The Fabry-Perot (FP) interferometer is an effective tool for measuring the wavelengths of terahertz waves. It exhibits the advantages of easy adjustment, simple structure, quick measurement, and high accuracy, making it a highly effective tool in the field of wavelength measurements. The FP interferential filter should satisfy the requirements of high reflectivity and low transmissivity. Interference fringes generated by low-reflectivity filters exhibit low precision and resolution, leading to increased errors in wavelength measurements. Natural nonmetallic materials exhibit low reflectivity for Terahertz waves. However, by depositing a periodic metal array on their surface, a frequency-selective surface (FSS) can significantly enhance their reflectivity for Terahertz waves. Currently, this is the preferred method for designing FP interferential filters. Extant studies indicate that the dimensions of FSS structural units decisively impact Terahertz transmission characteristics. Many researchers examined metal array structures, but the compatibility among performance parameters, such as reflectivity, applicable frequency range, and angle stability, has not been satisfactorily realized. To further explore the relationship between the FSS structure and FP interferential filter performance and address the issues presented in extant research, in this study, a new FSS structure is proposed.
The FSS structure proposed in this study utilize high-resistance silicon and copper. Firstly, in the frequency range of 1.0‒3.0 THz, the relationship between thickness of high-resistance silicon and resonance period is simulated using the HFSS software. A high-resistance silicon with a thickness of 100 μm is determined as the substrate material. In the frequency range of 1.2‒2.5 THz, the relationship between number of metal grid layers and Terahertz wave transmission characteristic is simulated, confirming the use of a single-layer grid for the FSS structure. The influence of various structural parameters on Terahertz wave transmission characteristics is analyzed, and parameter optimization is performed to determine the optimal size parameters. The current density and electric field intensity of the FSS structure under optimal parameters are simulated to analyze the reasons for its high reflectivity. Furthermore, the angular stability of the optimized structure is investigated. Finally, based on the size parameters, physical fabrication is conducted, and the transmission characteristics of the physical samples are measured using a Terahertz time-domain spectrometer (THz-TDS) in the nitrogen environment with 2% humidity. The measurement results are compared with the simulation results to validate their accuracy.
With respect to the FSS structure designed in this study, period, line width, and circular hole radius correspond to the main factors affecting the transmission characteristics. The reflectivity increases as the period and circular hole radius decrease, and the reflectivity increases with an increase in the line width (Fig. 6). When the Terahertz wave impinges vertically on the FSS structure, the copper metal undergoes plasmonic resonance, resulting in a significantly higher current density when compared with the surface of high-resistance silicon. Simultaneously, the electric field is mainly concentrated on the surface of the FSS structure, with only a minor portion of the electric field present in the high-resistance silicon and air media (Fig. 7). Hence, the FSS exhibits high reflectivity. The reflectivity escalates as the incident angle increases. Around the frequencies of 1.7 THz and 2.2 THz, the differences between the reflectivity at an incident angle of 45° and the reflectivity at normal incidence are 0.5% and 0.8%, respectively. The differences in transmissivity are 0.39% and 0.5%, respectively, which are within a reasonable range of variation (Fig. 8). Owing to the influence of the manufacturing process, some defects are observed in the physical filter and metal grid structure (Fig. 9). Through measurements, the physical filter exhibits reflectivity ranging from 91% to 98.4% and transmissivity ranging from 0.7% to 8% within the frequency range of 1.34‒2.34 THz, which are in good agreement with the simulation results (Fig. 10). Thus, the design requirements are satisfied.
This study presents a new FSS structure fabricated by depositing a single-layered metal copper grid on a high-resistance silicon wafer. A physical Terahertz FP interferential filter corresponding to this structure satisfies the requirements of high reflectivity and low transmissivity. Under vertical THz-wave incidence, the measured results are in good agreement with the simulation results. In the frequency range of 1.34‒2.34 THz, reflectivity ranges from 91% to 98.4% and transmissivity ranges from 0.7% to 8%. The structure also exhibits good stability within an incident angle range of 0°‒45°. This research improves the reflectivity of the filter, widens its applicable frequency range, and enhances the angle stability. Additionally, it provides new references for the study of FSS structures and related terahertz devices. Finally, given the influence of the manufacturing process, some defects exist in the physical filter, which may affect the interference of Terahertz waves during practical usage. In future studies, further improvements in terms of the coating processes can enhance the research findings.
