## 3 Results and Discussion
In TM mode, the first resonance (\(\omega_{1}\)) is very stable at 0.39 THz, while the second resonance (\(\omega_{2}\)) is gradually red-shifted from 1.71 to 0.82 THz by increasing the \(L\) value from 8 to 24 \(\upmu\)m, as shown in Figure 5b. The tuning range of the second resonance and the free spectrum range (FSR) is 0.89 THz. This characteristic is very suitable for THz filter and sensor applications. The tendency of resonances and \(L\) values of the eSRR device are summarized in Figure 5c. Figure 6 represents the corresponding E- and H-field distributions with \(L=12\)\(\upmu\)m. It is worth mentioning that the E-field energy is distributed on the upper and lower parts of the two metal bars and the H-field energy is distributed in the center  
Figure 4: E- and H-field distributions of eSRR with \(L_{1}=24\)\(\upmu\)m in TE and TM modes. Figure 3: Transmission spectra of eSRR with different \(L_{1}\) values in (**a**) TE and (**b**) TM modes. (**c**) The relationships between resonances and \(L_{1}\) values are shown in (**a**,**b**). of the two metal bars monitored at 1.33 THz in TM mode, which is obviously different from the case where the lengths of two metal bars are not equal (\(L_{1}\
eq L_{2}\)). The lower metal bar is connected to the suspending microelectromechanical systems (MEMS) comb driver moving along the y-axis direction to change the gap between two metal bars (i.e., \(g\) value) and determine tunability and applicability. The transmission spectra of the eSRR with different \(g\) values are shown in Figure 7. There are two stable resonances in TE mode, as shown in Figure 7a. In TM mode, the first resonance (\(\omega_{1}\)) is very stable at 0.39 THz and the second resonance (\(\omega_{2}\)) is gradually blue-shifted from 0.83 to 1.33 THz by increasing the \(g\) value from 0 to 12 \(\upmu\)m, as shown in Figure 7b. The tuning range is 0.50 THz. That means that FSR can be broadened by 0.50 THz. Therefore, the eSRR could be controlled to broaden the tuning of FSR by increasing the \(g\) value and narrowed by increasing the \(L\) value, which can be used in a tunable bandwidth filter. The tendencies of resonances and \(g\) values are shown in Figure 7c. Figure 5: Transmission spectra of eSRR with different \(L\) (\(L=L_{1}=L_{2}\)) values in (**a**) TE and (**b**) TM modes. (**c**) The relationships of resonances and \(L\) values in (**a,b**). Figure 6: E- and H-field distributions of eSRR with \(L=12\)\(\upmu\)m in TE and TM modes. values of the eSRR device are summarized in Figure 7c. In order to further understand the physical mechanism of electromagnetic responses of the eSRR, the corresponding E- and H-field distributions of the eSRR with \(g=6\)\(\upmu\)m are plotted in Figure 3.0 The E-field energy is distributed on the upper and lower parts of the two metal bars, and the H-field energy is distributed in the center of the two metal bars monitored at 1.30 THz in TM mode. Therefore, by moving the lower metal bar to a different position, the eSRR device exhibits different optical characteristics. This design provides an effective approach to realize the tunable THz filter, polarizer and sensor applications. Figure 3.1 E- and H-field distributions of eSRR with \(g=6\)\(\upmu\)m in TE and TM modes. Figure 3.2 Transmission spectra of an eSRR with different \(g\) values in (**a**) TE and (**b**) TM modes. (**c**) The relationships of resonances and \(g\) values in (**a**,**b**).