## 1 Introduction  
Due to the strong light-electron coupling at the interface of a metal and a medium, surface plasmon polaritons (SPPs) have enticing properties, such as the subwavelength confinement and surface enhancement of the optical field. Due to the benefits provided by these features, SPPs are widely used in designing on-chip photonics devices, such as optical switches [1], optical logical gates [2; 3], and other optical parameter detecting devices [4; 5], which will be important in future information processing and all optical communication. In these planar-integrated nanophotonics devices, on-chip manipulation and control of SPPs is of crucial importance. Therefore, a large number of studies have been conducted regarding the control of SPPs on metal surfaces with well-designed nanostructures [6; 7; 8] by manipulating optical parameters such as wavelength [9], phase [10; 11; 12], and polarization state [13; 14; 15] of the incident light. An optical router is an important component in communication systems. When designing optical routing devices for signal transmission, in general, the aim is to uniquely connect a certain launching path of the excited optical signal with a certain optical state of the incident signal. Therefore, the light parameters, such as wavelength and polarization state, which determine the SPPs' directional launching, are the key functionality for designing SPP-based on-chip photon routing devices. In 2013, [PERSON] et al. reported that the polarization-sensitive plasmonic apertures having the shape of an ear of wheat in a gold film enabled polarization-controlled tunable SPP directional launching [16]. Since then, controllable SPP directional launching [17; 18; 19; 20] and on-chip spin routing devices [21; 22; 23] have received significant research attention. However, most reported works have only realized two possible SPP directional launching paths by controlling the polarization states of the incident light, which limits the tunability. Developing highly adjustable on-chip routing devices is extremely important in practical applications. By comparison, in previous works [17; 18; 19; 20], the directional launching SPPs suffer from beam diffraction during the propagation. Suppressing this diffraction can effectively improve the anti-interference ability. Thus, the suppression of the on-chip SPP-beam diffraction effect has become a popular research topic in recent years. One of the most effective and feasible solutions is to excite a non-diffracting SPP beam. Although a large number of investigations on non-diffracting SPP beams have been undertaken [24; 25; 26; 27; 28], to the best of our knowledge, non-diffracting SPP-beam-based spin routing devices with high tunability in multiple degrees of freedom have not been reported.  
Therefore, this study focused on the highly adjustable non-diffracting SPP-beam-based spin router. A simple rectangle nano-slot array structure, which can generate a nondiffracting SPP beam in the positive/negative y-axis direction when the incident light undergoes left circular polarization (LCP)/right circular polarization (RCP), was theoretically designed to realize spin routing using Finite-Difference Time-Domain (FDTD) simulation. The simulation results show that the propagation direction of the excited nondiffracting SPP beam can be dynamically rotated counterclockwise or clockwise by changing the incident angle. Hence, the routing SPP beam can be coupled to different output waveguides to realize dynamic tunability. The broadband response of the device was also investigated.