*3.3. THZ-TDS Setup*

All of the experimental investigations were performed with a THz-TDS system (Menlo TeraSmart, Martinsried, Germany). The centered wavelength of the femtosecond laser that was used was 780 nm and the repetition rate was 100 MHz. The frequency resolution of the THz-TDS system was 1.2 GHz with a signal to noise ratio (SNR) of about 80 dB. The beam was guided between the transmitter and detector by off-axis parabolic mirrors.

#### *3.4. 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 (*fR*) 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

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.

**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 μm, 1000 μm, and 2000 μm, respectively. (**c**) The simulated and experimental transmission spectra for the THz PC waveguide with the gap distance (*g*) set as 500 μm, 1000 μm, and 2000 μm, respectively.

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]

$$\mathbf{Q} = \frac{\omega\_r}{\text{FWHM}}\tag{3}$$

where *ω<sup>r</sup>* is the resonant frequency (*ω<sup>r</sup>* = 2π*fR*) 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 μm, 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 5.** Characteristics of the THz PC waveguide. Influence of the gap distance (*g*) on (**a**) the resonant frequency *fR*, (**b**) Q factor, (**c**) the reflection loss (at *f* = *fR*), (**d**) the bandwidth (with FWHM and −60 dB bandwidth, respectively), and (**e**) the transmission loss with *f* = 0.1 THz, *f* = 0.2 THz, and *f* = 0.3 THz, 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 μm, 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. In addition, in order to better illustrate the performance of the transmission loss in diverse frequency conditions, the transmission loss was analyzed in detail at different frequencies within 0.3 THz, individually (Figure 5e). The results revealed that the transmission loss decreased with a larger gap distance, and a similar trend was also indicated by the simulation results. The above results show that the waveguide performance could be controlled by the gap distance. Thus, the optimum structural parameters of the waveguide should be selected according to the application requirements.
