## 3 Experiment and Results
### Measurement and Discussion  
As shown in Figure 4a, in the measurement of reflection loss, the signal is measured by receiver (1#), which is set at the reflection optical path (red arrows). For measurement of the transmission loss, the signal is measured by the receiver (2#) that is set at the transmission optical path (blue arrows). The RI of the photosensitive resin material used in THz frequency range were measured by the THz-TDS system. As shown in Figure 4b,c, the experimental and simulated loss of the proposed waveguide with different gap distances were obtained from 0.1 to 0.5 THz in the potential 6G telecommunication band. In the reflection spectra, it was found that the resonant frequency (\(f_{\text{R}}\)) of the waveguide shifted to the left with an increase in gap distance (\(g\)), which is shown in Figure 4b. The experimental and simulated resonant frequency were both in the region of 0.2 to 0.3 THz. The numerical simulation results showed that the reflection loss decreased by about \(-20\) dB, and the experimental results showed that it decreased by about \(-10\) dB. As shown in Figure 4c, in the transmission spectra from 0.1 to 0.3 THz, it was found that the experimental and simulated results demonstrated approximately the same loss, of less than \(-45\) dB, and it showed a remarkable decrease trend higher than 0.3 THz. The magnitudes of loss  
Figure 3: (**a**) Schematic of the SLA printing process and 3D printed object. Optical microscopy images of (**b**) the cross-section of the THz PC waveguide, (**c**) the periodically arranged unit cells of the THz PC waveguide, (**d**) the illac-shaped resonant cavities of the THz PC waveguide, and (**e**) one petal of the illac-shaped resonant cavities of the THz PC waveguide. (**f**) Statistical analysis of the size of four parameters after multiple measurements. observed in the experimental results did not exactly match that of the simulated results, and this discrepancy could be attributed to the detection limitation of the experimental apparatus. Overall, the numerical simulation results agreed well with the experimental results, which indicates that the proposed waveguide has a good controllable performance. To analyze the resonant performance of the waveguide, the Q factor and reflection loss at the resonant frequency were calculated. Q factor can be expressed by [30]  
\[\mathrm{Q}\ =\ \frac{\omega_{r}}{\mathrm{FWHM}} \tag{3}\]  
where \(\omega_{r}\) is the resonant frequency (\(\omega_{r}\) = 2\(\pi\)\(f_{R}\)) and FWHM is the full width half max of the resonant spectrum. As shown in Figure 5a, the resonant frequency has a shift of 33 GHz. Simultaneously, as shown in Figure 5b, with a gap distance of 500-2000 \(\mathrm{\SIUnitSymbolMicro m}\), the measured Q factor was obtained from 1.55 to 2. The reflection loss was further analyzed at the resonant frequency, as shown in Figure 5c. The difference in the reflection loss between the experimental and simulated results was about 10 dB. Figure 4: (**a**) Optical path for the THz-TDS measurement setup, the above experiments are performed in dry air. (**b**) The simulated and experimental reflection spectra for the THz PC waveguide with the gap distance (\(g\)) set as 500 \(\mathrm{\SIUnitSymbolMicro m}\), 1000 \(\mathrm{\SIUnitSymbolMicro m}\), and 2000 \(\mathrm{\SIUnitSymbolMicro m}\), respectively. (**c**) The simulated and experimental transmission spectra for the THz PC waveguide with the gap distance (\(g\)) set as 500 \(\mathrm{\SIUnitSymbolMicro m}\), 1000 \(\mathrm{\SIUnitSymbolMicro m}\), and 2000 \(\mathrm{\SIUnitSymbolMicro m}\), respectively. In order to better understand the transmission characteristics of the waveguide, the FWHM and the \(-60\) dB bandwidth were analyzed. As shown in Figure 5d, with a gap distance of 500-1000 \(\upmu\)m, FWHM was obtained from 0.2 to 0.3 THz. The \(-60\) dB bandwidth was obtained from 0.4 to 0.5 THz, which is within the detection limitation of most commercial THz-TDS systems.