Open and Close-Packed, Shape-Engineered Polygonal Nanoparticle Metamolecules with Tailorable Fano Resonances.

A top-down lithographic patterning and deposition process is reported for producing nanoparticles (NPs) with well-defined sizes, shapes, and compositions that are often not accessible by wet-chemical synthetic methods. These NPs are ligated and harvested from the substrate surface to prepare colloidal NP dispersions. Using a template-assisted assembly technique, fabricated NPs are driven by capillary forces to assemble into size- and shape-engineered templates and organize into open or close-packed multi-NP structures or NP metamolecules. The sizes and shapes of the NPs and of the templates control the NP number, coordination, interparticle gap size, disorder, and location of defects such as voids in the NP metamolecules. The plasmonic resonances of polygonal-shaped Au NPs are exploited to correlate the structure and optical properties of assembled NP metamolecules. Comparing open and close-packed architectures highlights that introduction of a center NP to form close-packed assemblies supports collective interactions, altering magnetic optical modes and multipolar interactions in Fano resonances. Decreasing the distance between NPs strengthens the plasmonic coupling, and the structural symmetries of the NP metamolecules determine the orientation-dependent scattering response.

Broadband, Angle- and Polarization-Invariant Antireflective and Absorbing Films by a Scalable Synthesis of Monodisperse Silicon Nanoparticles.

Optically induced magnetic resonances (OMRs) are highly tunable scattering states that cannot be reproduced in systems that only support electric resonances, such as in metals, lossy, or low-index materials. Despite offering unique scattering and coupling behavior, the study of OMRs in thin films has been limited by synthesis and simulation constraints. We report on the absorption and scattering response of OMR-based thin films composed of monodisperse crystalline silicon nanoparticles synthesized using a scalable nonthermal plasma growth technique and tractable simulation framework. The synthesis is solvent and ligand free, ensuring minimal contamination, and crystalline particles form with high yield and a narrow size distribution at close to room temperature. Using a scalable high-throughput deposition method, we deposit random particle films, without the need of a solid host matrix, showing near complete blackbody absorption at the collective OMR. This is achieved using 70% less material than an optimized antireflective-coated crystalline silicon thin film. The film exhibits strongly directional forward scattering with very low reflectivity, thus giving rise to angle- and polarization-insensitive antireflection properties across the visible spectrum. We find that, while commonly used effective medium models cannot capture the optical response, a modified effective medium accounting for multipole resonances and interparticle coupling shows excellent agreement with experiment. The effective permittivity and permeability are written in a mode and cluster resolved form, providing useful insight into how individual resonances and nanoparticle clusters affect the overall film response. Electric and magnetic-mode coupling show dramatically different behavior, resulting in uniquely different spectral broadening.