Ultracompact multi-mode add-drop multiplexer based on pixelated photonic-like crystals

The development of integrated photonic systems necessitates ultracompact multi-mode add-drop multiplexers to enable high-density node interconnection communication. Conventional cascaded directional couplers, however, suffer from prolonged coupling lengths and intermodal crosstalk during the evanescent wave coupling progress, imposing fundamental limits on device miniaturization and multi-mode adiabatic conversion. Here, we propose a groundbreaking single-level mode conversion mechanism enabled by pixelated photonic-like crystals. By leveraging subwavelength-scale dielectric scattering units, we achieve simultaneous mode field confinement and phase compensation, reducing the coupling length by two orders of magnitude (to 3.6 μm × 5.4 μm) while suppressing intermodal interference. Benefiting from this design, we demonstrate an ultracompact mode add-drop multiplexer supporting simultaneous add/drop of 1-3 TE modes with the operation broadband ≥ 38 nm and insertion loss < 4 dB. Furthermore, the devices enable routing of 11.25 Tbit/s QPSK-OFDM signals with bit error rates of 10^-5 at -13.5 dBm. This work verifies the feasibility of on-chip communication interconnection through the single-level mode conversion mechanism, offering a versatile solution for integrated photonic networks with potential extensions to LiDAR, quantum key distribution, and neuromorphic photonics.