Cluster-based two-dimensional gratings for structured illumination microscopy

Structured illumination microscopy is a widely used super-resolution technique that reconstructs images by encoding high-spatial-frequency information in an illumination pattern into the observable spectrum through the moiré effect. Periodic nanostructures, including two-dimensional metallic gratings, provide a versatile platform for generating structured illumination patterns. Previous studies have shown that two-dimensional metallic gratings can efficiently excite surface modes with free-space illumination, enabling the generation of structured optical field distributions. Here, we extend this platform by integrating a nanogranular Ag/TiO₂ cluster layer onto the silver grating to embed deeply subwavelength scattering features within the standing-wave illumination. An effective optical model for the nanogranular film is developed and validated through ellipsometry measurements and full-wave electromagnetic simulations. The cluster-integrated architecture preserves the excitation of angle-tunable quasi-standing modes while introducing highly localized near-field hotspots, with intensities approximately ten times larger than those of the bare silver grating, granting simultaneous control over the global phase of the illumination and the local field confinement. Blind-SIM reconstructions of a sub-diffraction emitter show that the additional high-spatial-frequency content introduced by the nanogranular layer effectively enlarges the optical transfer function and its shifted copies, yielding an average resolution enhancement of approximately 43% over the bare silver grating.