Compact and low-loss on-chip silicon crossing based on subwavelength grating slot waveguides.

We propose and experimentally demonstrate a compact crossing based on a subwavelength grating slot waveguide (SWG-SW), leveraging the anisotropic properties of two orthogonal SWG-SWs to suppress diffraction effects in the crossing region for the input TE polarization mode. The design utilizes the unique mode characteristics of the SWG-SW, enabling low-loss mode transitions through the crossing with a simple and compact hollow structure design at its center. By integrating the above crossing with a well-designed slot-to-SWG-SW mode converter, a crossing for the widely used slot waveguides is achieved, with a footprint of ∼ 18×18 μm. Simulation results show that the device achieves an insertion loss (IL) of <0.35 dB and a crosstalk (CT) of <-40 dB over a bandwidth range of 1500-1600 nm. Experimental results confirm that within the wavelength range of 1500-1580 nm, the IL is approximately 0.4 dB, and CT values are below -28 dB. To further improve the device performance, additional side grating arrays are incorporated into the design. The optimized structure with side gratings achieves IL of <0.13 dB and CT values around -35 dB. Experimental measurements of the device with side gratings demonstrate a stable IL of approximately 0.1 dB and CT values consistently below -35 dB, marking a significant improvement over the original design. To the best of our knowledge, this is the first experimentally verified crossing for slot waveguides. Further analysis indicates that the device exhibits an exceptionally broad bandwidth of approximately 576 nm (1257 nm to 1833nm) with IL < 1 dB and CT < -30 dB.