Inverse-Design-Assisted On-Chip GRIN Meta-Lens for Dual-Polarization Multimode Waveguide Crossing

On-chip meta-lenses provide a flexible approach to manipulate guided waves on a chip using gradient-index subwavelength slot arrays. These arrays have recently gained attention for their compactness and reliability in functional photonic components. However, the reported schemes have only been demonstrated for single polarization. Here, the gradient-index meta-lens is utilized to realize a universal multimode waveguide crossing, which is an important component in the mode division multiplexing circuit, allowing complex network routing. For a proof-of-concept demonstration, a compact and three-channel silicon waveguide crossing for supporting six modes (three TE-polarization modes and three TM-polarization modes) is proposed and experimentally demonstrated, which has less insertion losses (<0.7 dB) and low crosstalks (<−14 dB) in the wavelength range from 1500 to 1600 nm. Moreover, a five-channel dual-mode waveguide crossing is also demonstrated, revealing that our solution can theoretically be flexibly extended to support more modes and more transmission channels. Our study paves the way for realizing dense-integration and large-capacity on-chip multimode routing and multiplexing systems.