Suppressing Stitching Errors in Full-Sensor-Plane Color Routers via Optical Structural Similarity

Nanophotonic color routers promise higher photon utilization than absorptive color filters for complementary metal-oxide-semiconductor (CMOS) image sensors, yet most reported designs remain optimized for normal incidence, with efficiency dropping to about half of its peak by $\theta \approx 6°$ . Here, we present an angle-specific inverse-design framework in which the sensor plane is partitioned into local angular zones, and each zone is assigned a dedicated color-router unit cell. To enable practical assembly of these heterogeneous unit cells, we introduce an optical structural similarity (OSS) constraint that enforces permittivity-level continuity between neighboring cells during topology optimization without additional full-wave simulations. In a three-dimensional freeform library, the optimized unit cells achieve average optical efficiencies of 91.1%, 80.9%, and 83.9% for the red, green, and blue channels, respectively, with optical cross talk below 2.0%. OSS reduces the average stitching error from 24.6% to 5.99%, suggesting that it may provide an effective means of suppressing stitching errors in metasurface designs based on the locally periodic approximation. The framework further extends to a five-layer architecture with a comparable stitching error of 5.65%, establishing a practical route to oblique-incidence-robust, library-based color routing and, more broadly, to large-area metasurfaces requiring spatially varying functionality with robust inter-cell compatibility.