Quasiresonant Regime of Surface Plasmon for Broad Angular Responsivity of Plasmonic Diffraction

We propose plasmonic diffraction under quasiresonant regime of surface plasmon (quasi-SPR) that exhibits broad incident-angle responsivity. Numerical simulations reveal that quasi-SPR induces efficient and large-angle diffraction toward the transmission side over a wide range of oblique incidences by tuning the metal grating period to satisfy the Rayleigh anomaly and by optimizing the grating width and height to maximize diffraction efficiency at normal incidence. In a backside-illuminated silicon image sensor integrated with metal gratings, near-infrared absorption in silicon exceeded 40% for incident angles up to ±18° within a limited thickness of 3.3  μm. The angular absorption profile reflects the inherently broad response of quasi-SPR and provides a versatile approach to enhance light-matter interaction in compact optoelectronic devices.