Low-loss and broadband silicon-based multimode waveguide crossing at 2 μm using 2-D sub-wavelength holey metamaterials

Silicon-based multi-mode optical waveguide crossings with small size and high performance are crucial components for building photonic integrated circuits, but almost all the current devices operate around the near-infrared bands of 1.31 μm and 1.55 μm. To further enhance communication capacity, in this paper, a multimode (e.g., TE₀-TE₂) waveguide crossing operating at mid-infrared waveband of 2 μm, with good performance, is proposed and experimentally demonstrated using a full two-dimensional (2-D) sub-wavelength holey metamaterial (SWHM) waveguide, where the specific distributed 2-D hole arrays are inserted into two orthogonal multi-mode interference waveguides. Here, the introduction of SWHMs offers great design flexibility and tunability to generate the desired refractive index distributions within the crossing structure that enables tight trap of incident multiple TE modes in the through route over a large waveband around 2 μm, thus realizing an effective multimode crossing with low insertion loss, low crosstalk and large bandwidth. Simulation results show that the realized device has a compact size of 12.6 µm × 12.6 µm and its insertion losses are, respectively, < 0.37, < 0.28, and <0.32 dB for TE₀, TE₁, and TE₂ modes over an ultra-broad wavelength range of 1700-2200 nm, while the crosstalks are below -32 dB. The experiment results verify that the average insertion losses for the three modes are as low as 0.14 dB, 0.08 dB, and 0.20 dB, respectively, with the corresponding crosstalk below -25 dB at 2 µm. The provided device may find potential applications in on-chip optical communications and computing.