Mid-Infrared on-Chip Dichroic Filters Enabled by Adiabatic Evolution and Dispersion-Engineered Subwavelength Grating...

Broadband high-performance optical filters are fundamental building blocks for advancing mid-infrared (MIR) integrated photonics, particularly for applications in molecular sensing and free-space communication. Although dichroic filters offer robust high-roll-off performance in the near-infrared, their realization in the critical MIR atmospheric transparency windows (3-5 μm) has remained largely unexplored. In this work, we demonstrate a high-performance on-chip dichroic filter on a silicon-on-insulator (SOI) platform, specifically engineered for the 3300-3700 nm spectral range. By integrating a longitudinally segmented subwavelength grating (SWG) waveguide, we effectively mitigate slot-like propagation losses induced by sidewall roughness while simultaneously achieving precise control over mode dispersion. Through the synergistic adiabatic evolution and asymmetric directional coupler theory, the device achieves a maximum roll-off rate of approximately 1 dB/nm and an extinction ratio exceeding 12 dB at a measured cut-off wavelength of λ _c ≈ 3460 nm. The filters exhibit flat-top low-pass and high-pass characteristics over a 400-nm bandwidth with negligible insertion loss. Our results demonstrate strong agreement between theoretical analysis and experimental validation, providing a scalable solution for interchannel crosstalk suppression and high-SNR signal extraction in MIR photonic integrated circuits. This work fills a critical gap in the MIR toolkit, paving the way for sophisticated on-chip spectroscopic systems and robust free-space optical links.