Ultra-Broadband Multimode Waveguide Crossing via Subwavelength Transmitarray with Bound State

As a fundamental building block for photonic integrated circuits, waveguide crossing is playing an essential role for routing the increasingly complex network on chip. Driven by the demands for carrying a large capacity of data per waveguide, wavelengths and guiding modes are emerging as important degrees of freedom for optical multiplexing to embrace the advantages of signal parallelism. Although various waveguide crossing structures are proposed to either support broad spectra of wavelengths or a large number of modes, none of these are simultaneously achieved. Here, an ultra-broadband multimode waveguide crossing based on subwavelength transmitarray (SWTA) is proposed. The SWTA is well designed for high transmittance in the propagation direction and excellent confinement in its orthogonal direction by creation of a bound state. A three-mode crossing is designed and fabricated on silicon on insulator with a compact footprint of only 5.05 mu m x 5.05 mu m. The crossing is able to operate over a large bandwidth from 1.4 to 2.1 mu m with an excess loss of less than 0.77 dB. For a proof-of-concept demonstration of the structure scalability, the crossing is predicted to support up to ten modes. The ultra-broadbandwidth and compactness of this multimode crossing make it possible to densely cross connect the waveguides with large data capacity via wavelength and mode division multiplexing.