Inverse‐Designed Compact Lithium Niobate Mid‐Infrared Devices

Mid‐infrared (MIR) photonic integrated circuits have attracted increasing attention owing to their crucial role in applications such as communications, sensing, and LiDAR. Notably, thin‐film lithium niobate (TFLN) is a promising platform due to its wide transparency window and low propagation loss, yet its anisotropic etching often results in sloped sidewalls that degrade device performance. Here, we introduce an inverse design strategy with fabrication constraints that explicitly incorporates sloped sidewall effects into the optimization process. Using this approach, we experimentally demonstrate a compact, low‐loss polarization beam splitter, power splitter, and waveguide crossing operating across the 3670–3730 nm band for the first time. It establishes a robust design framework for high‐performance and miniaturized MIR photonic devices on the 1400‐nm TFLN platform with a 4.7‐µm‐thick silica buffer, paving the way for their applications in secure free‐space optical communications, biochemical detection, and LiDAR within the MIR spectrum.