# Plasmonics Induced Multifunction Optical Device via Hoof-Shaped Subwavelength Structure
## 4 Plasmonic Optical Devices Based on Periodic Hoof-Shaped Arrays  
### Design of Plasmonic Optical Switch  
Considering the induction of new transmission peaks in the mid-infrared range, we constructed a plasmonic optical switch (PO switch) by changing the polarization angle of the incident light. The parameters of the PO switch are shown in Figure 1b. In this paper, we consider that the center wavelength \(\lambda_{0}\) of the transmission peak is the working wavelength of the PO switch and the performance index of the contrast ratio is used to quantitatively describe the control effect of the PO switch on the lamp [29]:  
\[\eta=10\log_{10}\left(\frac{T_{on}}{T_{off}}\right) \tag{1}\]  
where \(T_{onb}\)/ is the transmission peak at the working wavelength \(\lambda_{0}\) of the switch when the switch is on/off. Furthermore, we change the length of \(S_{1}\) in the hoof-shaped structure for break the structure's symmetry in \(X\) direction. The parameters are as follows: \(S_{1}\) is set to 0 nm, 10 nm, 20 nm, 30 nm, 40 nm, 50 nm and other parameters remain unchanged. As shown in Figure 4a, the redshift of transmission peak occurs with the increase of \(S_{1}\), and the maximum redshift is 167 nm at \(\theta\) is 90. It is apparent that, when \(S_{1}\) is 50 nm, the maximum contrast ratio is 13.2 dB in Figure 4b. The results show that the symmetry of the structure is more complete with the increase of \(S_{1}\). Generally speaking, when the two resonance modes are asymmetric in the Fano resonance (Resonance mode must be generated or the resonance intensity must reach a certain threshold), the transmission peak display splitting [30; 31; 32]. Figure 3: (**a**) Transmittance spectrums of the three structures (U-shaped, rectangle and hoof-shaped) under the different angles of the incident polarization, the structural parameters of the three structures remain unchanged. (**b**) Electric field distributions of U-shaped structure, rectangle structure and hoof-shaped structure at 0 and 90 degrees. In other words, when the resonance modes of the two tips are asymmetric, the transmission peak will shift. At the same time, with the increase of \(S_{1}\), the space in the whole hole increases, which leads to the increase of accumulated charge and the increase of transmittance. We change the length of distance \(D\) in the hoof-shaped structure to study the influence of the EOT. The parameters are as follows: \(D\) is set to 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, and other parameters remain unchanged. As shown in Figure 5a, the transmission peak occurs blueshift with the increase of \(D\), and the maximum amount of blueshift is 900 nm at \(\theta\) is 90. We can find that the maximum contrast ratio is 14 dB when \(D\) is 90 nm in Figure 5b. By optimizing the geometric parameters of the structure, we obtain the transmittance of the transmission peak and the contrast ratio of the optical switch under the different conditions, which is more widely used in different applications. It can be seen from the figure that when there is a certain distance between the two structures, the coupling effect between the two structures is better, resulting in higher transmittance. Generally speaking, the closer the distance between the two holes, the stronger the coupling between the excited resonance modes. But in the hoof-shaped structure, when there is a certain distance between the U-shaped structure and the rectangle structure, the metal part in the middle can be regarded as a metal nanoparticle, that is, an electric dipole. When the incident light is irradiated, the interaction between photons and electrons will be greatly enhanced due to the existence of the electric dipole [33]. Therefore, in a certain range, the larger the distance between the U-shaped structure and the rectangle structure, the larger the size of the metal nanoparticles in the middle, the better the resonance coupling effect, that is, the greater the transmission.