx: THz CW transmitter, WG: waveform generator, VODL: variable optical delay line, THz-Rx: THz CW receiver, OAPM: off-axis parabolic mirror, BS: silicon beam splitter, GS: 2D galvanometer scanner, DPG: digital delay/pulse generator, Amp: current preamplifier.  
During the frequency sweep, frequency-domain THz interferograms are measured using the coherent homodyne detection method and trace averaging is used for noise reduction. The FFT results of the frequency-domain data are used as A-scan data. The total scan time depends on the number of points in the C-scan range and the number of averaged traces for each A-scan dataset. For example, it takes 100 s to acquire A-scan data for 100 \(\times\) 100 points with 5 averaged traces at a frequency sweep rate of 1 kHz and 4000 s for 200 \(\times\) 200 points with 50 averaged traces.