Ultra-broadband dual-polarization multimode silicon waveguide crossing enabled by subwavelength-grating-induced bound states

• Compact dual-polarization multimode silicon crossing with 12-mode operation, <0.95 dB loss, <−20 dB crosstalk over 350 nm. • Two-dimensional subwavelength grating arrays enable broadband index engineering and optical bound-state formation. • Mode-scalable design to 20 modes via single-step fabrication, setting benchmark for hybrid multiplexing photonics. We propose and experimentally demonstrate a compact, ultra-broadband and dual-polarization multimode waveguide crossing. This design employs a two-dimensional subwavelength grating array to regulate the equivalent refractive index around the crossing region, thereby forming bound states. To validate the structural model, we design and fabricate a waveguide crossing supporting 12 modes (TE 0 -TE 5 and TM 0 -TM 5 ) on the standard 220 nm silicon-on-insulator platform using a single-step etching process. The device features a compact footprint of 13 × 13 μm 2 and operates within a 12-mode dual-polarization high-performance bandwidth covering 1350–1600 nm, where all modes maintain insertion losses below 0.95 dB and crosstalk levels below –20 dB. For TE polarization, the operational range further extends from 1350 to 2100 nm, while TM polarization exhibits a broad spectral response from 1200 to 1600 nm. Experimental results show that the 12-mode waveguide crossing device achieves insertion loss < 1.08 dB/<0.63 dB (TE 5 /TM 3 ) and crosstalk < -20 dB in the 1500–1600 nm range. Notably, by flexibly adjusting the waveguide width and the subwavelength grating array parameter N 2 , this structure can be further expanded to support higher-order modes (such as TE 0 -TE 9 , TM 0 -TM 9 ), demonstrating outstanding mode-expansion capability. The proposed device combines broadband performance, dual-polarization support, and a compact footprint, setting a record among multimode waveguide crossings. It effectively addresses the key challenge of dense integration in polarization- and mode-division multiplexing systems, providing a solid foundation for high-capacity on-chip optical interconnects.