Ultra-compact asymmetric polarization beam splitter based on hybrid plasmonic waveguide and subwavelength grating utilizing particle swarm optimization algorithm

Compared with the traditional forward design method, the inverse design method has become a hot research topic because of its high degree of freedom and flexibility within the field of silicon-based photonic devices. In this work, we propose ultra-compact and ultra-wide bandwidth polarization beam splitter (PBS) by inverse design method on a standard silicon-on-isolator platform. The structure of the PBS consists of a hybrid plasmonic waveguide coupled to a subwavelength grating waveguide in an asymmetric directional coupler. The unique polarization diversity of the hybrid plasmonic waveguide enables TE and TM polarizations to be transmitted in two different layers, which is expected to break through the size limitation of photonic devices. The introduction of subwavelength grating effectively reduces the beat length of TE polarization and significantly broadens the working bandwidth of PBS. In order to optimize the parameters, the particle swarm optimization algorithm is employed, with the aim of searching to obtain a set of optimized parameters that satisfy both the TE polarization phase matching and the TM polarization phase mismatch. The numerical results demonstrate that the coupling length of the PBS is only 2.36 μm, and both the bandwidths for the TE and TM polarizations exceed 123 nm, which effectively covers the entire C-band. At the wavelength of 1550 nm, the extinction ratio for TE (TM) is 12.96 dB (20.13 dB) and the insertion loss is only 0.23 dB (1.12 dB). The fabrication tolerance of the PBS is also analyzed, and the results show that the device exhibits good tolerance to fabrication errors. The PBS obtained by inverse design has the characteristics of large bandwidth and small size, which is conducive to the development of photonic devices in the direction of integration.