Mid‐Infrared on‐Chip Dichroic Filters Enabled by Adiabatic Evolution and Dispersion‐Engineered Subwavelength Grating Waveguides

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.