Design and optimization of a compact polarizer on the surface plasmon polaritons based SOI waveguide

Abstract

Silicon-on-insulator (SOI) waveguide is a competitive and technological platform for photonic integrated circuits because of its compatibility with mature complementary metal oxide semiconductor (CMOS) process. Silicon nanowires enable ultra-sharp bending due to their high refractive index contrast between the core and cladding layer of the waveguide, consequently allowing for a small device size. Unfortunately, the high dielectric contrast makes SOI waveguides highly polarization dependent, leading to a lot limitations for more practical applications of the SOI based devices. On-chip polarizers are important components for design and development of many optical-fiber coherent communication and optic sensing systems, such as switches, integrated-optic gyroscopes and polarization independent receivers for coherent detection in which only a specific polarization state is allowed. In this letter, a compact and low-loss TE passed polarizer was proposed and designed and optimized based on surface plasmon polaritons SOI platform where a 220nm-thin silicon layer acts as the core layer on top of a 2-μm silica buried layer. The structure used for SPPs based polarizer consists of a dielectric waveguide and metal slab film with a low-index spacer layer separated. In such a structure, surface plasmon resonance is achieved by the evanescent wave from the light beam being totally internally reflected inside the high-index silicon core layer with phase matching conditions satisfied. The fundamental TM filed is coupled into the plasmon mode concentrated in the close vicinity of the metal/dielectric interface. In contrast, the fundamental TE mode is mainly guided in the silicon core region. As a result, the TM mode behaves a larger absorption loss due to the existence of the imaginary part in the metal permittivity, and the TE mode is slightly affected. For a proper design of the waveguide dimensions, the TM mode propagating through the polarizer will suffer significant attenuation while the TE mode keeps a low propagation loss, thus providing a high polarization extinction ratio. Effects of important design parameters, such as metal material, spacer thickness and refractive index, on the performance of the polarizer are compared and analyzed. FDTD simulation results show that, a 5 μm-long polarizer with SiN spacer and Cr cladding could provide a high polarization extinction ratio of more than 20 dB with an excess loss of TE mode less than 0.27dB at 1.55μm wavelength over a 140 nm broad bandwidth. And the insertion loss can be further reduced to less than 0.2 dB with a 10-μm polarizer length by increasing the buffer thickness. This compact and low-loss polarizer could be easily implemented with simple fabrication process.