Results and Discussions Firstly, the terahertz transmission spectrum of the sensor is numerically calculated using the finite integration technique (FIT), and there is an obvious resonance transmission dip at 0.776 THz (Fig. 1). Then, the influence of each structural parameter on the resonant frequency is analyzed, and the variation law of the resonant dip frequency with the structural parameters is obtained. At the same time, in order to investigate the applicability of the sensor, the influence of different incident angles and polarization angles on the sensor is further studied. It is found that the position of the resonant frequency and the transmittance is almost unchanged in the range of 0°-30°, which indicates that the sensor is very appropriate for the practical application of biological sensing. In order to further analyze the sensing performance of the sensor, this paper places a layer of the analyte with a variable refractive index on the surface of the sensor. Through the calculation and analysis of its different transmission spectra, it is found that with the increase in the refractive index, the resonance dip has a red shift, and the resonant frequency gradually decreases. After collecting the resonance peak frequency corresponding to the refractive index of each analyte, it is verified that the sensor has a refractive index sensitivity of 161.06 GHz/RIU (refractive index unit) and a figure of merit (FOM) value of 1.98 (Fig. 6) calculated by linear fitting. In order to investigate the influence of different cell numbers and substrate materials on the sensor's characteristics, the performance of the single split-ring sensor with the same structural parameters and the double split-ring sensor with silicon material as the substrate is compared. It is found that the refractive index sensitivity of the two sensors is lower than that of the double split-ring sensor designed in this paper. Finally, in order to verify the actual performance of the terahertz biosensor, the designed copper metal structure is fabricated on the quartz substrate by using the traditional photolithography technology and stripping process, and the sensor is successfully fabricated (Fig. 9). By using the continuous wave terahertz spectrum detection system (Topica Photonics AG, TeraScan 1550), the sensor is tested on different mass concentrations of bovine serum albumin (BSA) solution (Table 1), and the experimental results are shown in Fig. 11. The experimental results show that with the increase in mass concentration of BSA solution, the resonance dip has a red shift, and the resonant frequency decreases, which is consistent with the numerical results. However, the relationship between the frequency shift of the resonance dip and the mass concentration of the BSA solution is not linear, which is common in biological experiments. The Hill model is usually used to fit the relationship curve between the change of resonant frequency and the mass concentration of BSA solution. By using this model to fit the experimental results, the sensing sensitivity of 59.02 GHz/(ng·mm^-2) and the detection limit of 0.004 mg/mL are obtained.

Terahertz wave has become a promising technology for studying chemical and biological molecules due to its macromolecular fingerprint recognition, low photon energy, and high penetration characteristics. With the development of terahertz time-domain spectroscopy and portable terahertz spectroscopy tools, terahertz sensing technology is increasingly widely used in the fields of high sensitivity and on-site detection/recognition of trace biological molecules, promotion of protein synthesis, and cell division. However, there are problems such as low scattering cross-section and weak absorption due to the size mismatch between biomolecules/cells and terahertz wavelengths (30 μm-3 mm). Therefore, it is necessary to use enhanced terahertz resonance with subwavelengths to achieve strong light capture. Besides, metamaterials can be artificially designed to control electromagnetic waves, which can enhance the detection ability of terahertz waves.

From previous studies, it has been found that under the illumination of the incident light, the metamaterial with a metal split-ring structure will generate a very local and binding electric field at the split position so that it can greatly enhance the absorption cross-section of the biochemical detection sample located on the surface of the split-ring structure and realize the sensing detection of trace biochemical samples. Based on the analysis of metamaterials with a split-ring structure, a quartz substrate terahertz metamaterial biosensor with a double split-ring structure is designed in this paper. Through the frequency change of two equivalent capacitance inductor (LC) resonances in different refractive index environments, high refractive index sensitivity sensing is realized, and the detection of some biological molecules with different concentrations is achieved.

In this paper, a quartz substrate terahertz metamaterial biosensor with a double split-ring structure is designed and fabricated. It is found that there is an obvious resonance transmission dip at 0.776 THz (Fig. 1). The influence of different incident angles and polarization angles on the sensor is further studied. It is found that the position of the resonant frequency and the transmittance is almost unchanged in the range of 0°-30°. This paper also places a layer of the analyte with a variable refractive index on the surface of the sensor, and it is found that the sensor has a refractive index sensitivity of 161.06 GHz/RIU and a FOM value of 1.98 (Fig. 6). At last, the sensor is tested in different mass concentrations of BSA solution (Table 1), and the experimental results are shown in Fig. 11. By using Hill model to fit the experimental results, the sensing sensitivity of 59.02 GHz/(ng·mm^-2) and the detection limit of 0.004 mg/mL are obtained. The results of theoretical simulation and biological experiments show that the biosensor has good sensing performance, simple structure, small size, and stable performance. It can be used for the rapid detection of trace biomolecules and related application fields.