An ultra-compact and broadband high extinction ratio polarization splitting directional coupler using nanohole-based metamaterial waveguides

On-chip polarization beam splitters (PBSs) are necessary elements in the polarization diversity circuits and coherent optical communication systems to manage the polarization of silicon nanophotonic devices. However, achieving the low insertion loss (IL), compact footprint, broad bandwidth, and high extinction ratio (ER) of PBS at the same time remains challenging. Here we propose and experimentally demonstrate a compact and high-ER PBS with a directional coupler (DC) consisting of two metamaterial waveguides, which constructs the asymmetry of the DC-based PBS in a new way. For TM polarization, the proposed PBS performs as a DC, while for TE polarization, the device acts as two isolated waveguides since one of the metamaterial waveguides establishes a large bandgap of TE polarization. In simulation, an ultra-compact coupling region of $1.3 \times 6 \mu \mathbf{m}^{2}$ is achieved and the proposed PBS has an ER >20 dB and IL <0.5 dB in the wavelength range of 1500 - 1590 nm, which exceeds the entire C-band. The ER values at the central wavelength of 1550 nm are 48 dB and 36.6 dB for TE and TM modes, respectively. The experimental results illustrate that the fabricated PBS has excellent performance of ER >15 dB and IL <1.2 dB within a wide bandwidth of 90 nm.