# Terahertz Wideband Filter Based on Sub-Wavelength Binary Simple Periodic Structure
## 4 Production and Performance Testing  
In this paper, a sample was fabricated with inductively coupled plasma reactive ion etching technology. The three-dimensional and top views of the fabricated terahertz broadband filter are shown in Figure 6.  
According to the SEM measurement, the groove depth of the actual fabricated terahertz broadband filter was about 29.81 \(\upmu\)m, which is close to the design value of 29.60 \(\upmu\)m and is within the error tolerance. The period of the fabricated terahertz wideband filter was 127.19 (53.00 + 74.19) \(\upmu\)m, which is close to the design value of 127.90 \(\upmu\)m and is within the error tolerance. The duty ratio of the fabricated  
Figure 6: (**a**) SEM 3D and (**b**) SEM top view of the terahertz wideband filter.  
terahertz wideband filter was 0.417 (53.00/127.19) and the design value is 0.414, which is within the error tolerance.  
The sample was tested in Shanghai THz device testing company (Shanghai, China), and Advantest TAS7500 series terahertz band ultra-wideband spectral analysis system (Advantest, Shanghai, China) was used to test the filter performance. After the analysis unit received the analysis signal and transmitted it to the PC, the experimental test data were obtained. The test results of the sample were basically the same as the curve of the simulation results. As shown in Figure 7, the corresponding frequency point at the highest transmission efficiency peak was close to 4.07 THz, and there was a significant transmission efficiency through near 6 THz. However, the spectral line of the test results exhibited a multi-spike shape because the terahertz wave was generated by the ultrafast laser-pulsed nonlinear crystal, which resulted in the selection of the terahertz signal by the cavity.  
The peak structure of the above test data interfered with the calculation of 3-dB bandwidth to a certain extent, and also affected the quantitative comparison and analysis of experimental data and simulation data. In order to better perform data analysis, the measured results were polynomial-fitted, and the frequencies of the measured data were subdivided into many different frequency bands, where each frequency point corresponded to a transmission efficiency of the point, and the peak point of each pulse in each frequency band was selected and relabeled. Finally, the fitted envelope was compared with the pre-fit experimental value and the sample simulation optimization result, as shown in Figures 8 and 9:Figure 9 shows that the fitting curve agreed well with the sample test value, and the black fitting curve was completely in the middle of the red sample test data peak structure. In the measurement, the 3-dB bandwidth of the fitting curve was 2.18 THz, and the highest transmission efficiency was 38.1% in the main lobe. In the simulation, the 3-dB bandwidth was 2.19 THz, the highest transmission efficiency was 48.8%, as shown in Table 1. The reason for the declining transmission efficiency in the measurement consists of three main parts: firstly, the absorption of the material itself caused the loss of the transmitted wave energy; secondly, for the incident beam, the incident condition was approximately of normal incidence; finally, the collimation beam expansion process was not performed, and there was a space transmission loss of the terahertz beam in the atmosphere.