Far‐Field Readout of Coherent Plasmonic Coupling in Au Nanoisland Arrays

Non‐perturbative access to systems supporting coherent processes is important for both fundamental studies and emerging quantum technologies. Here, we show that encoded coherent optical processes in plasmonic nanoparticle networks can be interrogated through far‐field in‐plane scattering. The networks consist of Au nanoislands with random shapes and sizes arranged into long, straight chains (“island‐lines”) approximately 200 in length. Periodic repetition of these island‐lines enables Rayleigh‐anomaly‐mediated coupling–i.e., the formation of surface lattice resonances–establishing coherent nanoparticle networks driven by in‐phase excitation of nanoisland plasmonic dipoles. Using structured illumination to control the momentum‐space distribution of excitation, we selectively drive these lattice resonances and monitor the resulting in‐plane scattering patterns. The polarization and intensity distributions of the scattered light provide a far‐field readout of phase‐correlated collective responses, demonstrating how signatures of coherent processes can be extracted even when optical phase is completely lost. This platform provides a route for accessing collective plasmonic coherence with potential applications in sensing, optical encoding, and studies of coupled emitter–plasmon systems.