## 3 Results and Discussion
### Refractive Index Sensing Characteristics of Metamaterial  
It is known that the transmission spectra of terahertz metamaterials can drift with the change in the refractive index (RI) of the environment. In this work, three groups of metamaterials, the structure with only CW resonator, the metamaterials with unilateral symmetric structure, and metamaterials with unilateral asymmetric structure, were simulated. The quartz substrate and gold layer were used in the design of high sensitivity refractive index sensors for better stability and corrosion resistance properties. The sensing response came from the drift of LSP resonance peak/ EIT-like windows of different structures. A sensing dielectric layer of 3 \(\upmu\)m thick was added with the change of refractive index from 1 to 2. It was observed that as the RI of the dielectric layer increased, both the resonance peak and the transparent window were red-shifted. In addition, the FWHM of the EIT-like window showed slight broadening while the resonance intensity was also weakened (see Figure 9a-c).  
The performance of the sensor was mainly characterized by several important parameters, including the refractive index sensitivity (S), Q-factor, and figure of merit (FOM). \(S\) is defined dip frequency shift per-refractive index unit: \(S=\Delta f/\Delta n\). Q-factor is an important  
Figure 9: The influence of refractive index changes on the transmission spectra. (**a**) The transmission spectra of CW resonance peak varying with refractive index; (**b**) The transmission spectra of EIT-like window with unilateral symmetric structure varying with refractive index; (**c**) The transmission spectra of the EIT window with unilateral asymmetric structure varying with refractive index. With the increase of the refractive index of the dielectric layer, the resonance peak and the transparent window are red-shifted; (**d**) Linear fitting of the refractive index sensitivity against the drift of the peak frequency of three metamaterials.  
parameter that determines the resonance, dispersion, and loss properties of EIT-like metamaterials, and reflects the resonance intensity and resolution of the sensor. \(Q\) is defined as: \(Q=f/FWHM\). FWHM is the full width at half maximum of the resonance frequency. FOM obtained by dividing the sensitivity by FWHM used to characterize metamaterials sensing capabilities. FOM is described as: \(Q=S/FWHM\). Figure 9d shows the fitting of the refractive index sensitivity of three metamaterials. All the three curves show linear relationship as the RI increases, indicating a promising potential for sensing applications. For the CW-only structure, it could achieve a refractive index sensitivity of 63.6 GHz/RIU. However, for the unilateral symmetric structure metamaterial, it could achieve sensitivity of 84.4 GHz/RIU, Q-factors of 33.67 and FOM of 2.81. The refractive index sensitivity, Q-factor, and FOM of the metamaterial with unilateral asymmetric structure were found to be 102.3 GHz/RIU, 51, 5.115, respectively. These results show that, compared with the resonant peak, the EIT-like window is more sensitive to the refractive index of the external environment, especially for the unilateral asymmetric material structure (102.3 GHz/RIU).  
More specifically, the extracted results were compared with similar results reported in [10; 11; 17; 28; 29; 30], as shown in Table 1. It can be found that the two metamaterial structure sensors proposed in this work showed broader bandwidth and good sensing properties. EIT-like terahertz metamaterial refractive index sensors have a wide range of promising applications in environmental, chemical, and biological diagnostics as well as other related fields.