Terahertz filter is an important functional device for realizing terahertz imaging, terahertz communication, and other terahertz application technologies, and in-depth study of terahertz filters plays a great role in promoting the development of terahertz technologies. Therefore, the design of terahertz filters with a wide range of adjustable center frequency, sensitive bandwidth adjustment, deep modulation depth, simple structure, and multi-band transmission performance has become an urgent problem. In this study, a metamaterial terahertz band-stop filter based on two intersecting split-ring resonance (TI-SRR) is designed. It can effectively expand the bandwidth adjustment range, adjust the center frequency, reduce the transmittance, and make the stopband attenuate rapidly. At the same time, it has multi-band filtering and is easy to fabricate and process. The metamaterial terahertz band-stop filter has high application value in the field of new communication equipment and precision instruments.

In this experiment, the effect of each parameter on the performance of the filter is investigated by varying the line width, inter-ring spacing, and radius size of the TI-SRR. The transmission coefficients of the filter under each parameter are compared, and the design scheme with the best performance is summarized. The electric field and surface current distributions at the three resonance points of the metamaterial band-stop filter are analyzed, so as to investigate the working mechanism of the filter. In order to verify the calculation results of the theoretical model, physical samples of the filter are prepared by micro-nanolithography and tested using a terahertz time-domain spectroscopy (THz-TDS) system. The results of the actual measurement and simulation are compared to find the cause of the error.

Simulation experiments are carried out on several parameters of the metamaterial terahertz band-stop filter, while other parameters are kept unchanged. When the line width w gradually increases from 2 to 14 µm, the filter's first stopband bandwidth increases from 0.190 to 0.253 THz, and the relative bandwidth radually increases from 44.98% to 54.33%. The bandwidth range increases, and the center frequency is shifted to the high-frequency direction, while the filter's modulation depth deepens. However, as the line width increases, the resonance in the ring is strong, and clutter appears at high frequencies. The line width w is selected to be 10 µm (Fig. 2). When the spacing m gradually increases from 10 to 40 µm, the first stopband bandwidth of the filter decreases from 0.238 to 0.227 THz, and the relative bandwidth gradually decreases from 52.44% to 44.51%. The bandwidth of the filter decreases slightly, and the adjustment of the spacing allows the filter bandwidth to be adjusted precisely. The transmittance of the filter is gradually increasing, which is not favorable to the performance of the band-stop filter. Therefore, smaller spacing indicates a better filtering effect. However, as the spacing m decreases, the resonance in the ring becomes stronger, and clutter occurs at high frequencies. The spacing m is selected to be 10 µm (Fig. 3). When the outer radius R₁ gradually increases from 56 to 68 µm, the inner radius R₂ gradually increases from 56 to 58 µm, and the first stopband bandwidth of the filter increases from 0.222 to 0.261 THz. The relative bandwidth increases from 47.65% to 61.34%, and the effective bandwidth of the filter increases. The filter performance transmittance is reduced within the effective bandwidth, and the filter performance is improved. However, with the gradual increase in the radius, the filtering effect of the third resonance point becomes worse, and the outer radius R₁ is selected to be 60 µm after comprehensive consideration (Fig. 4). The physical samples of the filter are prepared by micro-nano lithography (Fig. 10). The measured results are shifted at each resonance point compared with the simulation, and the measured stop-band suppression effect is not as good as that of the simulation. Especially, the gap is obvious at the high frequency, but the overall curve trend remains consistent.

In this study, a terahertz band-stop filter based on metamaterials is designed to optimize the performance of the filter by varying the metal line width, inter-ring spacing, and metal open-ring radius, and the optimal design is concluded. The working mechanism of the filter is analyzed based on the electric field and surface current distributions of the terahertz filter. The physical filter samples are prepared by micro-nano lithography and tested by the THz-TDS system. Test results are comparable with the simulation results considering the errors in the test process. The band-stop filter has the advantages of a simple structure, adjustable center frequency, fine-tunable bandwidth, wide range adjustment, low stopband transmittance, and deep modulation depth.