# Numerical Study of a Polarization Selective Visual Optical Switch
## 1 Introduction  
Surface plasmons (SPs), as the carrier of optical information, has become a pivotal bridge between the micro-nano circuits and optical devices. SPs include surface plasmon polaritons (SPPs) and local surface plasmon polaritons (LSPs), which are surface electromagnetic waves formed by collective oscillation of free electrons interacting with incident light field in metal [1; 2; 3; 4]. In recent years, the enhanced optical transmission (EOT) effect brought by metal-based surface plasmons has been applied in different wavelengths, including visible light, near-infrared, and infrared ranges. In particular, when the response range of electromagnetic wave is in the visible light band, the diffraction, scattering and absorption of plasmonic structures can provide us extremely vivid information, especially in color. In general, when the geometric parameters of the micro-nano structure and the types of material change, the peaks of surface plasmon resonances will change (including the position, intensity, and splitting), making the materials present different colors, namely, structural colors. Based on the SPs, a large number of optical devices have been constructed, and its optical response can be designed by controlling the size, pattern and composition of the subwavelength units. For example, it is numerically found that the polarization state is a sensitive parameter in plasmonic EOT only when the gap size between triangular nanoapertures is less than 20 nm [5]. Through the construction of bullseye [6] and hoof-shaped [7] structures, the subwavelength metal structure has high transmittance and application prospect in enhancing optical transmission effect. Besides, these characteristics of the SPs have major applications [8; 9; 10] in ultra-high resolution imaging [11; 12; 13], inverse design [14], complementary metal oxide semiconductor (CMOS) digital integrated circuit [15; 16; 17], light emitting diode [18], steganography [19], and many other fields. Among of them, plasmonic opticalswitch (POS) plays a vital role in the construction of nanophotoniccircuits [7; 10]. In particular, as a basic logic element in photoelectric data processing, the advantages of small size and fast response of the POS can make it play an important role. For example, a multi-band multi-functional photonic device, in our previous work [7], has been proposed. By changing the polarization direction of the incident light, the plasmonic splitter and switch were both constructed at different wavelengths in the near infrared range. On the other hand, due to the local field enhancement effect of the SP excited by the oscillation between the photon and electron, it can break through the diffraction limit and greatly improve the color resolution [20]. At the same time, the EOT effect brought by surface plasmon can make the structure obtain higher transmittance, leading to the stronger color display. Up to now, plasmonic structural color has drawn more attention and various applications have been proposed [21; 22; 23]. For example, a structure combining silicon material and refractive index matching layer is proposed to improve the color richness and brightness [24]. However, the combination of surface plasmon devices and structural colors has not been studied in depth. Based on our previous work [7], when the working state of the optical switch can be directly represented by different colors, the device can be more operable and visual. In this paper, we present a composite structure based on filling different number of metal strips in the hollow circular holes. By rotating the polarization direction of the electric field of the incident light, the transmission peak within the visible light range shifts, and the material shows different colors. In addition, a plasmonic visual optical switch is constructed in the visible light range to directly characterize its working state by color, taking advantage of the phenomenon of displacement of transmission peak. Simultaneously, the contrast ratio of the device is more than 20 dB.