Inverse Design of Nanophotonic Color Router Robust to Oblique Incidence

Abstract

With continued pixel miniaturization, conventional micro-lenses and absorptive color filters in CMOS image sensors suffer from significant performance degradation and low photon influx as the pixel pitch approaches the diffraction limit. While numerous studies have explored nanophotonic color routers as a solution, most lack a systematic analysis of optical efficiency under broad incident angles, which is critical for practical CIS applications. In this work, an inverse design framework based on automatic differentiation is developed to achieve robustness against incident angle variations. The design maintains a high optical efficiency of 78% on average within a +/- 12 degrees incident angle range for unpolarized light.The trade-off relation between the acceptance angle range and the achievable optical efficiency of the device is also identified. The proposed optimized method is then demonstrated and can be further extended to design broadband broad-angle color routers. These findings pave the way for more efficient and versatile CMOS image sensors, offering substantial improvements over existing methods by ensuring efficient light utilization under realistic illumination conditions.