Ultra-Broadband and Ultra-Compact On-Chip Silicon Polarization Beam Splitter by Using Hetero-Anisotropic Metamaterials

The polarization beam splitter (PBS) is an essential component in the polarization diversity circuit to overcome the strong polarization dependence of silicon nanophotonic devices. Among various PBS structures, the asymmetrical directional couplers (ADCs) are most commonly used due to the best overall performances. However, it is still challenging to realize an on-chip silicon PBS with a small footprint, low loss, high extinction ratio, and, especially, broad bandwidth, simultaneously. The working bandwidth is limited for ADCs, since the wavelength difference will introduce deviations to both the effective indices and the coupling strength, which would distort the phase-matching property and the optimal coupling length. In this paper, an on-chip silicon PBS is proposed and demonstrated using effective medium anisotropy instead of configuration asymmetry to break the bandwidth bottleneck. The metamaterials with engineered anisotropy and dispersion are utilized to form a hetero-anisotropic slab. For TM polarization, the slab performs as a multi-mode interference coupler, while for TE polarization, the slab performs as two isolated waveguides. The fabricated PBS shows low loss <1 dB, high extinction ratio >20 dB, and a record working bandwidth >200 nm, with a footprint as small as 12.25 x 1.9 mu m(2).