Template-Assisted Capillary-Assembly of Crystalline Silicon Nanoparticles for All-Dielectric Nanoantenna

Nanoparticles (NPs) of high refractive index materials supporting Mie resonances in the visible to near infrared range have been attracting much attention as a versatile platform for manipulating light-matter interaction at the nanoscale. By properly assembling high-index NPs, the optical response can be tailored via the near field coupling between NPs. The coupling provides accessible electric and magnetic hot spots in an assembly, and modulates the directionality of the light scattering.<br/>A NP assembly can be produced by top-down microfabrication technology and bottom-up self-assembly technology. The former one has high accuracy and high reproducibility, but reducing the gap between NPs below 10 nm, which is crucial for near field coupling of the resonance modes, is still not straightforward. On the other hand, an assembly of almost touched NPs can be easily produced by the latter process. The research on self-assembly of NPs has been preceded by plasmonic NPs and a variety of structures have been produced with high controllability and reproducibility. <br/>In this work, we develop a process to fabricate linear and zigzag arrays of silicon NPs by using a template-assisted capillary-assembly method. For the successful application of the method for controlled formation of a silicon NP assembly, a high-quality precursor, i.e., an agglomeration-free solution of silicon NPs with uniform size and shape, is crucial. We have developed a solution of perfectly spherical silicon NPs dispersible in alcohol by thermal disproportionation of silicon monoxide at very high temperature (&gt;1475°C) and chemical etching, and by applying density gradient centrifugation process for the size separation [1]. For different length linear arrays of almost touched silicon NPs about 140 nm in diameter produced by the process, we measured angle- and polarization-resolved scattering spectra. We show that coupling of the electric dipole and magnetic dipole resonances between NPs strongly modifies the scattering spectra. For example, coupling shifts the resonance energy of the electric dipole mode, resulting in almost perfect overlap of the electric and magnetic resonances in a linear oligomer of silicon NPs. This appears as the Kerker-type forward scattering near the peak of the magnetic dipole resonance. We will also discuss the dark modes in coupled silicon NP arrays.<br/>[1] Sugimoto, H. et al., Adv. Opt. Mater., 8 (12), 2000033. (2020)