# Numerical Study of a Polarization Selective Visual Optical Switch
## 3 Results and Discussions
### The Influence of the Number of Metal Strips on the Enhanced Optical Transmission
Moreover, we consider that the center wavelength of the transmission peak is the working wavelength of the POS, and the performance index of the contrast ratio is used to quantitatively describe the control effect of the POS on the lamp [28]:  
\[\eta=10\log_{10}\left(\frac{T_{on}}{T_{off}}\right) \tag{7}\]  
where \(T_{on\text{off}}\) is the transmission peak at the working wavelength of the switch when the switch is on/off. As shown in Figure 2b-d, when the number of metal strips is 1, 2, and 3, the contrast ratio is 8 dB, 13.8 dB, and 20.3 dB, respectively. This is 6 dB higher than our previous work. More obviously, the different working states of the switch are directly represented by the color presentation [7]. It should be noted that when the number of metal strips is changed, the working wavelength of POS also shifts. This means that once the device is made, its working wavelength will be determined. However, in practical application, we combine the background color of the environment to select the geometric parameters of the device, so that the color produced by POS is not affected by the background color. Here, we give the relative electric field intensity distribution diagrams under the case of different number of metal strips. As shown in Figure 3a-c, when the polarization direction of the relative electric field is parallel to the _X-axis_, the charge is mainly concentrated at the sharp angle of the metal strip. This is due to the fact that the spine of the structure is dominated by the LSP resonance mode. Figure 2: Transmittances of different polarization directions (Parallel to _X-axis_ is 0\({}^{\circ}\); Perpendicular to _X-axis_ is 90\({}^{\circ}\)) under the different number of metal strips. (**a**) Hollow circular hole; (**b**) one metal strip; (**c**) two metal strips; (**d**) three metal strips. The inset is CIE1931 color space inversion map distribution. When the polarization direction of the relative electric field is perpendicular to the _X-axis_, the results show that in addition to a large number of charges accumulated at the sharp corner of the metal strip, there are also a large number of charges between the metal strip and the metal strip in Figure 3d-f. It can be seen from the relative electric field distribution diagrams that when the polarization direction is perpendicular to the metal strip, the space between different metal strips is equivalent to a Fabry Perot (FP) cavity. Therefore, when the polarization direction is rotated from parallel to perpendicular to the _X-axis_, due to the existence of the FP cavity, the resonance modes excited by the metal strip interact with each other, resulting in the shift of the transmission peak. In other words, the change of polarization direction change the coupling degree between the metal strips in the FP cavity, resulting in the shift of transmission peak [7]. The Influence of the Changing between the Different Geometric Parts of the Structure on the Enhanced Optical Transmission  
In order to further explore the influence of the interaction between different geometric parts of the structure on the EOT, we firstly study the structure of hollow circular hole. As shown in Figure 4(a1), we change the radius \(R\) of a hollow circular hole. It is found that the amplitude of the transmission peak decreases slightly with the decrease of the area of the hollow circular hole. Obviously, the decrease of the area of the circular hole leads to the decrease of the incident light participating in the resonant mode. In addition, we also studied the angle between the incident light and the normal direction of the material surface. As shown in Figure 4(a2), the results are more obvious when the included angles is 0, 5, 10, 15, 20, 25, and 30 degrees respectively. The only change is the transmittance, which has no effect on the position of the transmission peak. In a single circular hole structure, the SPPs resonant mode plays a major role in EOT effect [26]. In this model, the period \(P\) of the structure is the main geometric parameter to be adjusted.