Dependent scattering and plasmon coupling in concentrated suspensions of optically hard nanoparticles

This study establishes the accuracy and efficacy of the recently developed radiative transfer with reciprocal transactions (R²T²) method for quickly simulating radiation transfer through concentrated thick suspensions of optically hard nanoparticles featuring a large mismatch in refractive and/or absorption indices compared with their surrounding medium. Concentrated suspensions of optically hard nanoparticles exhibit strong light scattering and dependent scattering effects including both near-field interactions among particles and interferences of scattered waves in the far-field. Concentrated suspensions of metallic nanoparticles also exhibit plasmon coupling effect that leads to widening of absorption peak and red-shift in the peak surface plasmon resonance wavelength. However, predicting these complex interactions between EM waves and particles in thick and concentrated suspensions by explicitly solving Maxwell's equations is computationally intensive, if not impossible. Conventional solutions like Lorenz–Mie theory combined with independent scattering approximation do not account for dependent scattering and plasmon coupling. Furthermore, the dense medium radiative transfer theory is a far-field approximation that does not account for near-field effects, leading to significant errors in predictions, as illustrated in this study. By contrast, the R²T² method's predictions showed excellent agreement with the solutions of Maxwell's equations obtained using the superposition T-matrix method for thin films containing optically hard particles. The method also rigorously accounted for multiple scattering as well as plasmon coupling in thick concentrated suspensions. These results could facilitate the design of plasmonic suspensions used in various energy and environmental applications.