Unconventional Optical\nMatter of Hybrid Metal–Dielectric\nNanoparticles at Interfaces

Optical matter, a transient arrangement formed by the\ninteraction\nof light with micro/nanoscale objects, provides responsive and highly\ntunable materials that allow for controlling and manipulating light\nand/or matter. A combined experimental and theoretical exploration\nof optical matter is essential to advance our understanding of the\nphenomenon and potentially design applications. Most studies have\nfocused on nanoparticles composed of a single material (either metallic\nor dielectric), representing two extreme regimes, one where the gradient\nforce (dielectric) and one where the scattering force (metallic) dominates.\nTo understand their role, it is important to investigate hybrid materials\nwith different metallic-to-dielectric ratios. Here, we combine numerical\ncalculations and experiments on hybrid metal–dielectric core–shell\nparticles (200 nm gold spheres coated with silica shells with thicknesses\nranging from 0 to 100 nm). We reveal how silica shell thickness critically\ninfluences the essential properties of optical binding, such as interparticle\ndistance, reducing it below the anticipated optical binding length.\nNotably, for silica shells thicker than 50 nm, we observed a transition\nfrom a linear arrangement perpendicular to polarization to a hexagonal\narrangement accompanied by a circular motion. Further, the dynamic\nswarming assembly changes from the conventional dumbbell-shaped to\nlobe-like morphologies. These phenomena, confirmed by both experimental\nobservations and dynamic numerical calculations, demonstrate the complex\ndynamics of optical matter and underscore the potential for tuning\nits properties for applications.