# Terahertz Wideband Filter Based on Sub-Wavelength Binary Simple Periodic Structure
## 5 Conclusions  
In this paper, a rigorous coupled wave method and a genetic algorithm were used to propose a sub-wavelength binary simple periodic structure broadband filter design method which effectively realizes the filtering effect of broadband and high diffraction efficiency.  
A center frequency of 4.07 THz was designed and filtered. A terahertz wideband filter with a frequency range of 2.94-5.27 THz was used to fabricate samples and tested using a binary optical process. The experimental results were consistent with the design results. As shown in Table 2, the highest transmission efficiency in the main lobe was close to 50%, the highest transmittance of the reference double cone filter was increased by more than 10%, the 3-dB bandwidth was 2.33 THz, and the width was 0.13 THz. The influence of the variation of the grating structure parameters on the filtering effect of the filter was studied. The variation of the structural parameters had a great influence on the bandwidth. It was especially sensitive to the variation of the groove depth. Groove depth error control should be strengthened during the production. The fabrication error of the terahertz broadband filter structure parameters was analyzed, and the most suitable fabrication parameters are given: a grating period of 127.9 \(\upmu\)m; a duty ratio of 0.414; and a grating groove depth of 29.60 \(\upmu\)m. The allowable structural parameter redundancy ranges include: a grating period of 127.9 \(\upmu\)m (\(\pm\)2.5 \(\upmu\)m); a grating duty ratio of 0.414 (\(\pm\)0.002, minimum line width 53.0 \(\upmu\)m, redundancy \(\pm\)0.4 \(\upmu\)m); and a grating groove depth of 29.6 \(\pm\) 0.7 \(\upmu\)m. The current production technology can fully realize the structure. The device was fabricated and tested with a 3-dB bandwidth of 2.18 THz, which  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Value** & **3-dB Bandwidth** & **Maximum Transmission Efficiency** \\ \hline Designed & 2.21 THz & 49.1\% \\ \hline Sample (simulated) & 2.19 THz & 48.9\% \\ \hline Sample (tested) & 2.18 THz & 38.1\% \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of actual production and theoretical design of the filtering effect of the terahertz wideband filter.  
Figure 9: Comparison of sample simulation curve and fitted curve.  
is consistent with the design results, proving the correctness and feasibility of the design method. Additionally, there was a maximum transmission efficiency of 38.1% in the main lobe, which is less than the simulation and the reasons for this are discussed.  
In this paper, a high-performance terahertz wideband filter was realized that overcomes the limitations of the current terahertz filters with a simple structure, a lower cost, and easier production. At the same time, tolerance analysis and manufacturing error analysis of the structural parameters were carried out. The actual sample preparation and performance test showed that the test results were consistent with the theoretical design, and can be applied in the fields of terahertz communication, imaging, and other fields.  
Conceptualization, P.Y., L.L. and X.P.; Data curation, H.H.F., Z.Y. and X.Y.F.; Formal analysis, Y.T.; Funding acquisition, H.H.F.; Investigation, P.Y.; Methodology, P.Y.; Project administration, L.L. and X.P.; Resources, Y.T.; Software, Z.Y. and X.Y.F.  
This work has been supported by grants from the National Natural Science Foundation of China (No. 61275167), the Basic Research Project of Shenzhen (JCYJ20130329103020637, JCYJ20140418095735591, JCYJ20160603172549843, JCY20170817102315892 and JCYJ2017081701827765).  
The authors declare no conflict of interest.